97 Grand Prix With Intermittent Miss

Hello Everyone,

I have been following this forum for some time now and
finally decided to join. I have a 97 Grand Prix GTP with
110,000 thousand miles on it. I am having a problem with
and intermittent miss which is going to drive me in the nut
house. I had a friend hook up a scanner and we discovered
number 3 cylinder had over 200 missfires and number 4
cylinder had over 16,000 missfires in history. When scanning
in real time mode number 4 cylinder was actively missfiring.
So I changed all 3 coil packs, plugs and wires. Car starts much
faster now when cold and idle quality has improved significantly.
However, the miss still persists. I have done a fuel pressure
check and the readings appear to be good with the vacuum
applied and disconncted from the FPR. The fuel pressure is holding
solid after the 2 second initial prime. I have removed the throttle
body and did a thorough cleaning with electrical components removed
of course, and the air filter was also replaced. The problem still persists.
Fuel pump and fuel filter are newer as they were replaced at around
70,000 due to internal check valve failure inside pump. The miss seems
more pronouced when the TCC is engaged suppose something to do with
the direct connection. The TCC does engage around 35 mph and when you
tap the brake it disengages. It will also release when accelerating briskly.
When in the higher RPM range such as flooring the pedal you can then really
feel the car miss and on occasion the car backfires. My thinking is either
the crankshaft sensor or MAF sensor but don't want to keep throwing parts
at it till I am reasonably sure of either one. The PCM is not giving up any
secrets either, no SES light and no codes stored checked several times.

Thanks for any info you are willing to part with and sorry for the long winded
post. I just thought I would give all the facts so a reasonable conclusion
can be reached.

Once Again Thanks Everyone,
Steve

How about fuel system cleaner? Not just any. Try Chevron Techron. Red bottle. Get two of them. Do 12 gallons of fuel with one bottle, run it out and repeat. I think You'll be pleasantly surprised.

How about fuel system cleaner? Not just any. Try Chevron Techron. Red bottle. Get two of them. Do 12 gallons of fuel with one bottle, run it out and repeat. I think You'll be pleasantly surprised.

I have run 2 cans of B+G 40 fuel systems cleaner which from what I
understand is hard to beat as far as cleaning the fuel system, and still
no joy !!!

How long have you owned this car?
Do you run 87 octane in it on a regular basis?

I would strongly suggest doing a compression test. The #4 cylinder is the one most prone to detonation damage due to running 87 octane if you or the previous owner have made this mistake. It's also the most common cylinder to develop head gasket issues on these engines due to being in the center in the rear (less external surface area and airflow). Since you also have misfires in Cyl #3 (the center one in the front bank), it points even more toward possible head gasket issues.

How long have you owned this car?
Do you run 87 octane in it on a regular basis?

I would strongly suggest doing a compression test. The #4 cylinder is the one most prone to detonation damage due to running 87 octane if you or the previous owner have made this mistake. It's also the most common cylinder to develop head gasket issues on these engines due to being in the center in the rear (less external surface area and airflow). Since you also have misfires in Cyl #3 (the center one in the front bank), it points even more toward possible head gasket issues.
Good sugestion, but would I not have coolant
loss or the typical white smoke in the exhaust
from a blown head gasket. I have neither at this
time. I have checked the oil and it does not exhibit
the charistic coffee and cream look if water were mixing
with the oil.

Thanks for the quick response !!!

Steve

Good sugestion, but would I not have coolant loss or the typical white smoke in the exhaust from a blown head gasket.

In a word No. What if a headgasket blows between cylinders?

I realize my opinion doesn't mean much, but I agree with Rich, with any missfire problems, especially ones that can be narrowed down to a specific cylinder like #3 or #4. A person can waste a TON of money throwing parts at it and feel like a real idiot when they finally realize that a valve is burnt or a piston is cracked.

The fuel pressure is holding solid after the 2 second initial prime.

Whats this mean? You have to turn the key on, off, then back on in order for the fuel system to build and hold adequate pressure? Thats not normal, the pressure should build and hold with the first key on prime.

In a word No. What if a headgasket blows between cylinders?

I realize my opinion doesn't mean much, but I agree with Rich, with any missfire problems, especially ones that can be narrowed down to a specific cylinder like #3 or #4. A person can waste a TON of money throwing parts at it and feel like a real idiot when they finally realize that a valve is burnt or a piston is cracked.



Whats this mean? You have to turn the key on, off, then back on in order for the fuel system to build and hold adequate pressure? Thats not normal, the pressure should build and hold with the first key on prime.

When the key is turned on the first time the pressure builds and holds,
I don't have to keep turning the key on and off to build up pressure.
Disconnected MAF sensor and ran car with no real change, car still
intemittently missfires. Popped hood and noticed that both exhaust
manifolds were glowing as if someone applied a blow torch to them. Allowed
car to cool down and reconnected MAF sensor and then ran car some more
to verify that the glowing manifolds was not a result of the sensor being
disconnected. Once again both manifolds were glowing a nice orange
color. Is it possible that the crank sensor is starting to fail allowing
combustion to take place in the exhaust manifold ?

Once Again Thanks Guys for Your Input !!!
Steve

Nope, I'm not quiet sure....

I know cat converters will glow red hot if the motor runs really rich.

But I think you may have a lean condition. Have you replaced the fuel filter? What are your exact fuel pressure readings?

Nope, I'm not quiet sure....

I know cat converters will glow red hot if the motor runs really rich.

But I think you may have a lean condition. Have you replaced the fuel filter? What are your exact fuel pressure readings?

I guess I should be more specific when describing stuff. The exhaust
manifold only glows red just a ways below number #5 cylinder and just
below number #4 cylinder. I don't believe I have intake manifold gasket
issues as the car runs dead to rights at 700-750 rpm at idle. I know I
should probably check with a vacuum gage but I just don't have the
luxury of spare time as I am taking care of ageing parents. I did stop by
the Pontiac dealership today and discussed the symtoms with service
advisor and he felt that if crank sensor was failing it should throw a code.
He did mention that perhaps it might be leaking fuel injectors or possibly
faulty injectors which I am more inclined to agree with. The fuel readings
which you are wondering about are dead on per the GM service manuals.
The pressure holds well at first key on. It can go 30-40 minutes before
losing some pressure. Not sure about the cat though, if I step on the gas
hard wow does the car ever move out. No power drop off what so ever,
the car easily does 70 just like it was new. I just have this darn missfire which
is random at best. As I mentioned earlier time is a luxury I dont have much
of so I have decided to let the Pontiac dealership down the road see what
they can find with their scanning equipment. Could be the best 100.00
dollars spent.

Appreciate your interest in this problem and I will update tomorrow,

Thanks Again, Steve

Tim, don't short change yourself, give yourself some credit as your opinion does matter. You give solid advice based on what you've learned both hands on and from what you've read. You've been an asset to AF since you owned your Lumina, and nothing has changed since you got your GTP and came over here.

Srlash, not all head gasket leaks result in coolant loss or coolant/oil contamination. As Tim stated, you could have a compression leak between cylinders. Burnt valves and pistons can also cause compression loss and a resultant mis-fire. Cylinders 4 and 5 are the most prone to develop these as well, again due to the fact that they run a little hotter than the other 4 cylinders. It sucks you don't have time and had to take it in, but please let us know the outcome. We hate threads that have no resolution and your answer may help others in the future.

Tim, don't short change yourself, give yourself some credit as your opinion does matter. You give solid advice based on what you've learned both hands on and from what you've read. You've been an asset to AF since you owned your Lumina, and nothing has changed since you got your GTP and came over here.

Srlash, not all head gasket leaks result in coolant loss or coolant/oil contamination. As Tim stated, you could have a compression leak between cylinders. Burnt valves and pistons can also cause compression loss and a resultant mis-fire. Cylinders 4 and 5 are the most prone to develop these as well, again due to the fact that they run a little hotter than the other 4 cylinders. It sucks you don't have time and had to take it in, but please let us know the outcome. We hate threads that have no resolution and your answer may help others in the future.

Richtazz and Tblake,

I did not mean to question your suggestions as far as a blown head gasket
as I am certainly not an expert by any means. I hope I have not offended
either of you and I do appreciate both of your input. I had never heard of
a blown head gasket between cylinders so now I have learned something new
which is what this forum is all about. Pontiac dealership called and I will let
you know what they said, "it is not going to be easy to fix this car". Gotta
run right now, old people are keeping me on the run !!!

Steve

No offense taken Steve. You've got a bit of a head scratcher here, so we're throwing it all out there, just in case you didn't think of something.

On the keep it simple side of things, I would also double check the spark plugs to make sure that you didn't accidentally crack one while installing them. It's a fairly common occurrance with the rear plugs, since they're so easy to get to :eek7: !

So Steve, the dealership called you back an said the car wont be easy to fix? Uh Oh, doesnt sound good!

BTW, thanks for the comments Rich, I really appreciate it. However I doubt I will ever be as knowledgable as you moderators/advisors.

So Steve, the dealership called you back an said the car wont be easy to fix? Uh Oh, doesnt sound good!

BTW, thanks for the comments Rich, I really appreciate it. However I doubt I will ever be as knowledgable as you moderators/advisors.

Hello Again Guys,

Well picked up the car from the dealership, and spoke with the
service advisor and here is what he had to say. I was only charged for
1 hour of diagnostic service and they spent a total of 3 hours with the car.
They are pretty decent people to deal with. Now for the problem at hand;
the car has a steady missfire at 3000 rpm. A little less or more rpm and the
miss will stop. They checked everything from soup to nuts. They did a
compression check on all cylinders was informed compression was good on
all cylinders. They checked fuel injectors and said they all passed as well.
They double checked my work as well, swapping coils,checking plugs for
cracked insulators like you guys suggested and still nothing concrete. The
crank sensor, mass air flow sensor and all the other sensors pass all the
diagnostic checks and none of the sensors are throwing any codes. Oh by
the way, the missfire is on number 4 cylinder. They did not mention if they
performed a vacumm check I can only assume that one was done. They said
that what I have is very unusual in that #4 cylinder only missfires at 3000 rpm.
They hinted that perhaps the PCM was failing but did not want to suggest that
as this part is extremly expensive. Any suggestions guys as I am sure as hell out
of ideas. Oh sorry they did say that the intake manifold gasket was ok, age
catching up wih me HA.

Thanks for any info !!!
Steve

Wow, that is an interesting problem....

I'm stumped!

Will it missfire in park at 3000rpm's?

You can do like Rich stated in a different post.... Use an inductive timing light on the number 4 sparkplug wire, have a buddy hold the rpms at 3000 so it misses, then aim the timing light at the underside of the hood as you watch the spark pattern by the light flashes or lack of.

This might help you narrow it down to either ignition or fuel related. It sounds as though your motor is mechanically sound, so thats good!

If you dont have a timing light, I think you should be able to rent one from a place like autozone, usually free if you return it the same day. Just do your diagnosis in the parking lot and return it within 15 minutes.

Wow, that is an interesting problem....

I'm stumped!

Will it missfire in park at 3000rpm's?

You can do like Rich stated in a different post.... Use an inductive timing light on the number 4 sparkplug wire, have a buddy hold the rpms at 3000 so it misses, then aim the timing light at the underside of the hood as you watch the spark pattern by the light flashes or lack of.

This might help you narrow it down to either ignition or fuel related. It sounds as though your motor is mechanically sound, so thats good!

If you dont have a timing light, I think you should be able to rent one from a place like autozone, usually free if you return it the same day. Just do your diagnosis in the parking lot and return it within 15 minutes.

When I started having this missfire issue originally, my friend scanned it
in real time and he nor I could not understand how #4 cylinder could be
missfiring at idle but when raising the rpm to about 2000-3000 rpm it
would not missfire. Now after changing the coil pack, plugs and wires it
appears we have the exact opposite, in that it will only missfire around
3000 rpm. Oh by the way the service manger did advise that they did
check the ignition system from top to bottom including the ignition
module with no faults present. I did try the 3000 rpm in park but it
doesn't appear to be missfiring. It seems like you need a load on the
motor for the missfire to rear its ugly head. I will bug my friend with his
scanner and report back here with the results. Perhaps this problem is
related to the PCM. I dont really see how 1 cylinder could be affected
by a bad crank sensor, mass air flow sensor or any of the other sensors
for that matter.

Thanks,
Steve

I agree, something has to be going on... A flaky CPS or MAF will not only cause one cylinder to missfire.

Just for arguments sake, if I were you I'd pull out that #4 plug again and inspect it. I know a lot of forum members have recently had back luck with brand new delco 41-101 plats cracking. Not sure what type of plugs you put in. This might make sense though. When at load and combustion chamber pressures ar the highest, the spark has a hard time jumping the plugs gap, and might be arching out through a crack in the ceramic of the plug.

Maybe even swap the #4 sparkplug with an easy one to reach and see what cylinder is missfiring now. Not to say the dealership didnt do this, but you never know hoe "thourough" they were at checking things over.

I agree, something has to be going on... A flaky CPS or MAF will not only cause one cylinder to missfire.

Just for arguments sake, if I were you I'd pull out that #4 plug again and inspect it. I know a lot of forum members have recently had back luck with brand new delco 41-101 plats cracking. Not sure what type of plugs you put in. This might make sense though. When at load and combustion chamber pressures ar the highest, the spark has a hard time jumping the plugs gap, and might be arching out through a crack in the ceramic of the plug.

Maybe even swap the #4 sparkplug with an easy one to reach and see what cylinder is missfiring now. Not to say the dealership didnt do this, but you never know hoe "thourough" they were at checking things over.

I installed ru ready for this a set of Autolite Double Platinum plugs in the car.
Wanted to put the iridiums in but no one stocked them, had to special order,
cost I guess. Actually swapping plugs is a darn good idea or perhaps I will
just buy a single plug and try that. I never seen anything like this in my life
with this 1 cylinder missfire and then only under certain conditions. I will
keep you all in the loop on this one, it may take a day or two till I have my
friend scan the engine and to change the #4 spark plug.

Thanks, steve

autolite double plats arent bad plugs. I have AL single plats in my car, and it seems to run really good with them.

You could get a new plug to stick in there, or you could just swap in an old one if you didn't thow them out yet.

autolite double plats arent bad plugs. I have AL single plats in my car, and it seems to run really good with them.

You could get a new plug to stick in there, or you could just swap in an old one if you didn't thow them out yet.

Swapped coils and installed new spark plug in
number 4 cylinder, still same problem. I am now
thinking of changing the ignition module.

Steve

Swapped coils and installed new spark plug in
number 4 cylinder, still same problem. I am now
thinking of changing the ignition module.

Steve

yeah, I was thinking that was a posibility too, but why not have it tested?

Take it to advance or autozone, they can hook it up to a machine to test it. When they are done, have them do it again to get it nice and hot.

Very strange problem!

If it runs good at idle and above 3000 rpm, then your fuel injector should be working fine. The only other thing I could think of would be your PCM itself.

Ever consider a modified PCM like a DHP from pfyc.com? Now might be the time....

yeah, I was thinking that was a posibility too, but why not have it tested?

Take it to advance or autozone, they can hook it up to a machine to test it. When they are done, have them do it again to get it nice and hot.

Very strange problem!

If it runs good at idle and above 3000 rpm, then your fuel injector should be working fine. The only other thing I could think of would be your PCM itself.

Ever consider a modified PCM like a DHP from pfyc.com? Now might be the time....

Hi Guys,

Just a quick update on my problematic car. I called Autozone and Advance
Auto here in the Wash D.C area and no one can check the ignition module on
my car, maybe this is a service no longer offered. Anyway, ...tried spraying
wires on my car with water no arcing was seen. I have changed the number
4 injector and the problem persists. I have once again hooked up a scanner
in real time and number 4 cylinder will intermittently misfire. I think I will try
to ohm out the number 4 injector wiring while moving the wire bundle around
to see if I can shake any gremlins loose. Next step if that doesn't produce
results is a new ignition module I suppose. Question, is it possible that a failing
ignition module would only affect 1 cylinder? I was under the impression that
it would have more issues than just 1 cylinder missfiring. I have also taken some
vacuum readings, 18in/hg at idle with very little needle movement.

Any Ideas Guys,

Thanks Steve

I see we have another head scratcher. I get the feeling of deja vu on these ones. :eek:

To start what is the engine vacuum reading with load? Take it up to a range of 2000-3000 rpms for several seconds and see what the gauge does.

If you have access to a full function odb-ii scanner what are the short and long term fuel trim readings. Also, what does the MAF test read?

I see we have another head scratcher. I get the feeling of deja vu on these ones. :eek:

To start what is the engine vacuum reading with load? Take it up to a range of 2000-3000 rpms for several seconds and see what the gauge does.

If you have access to a full function odb-ii scanner what are the short and long term fuel trim readings. Also, what does the MAF test read?

Thanks for your time Bnaylor,

At idle with the car in park at 750 rpm 17.5 - 18/hg.
Snapping the throttle open, reading goes to 0/hg then
to 22/hg very quickly then returns to 17.5 -18/hg.
Slowly raising the throttle to 3000 rpm and maintaining
3000 rpm reading will rise to 20/hg and then slowly settle
back down to about 17.5 - 18/hg. Please note needle movement
is very smooth in all the tests above with no flicker what so
ever. Long term fuel correction is all over the place, today
with no engine missfire it was .7 Yesterday with the chronic
missfire it was -10. Ran the car on the freeway today at a
steady speed of 60 mph with the scanner hooked up in real time.
MAF sensor was toggling up and down properly in relation to
inclines and going down hills. Pre Cat o2 sensor is toggling
quickly between .100 - .900mv while the post Cat o2 sensor is
toggling slower. All other sensors such as EGR,IAT,coolant,MAP
and TPS sensors are toggling as well in relation to engine load
and speed of the car. Ohmed out the number 4 injector wires back
to their source;the underhood electrical center and the PCM. Moved
wires, to verify good connection. Ohmeter never read more than
0.0 ohms indicating both wires should be good. Pontiac mechanic
thought it might be a PCM driver circuit for the number 4 injector
which is intermittently going bad. Any suggestions? Could it possibly
be the ignition module?

Once Again Thanks,

Steve

Both the PCM and Ignition module are expensive parts to just be throwing at it....

Remember my days at the tech college, my instructor fabricated sort of a noid light out of a resistor, LED, injector plugs, and wires.

Basically at one end it would connect to the injector and the injector harness at the car end, so escentially this "tool" was completing the injector circuit and had an extra pig tail hainging off about 10 feet of wire. At the opposie end was a resistor and LED that you could move into the passenger compartment with you. Thus allowing you to monitor the PCM injector pulses with this LED noid light while your driving your car. Hope that makes sense.

I think he gave me a sheet/schematic on how to construct this with the correct ohm value of the resistor as well as gave radio shack part numbers.

I was always going to do it, but had no time. If this is something you think would be worth your time, I will try and dig up the sheet.

Vacuum looks good so we can rule out any exhaust backpressure issue or a bad CAT converter. However, the fuel trim indication at misfire is not good and rings a bell.

To be honest with you we would be speculating on whether the ICM or the PCM modules are the cause in this case. If the dealer ran a Tech 2 scan tool then you would figure that would have been detected. Plus wouldn't want you to waste any money or give you any false hope.

In the meantime we covered a similar recent issue with another member. Short/long term fuel trims way off and a persistent misfire. Maybe some of your answers are there so I would recommend reviewing the two threads posted on the issue. We darn near wrote a book on it. However, there was no resolution that we know of. See links below.

Click here (http://www.automotiveforums.com/vbulletin/showthread.php?t=903976)

Click here (http://www.automotiveforums.com/vbulletin/showthread.php?t=900921)

.... However, the fuel trim indication at misfire is not good and rings a bell.

Dare I ask what your getting at here Bob?

Dare I ask what your getting at here Bob?

:uhoh:

Your guess is as good as mine Tim. Use your imagination. :lol:

:uhoh:

Your guess is as good as mine Tim. Use your imagination. :lol:

Hey Guys,

I ran across this while doing a Google search. It sure does seem
like it fits my situation to a capital T. Let me know what you all
think. Link follows below....

http://www.asashop.org/autoinc/sept99/techtotech.htm

Thanks Guys,

Steve

Was a compression test ever done on this car?

Was a compression test ever done on this car?

Checkout Post 14 Rich. Also, from what I can see they checked fuel injectors meaning a balance test was probably run which is standard procedure.


Hello Again Guys,

Well picked up the car from the dealership, and spoke with the
service advisor and here is what he had to say. I was only charged for
1 hour of diagnostic service and they spent a total of 3 hours with the car.
They are pretty decent people to deal with. Now for the problem at hand;
the car has a steady missfire at 3000 rpm. A little less or more rpm and the
miss will stop. They checked everything from soup to nuts. They did a
compression check on all cylinders was informed compression was good on
all cylinders. They checked fuel injectors and said they all passed as well.
They double checked my work as well, swapping coils,checking plugs for
cracked insulators like you guys suggested and still nothing concrete. The
crank sensor, mass air flow sensor and all the other sensors pass all the
diagnostic checks and none of the sensors are throwing any codes. Oh by
the way, the missfire is on number 4 cylinder. They did not mention if they
performed a vacumm check I can only assume that one was done. They said
that what I have is very unusual in that #4 cylinder only missfires at 3000 rpm.
They hinted that perhaps the PCM was failing but did not want to suggest that
as this part is extremly expensive. Any suggestions guys as I am sure as hell out
of ideas. Oh sorry they did say that the intake manifold gasket was ok, age
catching up wih me HA.

Thanks for any info !!!
Steve

Thanks Bob, missed that one.

It's looking more and more like a PCM issue.

Thanks Bob, missed that one.

It's looking more and more like a PCM issue.

No problem Rich. To be honest I really don't know. On that last thread I pointed out to all, that particular member replaced his PCM and the ICM modules. Didn't make any difference. In my case not an issue because I have a spare PCM module, ICM and coils so I would probably have tried them out just to rule them out.

Sir,

How about you be nice and send me your spare PCM and
ICM and I will let you know if it corrects my missfire.... HA
just kidding. On a more serious note I have been able to
negotiate a remanufactured PCM from the local Pontiac
dealership for 250.70 including the core charge of 63.00
so ultimate cost would be 187.70 + tax. My friend has a
Genisis scanner and he informs me that he has the software
that will allow us to reprogam the PCM back to factory specs.
At this time I agree with you BNaylor, I am not so sure that
the PCM or ICM is at fault so perhaps I will just sit on this a
while longer and see what developes and see if any pattern
starts popping up. From what I understand most PCM's that
are replaced, 75-80% have no problems when they are put on
a bench. In other words they are extremely reliable.

Once again guys thanks for your help !!!

Steve

Hey Guys,

Does anyone know if the PCM's on the 97 Grand Prix GTP's
can cut the fuel delivery to an individual cylinder if it detects
a missfire. I do know that if a missfire is serious enough it will
flash the SES light to let you know you are damaging the cat,
just wondering about the fuel cut off. I am thinking that the fuel
cut off is probably used on the newer cars.

Thanks Guys, still haven't figured out the number 4 cylinder missfire.

Steve

Wow, its been so long! Can we recap?

Have you checked....
-compression
-spark plug (for cracks)
-spark from coil (swap coil around)
-injector pulses (noid light)

Your friend has a scan tool, can he hook it up to see whats going on?

Wow, its been so long! Can we recap?

Have you checked....
-compression
-spark plug (for cracks)
-spark from coil (swap coil around)
-injector pulses (noid light)

Your friend has a scan tool, can he hook it up to see whats going on?

Hey All,

Compression check good according to Pontiac dealership. Tried 3 new
plugs in number 4 cylinder no help. Swapped coils and bought new coil,
problem still with number 4 cylinder missfiring. Bought used PCM already
programmed with latest software from GM;no joy, cylinder 4 still missfires
randomly. Dont think I could have two PCM's with the same problem. So
we move on from here. Hooked up LED light to the fuel injector connector
and ran wire inside of car to observe injector under load. Drove car until
missfires, LED light pulsed normally. Appears injector driver circuit is ok.
Next ran same led wire to primary wiring below number 4 coil pack, and
observed under a load. While car is missfiring LED light pulses normally
once again. So where does that leave me. Checked hot side wire for injector
while driving to make sure circuit is not breaking down. While missfiring
voltage stays at a constant 14 volts and does not drop off. this car will not
give me a break, any help would be appreciated !!!

Steve

Next question, is the car actually misfiring, or are you just getting a code for it?

Next question, is the car actually misfiring, or are you just getting a code for it?

Hi Rich,

Thanks for the quick follow up. When the car
first started this missfiring about 2 months ago
it was just a slight rough idle and number 4
cylinder missfire. I was not getting any codes
or any SES light. Now within just the last week
I am getting a flashing SES light for about 30
seconds followed by a steady SES light. This is
still intermittent at this point. The missfire is
alot worse when flooring the pedal for about
5 seconds. And yes this missfire can be readily
felt. I also have a small scanner hooked up and
it shows number 4 actively missfiring.

Thanks Again Rich,

Steve

Steve, have you tried swapping the injector from number 4 to a different cylinder?

The more I think about it, the more I wonder if the dealership did an actual compression check or did a relative compression check. If it were me, I'd check compression on all 6 over again.

The more I think about it, the more I wonder if the dealership did an actual compression check or did a relative compression check. If it were me, I'd check compression on all 6 over again.

Hi tblake,

I went ahead and bought a fuel injector
about a month ago for number 4 cylinder,
no help. I do tend to wonder if in fact they
did do a decent compression test. Checked
the work performed invoice and it states they
did a compression check. At this point would
it be better to have someone do a cylinder
leak down test, what do you think. I think I
have covered everything electical. I do wonder
at this point if perhaps there is mechanical
issue with the engine !!!

Thanks Steve

A leakdown wont tell you anything if compression is good. If I were you, I'd just rent a compression guage from advance and check it out myself. To get the most accurate reading, do the following

-pull out all the plugs
-disable ignition (by unplugging ignition module)
-I'd even unplug the main fuel injector harness
-block throttle open with a screwdriver or something
-attach battery charger

Install the compression guage in each cylinder with a little oil to lubricate the threads so it comes out easy. Turn the car over for at least 6 puffs of air. the first puff should be at least 50% of the final reading. Let the guage sit on each cylinder to see if pressure stays. Record the readings.

I would check knock sensor(s) and see if they have the proper torque. I would probably replace them if the price is right.

I can't recall the variants which are used over the years...some actually are bathed in coolant and obviously you can't check the torque on those...but others bolt to a boss on the outside of the cylinder block and if they get loose, the resulting vibration can simulate false knock events.

If the knock sensor is freaking out...it may be pulling out spark like mad in an attempt to compensate...this would explain your red-hot manifolds when I wouldn't think they should be that way.

A buddy of mine and I as well have had the same problem with our SC 38's. Our problem was that the MAF plug, pin B was breaking down under heat. GM carries a replacement plug that you splice into your harness. Cost about $40. Fixed his problem but not mine. Im having the exact same problem as you. Hope this helps

You can also purchase the connectors from Mouser, at a considerable reduced price. For example I paid for the blower motor resistor connector with pins, secondary lock, if my memory serves me correctly I paid around $5.00 complete. That was the housing, pins and the secondary wire lock.

I would check knock sensor(s) and see if they have the proper torque. I would probably replace them if the price is right.

I can't recall the variants which are used over the years...some actually are bathed in coolant and obviously you can't check the torque on those...but others bolt to a boss on the outside of the cylinder block and if they get loose, the resulting vibration can simulate false knock events.

If the knock sensor is freaking out...it may be pulling out spark like mad in an attempt to compensate...this would explain your red-hot manifolds when I wouldn't think they should be that way.

Hey Guys,

Just a quick update on my car with the number 4 cylinder misfire. I put a
scanner on my fathers 2003 Caddy and checked the timing advance.
Pretty interesting what I discovered. In closed loop at idle the car will
advance timing anywhere from 8-10 degrees. When raising the idle it will
increase up to 20-30 degrees depending on rpms. Note these are all
positive numbers. Now for my Grand Prix, at idle in closed loop the car
reads -22 degrees yes this is no typo. When raising the rpm the car reads
upwards of -30 degrees advance. Wow is this even possible that the car
could run with this much spark advance being pulled out. I guess this
would explain the glowing exhaust manifolds. So I guess either a knock
sensor is defective or it is hearing some abnormal knocking from the engine.

Let me know what you guys think !!!

Steve

Yes, I think you better have a look at the knock sensor. I'm not even sure where it is. I'll see if I can find the location for you. BTW, does the 3800 have 1 knock sensor or two?

Yes, I think you better have a look at the knock sensor. I'm not even sure where it is. I'll see if I can find the location for you. BTW, does the 3800 have 1 knock sensor or two?

Hey tblake,

It has 2 knock sensors 1 by number 3 cylinder and 1 by number 4.
These readings don't make any sense, I can't believe that the car
wouldn't set a code with these readings much less run. I did double
check my fathers car with my car and the readings are accurate.
Negative 22 degrees at idle for my car versus 9 degrees for my dad's
car. !!!

Steve

I'm not sure how much the knock sensors are, but if I were you, I would maybe just replace them. Are they easy to access? If they are the same part number, you could swap them around, and then see what happens. ie: does the same cylinder seem to be misfiring....

I took a digimoto scan tool reading with my laptop of my 2000 GTP back in febrary. Here is a snapshop of the scan tool data to compare yours too. My car was running just fine and I was just toying around with in.

http://img181.imageshack.us/img181/3069/sensorsix1.jpg

[quote=tblake]I'm not sure how much the knock sensors are, but if I were you, I would maybe just replace them. Are they easy to access? If they are the same part number, you could swap them around, and then see what happens. ie: does the same cylinder seem to be misfiring....

I took a digimoto scan tool reading with my laptop of my 2000 GTP back in febrary. Here is a snapshop of the scan tool data to compare yours too. My car was running just fine and I was just toying around with in.

Hey tblake,

I have the GM service manuals and tested both of the knock
sensors and they both appear to be good according to the manual.
There is 3 tests you run,make sure you have 5 volts at the connector,
check the resistance of the sensor should be close to 100k ohms and
the AC voltage test. The darn things passed all three. Oh well I guess
I will just keep plugging away at this problem. I wonder if the the
scanner I am using is not displaying the correct info. I see you have
22 degrees at idle and I have -22 degrees, although the scanner
appears to work properly on my fathers car !!!

Steve

Odd problem isnt it?

This is a high-effort thing to try...but a possibility is to remove the front cover and see about the base engine timing from crank to cam.

There should be a nylon tensioner "foot" which takes up slack on the slack-side of the chain drive, if it is damaged or missing there's a possibility you have jumped base time on the engine :eek:

I can't recall ever removing a front cover in the vehicle before...so you'd probably be into dropping the cradle or pulling the engine. I am also wondering out loud if you could do a study by rolling the engine over to check crank position vs. the pushrod displacement, check via dial indicator or other.

The red-hot manifolds hint at some kind of timing shift where either the ECM is modifying the spark time unexpectedly or else the base time of the engine is wrong.

Think a good compression check would point out jumped cam timing?