Werid missfire on #3

i have a 2000 grand prix gt with a ss m90 supercharger kit with #42lb injectors. today i hooked it up to the scan tool and #3 was missfiring bad but when u rev it up the missfires are gone all new plugs wires injectors are good coil pack is good but but it said it only had 12lbs a vac so i did a smoke test on it and didnt show any smoke leaking out of the intake or tb so could it be a lower intake? but the thing is i put new lower intake gasket on 10k miles ago. compresstion is good too.

the coil pack could be going bad. an easy check is to swap it with one of the other coil packs to see if the misfire moves or not. the lim gasket should not have went bad after 10k miles unless installed incorrectly.

Next step would be a compression check in my book.

A lot of people skip a compression check when a missfire code shows up. They chase parts around replacing a lot of stuff, and finally do a compression check to find out they need a new motor!

What brand of plugs and wires did you install? double check to make sure you didn't accidentally crack a plug installing it. What does the #3 plug look like (dry, wet, sooty, etc...)?

Wasn't there a recent prior thread/post addressing some of the issues now addressed? See link below.

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

Wasn't there a recent prior thread/post addressing some of the issues now addressed? See link below.

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

yes and no that was about a intake problem i found that out and fixed it. and did a compresson test and everything u could think of thats what i dont get..

but but it said it only had 12lbs a vac so i did a smoke test on it and didnt show any smoke leaking out of the intake or tb so could it be a lower intake? but the thing is i put new lower intake gasket on 10k miles ago. compresstion is good too.

Regardless a reference link to the prior thread would have been helpful so members know the full story about this issue. Also, cuts down on confusion, redundant answers and everyone spinning their wheels.

Also to clarify what is 12 lbs a vac? :confused: Are you referring to boost or engine vacuum?

Regardless a reference link to the prior thread would have been helpful so members know the full story about this issue. Also, cuts down on confusion, redundant answers and everyone spinning their wheels.

Also to clarify what is 12 lbs a vac? :confused: Are you referring to boost or engine vacuum?


Im sorry its just no one left me a message back and i rly need new ideas what to look for ive been going nuts the last few weeks trying to find out whats going on with it. I looked it up on the scan tool and it said it was around 12lbs engine vacuum and the fuel trim are at -10. so i thought i was a small vac leak so i did a smoke test of it and didnt find any leakes and its just showing number 3 with alot of missfires.
The test i have done on it is

compression= good
fuel injector pulse is good
switched fuel injectors around and no change
Plugs are good and wires
coil pack is good
tested the spark thats good


"so any ideas of what it could be would help me out alot!!!"

Did you swap coils around and see if the missfire changes cylinders?

Sometimes coils can short out internally so that they still produce spark in a tester, but when the spark has to jump a gap in the combustion chamber with 10 to 1 compression ratio, it doesnt happen.

You can also ohm check the coil on the primary and secondary sides. Not sure what the spec is, but you can compare they to the other 2 coil packs. Bob might know the spec.

Also did you remove and inspect the spark plug for cracks as what was stated above?

Noid light in the injector harness......

General specs for stock coils is below. If MSD like Blaster a little tighter on coil primary resistance. Around .4 ohms.

Ignition coil primary resistance = 0.5 to 0.9 Ohms
Ignition coil secondary resistance = 5K to 10K Ohms

On the issue of the 12 in-hg or 12 lbs?? of vacuum. What does a regular automotive vacuum gauge 0-30 in-hg read instead of the scanner?

Thanks for the coil resistance Bob.

By serious issue you mean.... Out of time camshaft or major vacuum leak?

I was also wondering if that scanner is giving a true reading of engine vacuum. Best for the OP to just go rent an automotive vacuum guage.

Well, I'm old school Tim. I don't don't rely on scanners for getting a true reading of engine vacuum which will vary depending on where you take the reading. I believe they get their reading off or extrapolated off the MAP sensor. Real big expenditure for a gauge. $25.00?

Just a comment. What I meant was with a hot cam with radical specs engine vacuum may be low but not so far out of the 15-22 in-hg seen on a vacuum gauge. Also, keep in mind this member has the SS M90 kit installed and one of the original issues was the fuel trims were off.

I'd almost start to wonder if that PCM got flashed properly for the ss kit and high flow injectors.

I'd almost start to wonder if that PCM got flashed properly for the ss kit and high flow injectors.

I recall we asked him in the original thread and he stated yes. But we don't know what the programming specs ZZP used were. I'm not a fan of ZZP PCM modules and when you do engine changes or mods such as this then it is better to tune on your own. Like using a DHP Powertuner or HP Tuner.

Also, I would run the regular vacuum test to check for excessive exhaust backpressure which if too high can cause unexplained misfires. You run the engine up to around 2000-2500 rpms and hold steady while observing the vacuum gauge. If it drops or moves towards zero then there may be a restriction in the exhaust like the CAT converter.

Also, out of curiosity was a crank variation (case learn) performed when the ZZP PCM was installed?

I recall we asked him in the original thread and he stated yes. But we don't know what the programming specs ZZP used were. I'm not a fan of ZZP PCM modules and when you do engine changes or mods such as this then it is better to tune on your own. Like using a DHP Powertuner or HP Tuner.

Also, I would run the regular vacuum test to check for excessive exhaust backpressure which if too high can cause unexplained misfires. You run the engine up to around 2000-2500 rpms and hold steady while observing the vacuum gauge. If it drops or moves towards zero then there may be a restriction in the exhaust like the CAT converter.

Also, out of curiosity was a crank variation (case learn) performed when the ZZP PCM was installed?

No only tech 2 scan tools can do that i have a mentor scan tool yah this is the 2nd computer zzp sent me. coil packs are good i tested them awhile ago and i have a zzp stage gt1 cam in there that could of lowered the engine vac too but its juts werid just #3 doesnt make sense for me i went to college for automech and tested everything that u rly could test and came up with nothing. thats why ive been putting post on here.

No doubt it is a head scratcher! Glad I don't have the problem. :grinno:

Anyways, while we are on the subject of Tech II and if your scan tool has the capability what does the Misfire History Cyl # counter tell you? What ever I guess we can all agree that there is sufficient variation in the crankshaft rotation speed for the PCM to be able to detect a misfire condition. We're assuming you are getting a consistent P0303?

Also, are you still using the Bosch +4 plugs?

No doubt it is a head scratcher! Glad I don't have the problem. :grinno:

Anyways, while we are on the subject of Tech II and if your scan tool has the capability what does the Misfire History Cyl # counter tell you? What ever I guess we can all agree that there is sufficient variation in the crankshaft rotation speed for the PCM to be able to detect a misfire condition. We're assuming you are getting a consistent P0303?

Also, are you still using the Bosch +4 plugs?

ah yah tell me about it im getting pretty mad about it.
No not rly becuase im not running it that much ive had it parked in the garge for the last 3 to 4 weeks because im not going to drive around with that problem and make it worse your u know what i mean and history missfire is pretty high on it. ive gone threw 1 tank of gas in so far just letting it run and testing it no driving realy. and yes im still using the bosh +4 in it. You know because number 3 missfire is causing the other cylinders to missfire too because the computer is trying to correct number 3 missfire. but i know for a fact there something going on in number 3 because it missfires first and goes up to 20 before the others start to missfire.

"But when the engine in under load all the missfires are down to 0 it only misses when it idling" if that helps u at all

About the tech II its got programs in there like crank case relearn on there so your computer can relearn whats going on and the mentor scan tool doesnt have them programs in the scan tool.
oh yah here is another thing there oil in the intake i heard if theres oil in the intake that means the lower intake gasket could be leaking?

Oil in the intake? What part of the intake? Near the throttle body?

Oil in the intake? What part of the intake? Near the throttle body?

Umm in the supercharger and lower intake gallery mostly in the the lower intake

Yeah that doesn't sound good. What does the the spark plug read indicate? Any fouled plugs? Did you get the LIM from ZZP for the SS M90 kit?

On the misfire issue so you are getting companion cylinder misfire? Any cylinders with little or no misfire history stored? If you are getting any cylinders that are not misfiring then the values should only be less than 1/2 as great as the misfiring cylinders. Rule of thumb is if the missing cylinders are companion cylinders then the condition is most likely linked to the ignition system. After that time to look at a mechanical issue causing the misfires.

Yeah that doesn't sound good. What does the the spark plug read indicate? Any fouled plugs? Did you get the LIM from ZZP for the SS M90 kit?

On the misfire issue so you are getting companion cylinder misfire? Any cylinders with little or no misfire history stored? If you are getting any cylinders that are not misfiring then the values should only be less than 1/2 as great as the misfiring cylinders. Rule of thumb is if the missing cylinders are companion cylinders then the condition is most likely linked to the ignition system. After that time to look at a mechanical issue causing the misfires.

spark plugs all look normal no plugs fouled out. no i got a LIM from napa and the companion cylinder thats missing is on the firing order for the next cylinder. and yah i know ive been thinking mechanical too in a few weeks im going to go and replace that head with a diffrent one i have and just redo the intake gasket and all that maybe that will fix it

What kind of lower intake did you get fron napa?

I dont think they will have the right one.

Did you get one from a L67? Because I dont think it will line up to the L36 heads.

I thought that SS kit came with everything you needed....

oh nevermind i thought he was talking about the intake gaskets b.c zzp got new alum lower intake gasket so i thought he was asking if i got them, and no i got zzp intake the upper is all mech out.

Ok, I was just making sure you got the ZZP modified lower intake.

I'm stumped.....

I'm stumped.....

:lol:

When you're stumped Tim you have to go back to the basics and that includes the documentation. Out of curiosity I reviewed the FSM for any and all possible solutions to the P0300 and P030X misfire DTCs and what a list of possibilities, most we already covered. The part where it references possible mechanical issues is something else.

To the OP you did try other ignition wires and other plugs like maybe the Autolite 103s? And I mean not one at a time but all in shotgun (all cylinders)? I'm not a fan of Bosch spark plugs either especially the gimmicky +4 type.

yah i know what u mean about bosh +4 ive been meaning to change them but ive just been running them in there until i found out what was going on. and yes new plugs and wires. oh yah i was looking over my autos book from college today and i was thinking maybe something can be going on in the ignition control module u know the wirehaness that goes to it and the wires go behind the braket for it u know just maybe one of the wires that fire ing coil for #3 just maybe the wire being pinched or being grounded out i guess im at the point where i cant out rule anything. im thinking its something electrical causing it.

oh yah what did u think about oil in the intake?

With your SS M90 mod didn't you have to get a special bracket for the ICM/coil packs or relocate it?

On the oil issue I've seen it before. Not necessarily the port gaskets causing it although it never hurts to replace them but possibly the LIM bolts. You have to use sealer on them.

With your SS M90 mod didn't you have to get a special bracket for the ICM/coil packs or relocate it?

On the oil issue I've seen it before. Not necessarily the port gaskets causing it although it never hurts to replace them but possibly the LIM bolts. You have to use sealer on them.

yah there a special braket for that and nevermind checked the wires on that and its all good.

no i havnt put sealer on them are u suppose to on the intake bolts?
oh yah talked to one of my buddy today and he works at a dyno tune shop in my town and he said sometimes if you do that stuff to your car u still might need a good tune and could be something with the computer.
oh yah ? what bad stuff have u heard from zzp computers?

yah i know what u mean about bosh +4 ive been meaning to change them but ive just been running them in there until i found out what was going on. and yes new plugs and wires. oh yah i was looking over my autos book from college today and i was thinking maybe something can be going on in the ignition control module u know the wirehaness that goes to it and the wires go behind the braket for it u know just maybe one of the wires that fire ing coil for #3 just maybe the wire being pinched or being grounded out i guess im at the point where i cant out rule anything. im thinking its something electrical causing it.

oh yah what did u think about oil in the intake?

Did you ever ohm the coils primary and secondary sides? Swap he injectors? Check for injector pulses with a noid light?

You can pull the ICM off and have a place like autozone test it. Seems odd that the ICM would go bad right after a top end swap.

Just curious, How did the car run before you put the SS kit in?

Not sure if I asked you yet, but have you in fact done a compression check on all cylinders? Could you post your results?

Do you have access to a timing light? You can hook the inductive pick-up on the # 3 wire and aim the light at the underside of the hood to watch the spark pattern. This would finger or eliminate the secondary ignition system as the cause of the misfire. If the flashes are consistant, the ignition system isn't the issue. If there are gaps in the flashes, then it's definitely a spark related issue. That way, you can concentrate your efforts on mechanical or fuel related causes for the mis-fires if the spark pattern checks out.

yes switched the injectors around and did test all the coil packs and did a injector pulse test did that awhile ago and all of them looked good compression was like 155 on all of them its not just a top end swap put a cam in there but only gt stage 1 that shouldnt cause a missfire like that for that cam and ive rebuilt the whole motor everything is new on there and i did eveything too the book and did every test u can do when putting the motor back together i know thats build right.

about the computer isue sometimes they need just a little better tune and yes everything worked right with the stock motor. im just going to wait to make enought money for a dyno and tune for the car i think they can tell me more what going on there.

okay yah thanks never rly thought that about the timing light ill try that out today.

Not related to the misfire issue, but the inside bolts in the LIM should be sealed as oil can migrate up the bolt, in my own 98 3800, I had quite a puddle in there and was using about 600/qt, when doing UIM I discovered the puddle of oil, sealed both inside bolts now getting , 2k /qt, this is a 230k engine. I did not see a visible sign of oil fouling here but the ports were oily.

okay will do thanks

A rounded off exhaust camshaft lobe can give you adequate compression, but your car will missfire....Just a thought

yah its its got a new cam in there probably only 50miles on the new motor

Who installed the Cam? Just curious...

i did why?

I dont know, just thinking if it came with the rebuilt engine I'd wonder about it.

You did use engine assembly lube, right?

ah yah umm i rebuilt the motor my self at engine class in college i know i did everything right by the book there wasnt one thing i over looked

I dont doubt you at all, Just was wondering...

No idea about this one anymore, head scratcher.....

i have a 97 gtp and my friend as a 98 gt and we both get misfires on #3 but the car is not actually misfiring. it must be the pcm

Just a question, do you have a scan tool that can monitor misfires? If so, exactly how many mis-fires are we talking, compared to the other cylinders?