97 Grand Prix With Intermittent Miss

[srlash]

Originally Posted by tblake

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

[tblake]

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.

[srlash]

Originally Posted by tblake

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

[tblake]

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.

[srlash]

Originally Posted by tblake

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

[tblake]

Originally Posted by srlash

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....

[srlash]

Originally Posted by tblake

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

[BNaylor]

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

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?

[srlash]

Originally Posted by BNaylor

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

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

[tblake]

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.

[BNaylor]

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

Click here

[tblake]

Originally Posted by BNaylor

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

Dare I ask what your getting at here Bob?

[BNaylor]

Originally Posted by tblake

Dare I ask what your getting at here Bob?

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

[srlash]

Originally Posted by BNaylor

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

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

[richtazz]

Was a compression test ever done on this car?