Werid missfire on #3

[grandprix00]

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.

[accuab]

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.

[tblake]

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!

[richtazz]

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...)?

[BNaylor]

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

Click here

[grandprix00]

Originally Posted by BNaylor

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

Click here

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..

[BNaylor]

Originally Posted by grandprix00

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? Are you referring to boost or engine vacuum?

[grandprix00]

Originally Posted by BNaylor

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? 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!!!"

[tblake]

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?

[tblake]

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

[BNaylor]

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?

[tblake]

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.

[BNaylor]

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.

[tblake]

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

[BNaylor]

Originally Posted by tblake

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?