Yet Another Idle Misfire Thread - except I've tried all the obvious steps

[komrad36]

Hi all,

I've been feeling a slight shake at idle so I plugged in my scan tool to reveal occasional misfires, only in cylinder 4, at idle or when load drops, i.e. when letting off the throttle. They're very slight, maybe 2-4 per 10 seconds. Not severe enough to set any DTCs. No other DTCs stored. Runs great otherwise, no knock or misfires at WOT. Stock.

Not a huge worry, but I thought I'd see if you all had any suggestions.

What I've replaced with brand new AC Delco (unless noted otherwise) components over the course of working on the car for other things, and/or swapped with known good parts from friend's car, and/or confirmed working from scan tool measurements:

-MAF
-IAT
-TPS
-MAP
-no vac leaks: all new silicone vac lines, including digital climate control line, and fuel trims between -12 and 0 at idle depending on whether car is warm (is the -12 normal for warm, long idle?)
-ICM
-ZZP Gen 4 wires
-Autolite XP606 plugs, .060
-coil packs
-compression: ~160 psi in all cylinders, dry, and all within 5 psi of each other

Thoughts of what it could be:
-leaky injector?
-partially clogged cat? or would this only manifest at high mass flow?

What do you all think?

Thanks for your time,
Komrad36

[97grandprix1]

I'd switch the plug with a different cylinder then test

switch the wire with a different cylinder then test

I know you said obvious stuff was checked but I've had plugs AND wires fail right out of the box more than 1nce and your idea of obvious may be diff than mine. good luck

[komrad36]

Good call on that; I've heard of that happening too. Thanks for the reply. I did try that, replacing the wire in question with both a new AC Delco and then ZZP. Same with the plug; I used a 41-104, then an Autolite XP606. I love the XP606s and will stick with them - but neither swap cured the occasional misfire.

OH! I knew I forgot something. I compression tested all 6 cylinders (dry) and got ~160 psi for all 6. All within 5 psi of each other. Not bad for 152,000 miles on this engine! Must be the yearly SeaFoam treatment....

Thanks again. Any other ideas anyone?

[komrad36]

I also cleaned around the spark plug hole, cleaned and chased the threads, and installed the new one properly. I inspected the old plug and saw nothing unusual or undesirable on it.

[97grandprix1]

yer headed the right direction, backwards. I don't know about these cars in particular too well, just bought mine, can you switch coils per group or are the wiring plugs specific? starting at the plug and moving out is all I can think of, its too specific of a problem to warrant major engine sensor changes imop. check the wires controlling the coils/ every ghost I've found was something small when I was thinking big

[komrad36]

Coils are fully interchangeable, and each powers 2 spark plugs. I've moved all 3 coils around to no effect and later installed brand new OEM units as part of 150,000 mile tune-up.

Congrats on purchase! May it provide you many years of fun.

[komrad36]

wires controlling the coils

New ICM; backplate cleaned with degreaser, then water, then contact cleaner, then dried; gang connector cleaned with contact cleaner and dried.

[97grandprix1]

sweet

[Turbocharged400sbc]

I recently had a car with a constant idle stumble/mis

after years of diagnostics, and even a whole new engine (tossed in a built motor)...it still did it.

changed bin's...did everything....

replaced PCM and reflashed with same bin file.... misfire gone. haven't seen it since...old pcm is now in the trash

this is a 2002 with it just now hitting over 100k....diagnose all other things first, but its a possibility

[komrad36]

That is very interesting. I don't have another PCM to try, but maybe I'll stop at a JY someday. Thanks for the idea. Do you think injector is a possibility? I'm wondering if -12% LTFT at long idle might be a clue to leaky injectors.

[Turbocharged400sbc]

injectors are a definite possibility

you can undo the rails and with the coils removed (need ICM but you dont want spark) flip it over and plug in the inj clips and crank the car to check for any obvious spray pattern issues...sorta the hillbilly way, just be safe.

ive been finding all sorts of things as these cars get older....conector terminals with weak connections, corrosion, etc.

but that PCM threw me for a loop...ive rarely seen a GM obd2 pcm failure...especially one thats been with the car its whole life....

[komrad36]

I think I'll attempt the LIM job this summer and will check injectors then since I can properly clean around them first. Thanks!

Yeah, whenever I do any electrical work I try to clean all the connections with contact cleaner and then use plenty of dielectric/silicone grease to seal them up.

What are your fuel trims at long idle? I don't have another car to compare to.

Thanks,
Komrad36

[Turbocharged400sbc]

fuel trims on yours arent too far off

trims are adjustments to programmed tables and are there to allow the pcm to adjust for failing/skewed sensors/weather. even a 20k old O2 will have its trims changed with a new O2. that said id do yer lim...recently did a turbo 97 bonnie...man vac at idle junped nearly 3 inches and we had to reshim the bov spring to keep it closed.

paper tb gaskets that are old are well known to crack and leak coolant/air to affect fuelling.

reality is that a regasket job is likely to cure lots of ills since you will find em all....

[Turbocharged400sbc]

ignore tb gaskets...yer NA...but those flanges warp from overtightening and can leak too...

[komrad36]

I'm S/C

New TB gasket after a cleaning a while back, new upstream O2 after PCM threw a DTC. I don't know how it was still working up until then. Looked like the end was welded shut. No O2 or anything else getting in that thing. Took a can of PB and a 4 foot breaker bar to get it out lol. I should probably do the downstream as well, since I'm getting intermittent P0421 cat efficiency, but only after changing the upstream. Makes me think it might not be the cat

Wow, I really should do the LIM yeah. I run G05 coolant now, but previous owner left original DEXCOOL in for 130k :/
It's gonna be nasty.

What should vacuum be at idle? I'll go take a look at what I'm showing. Thanks!

[BillBoost37]

Another thing to consider if you think vacuum leak may be an issue, the EGR tube where it goes into the LIM. They love to crack and the age of yours, I'd give it 99% chance of having a crack.

I don't think that'll be your issue though. Likely a lim gasket or let's also remember #4 is the middle cylinder and gets warmer than 2 or 6. Sometimes that can change things.

[komrad36]

Interesting, thanks. I don't suspect a global vac leak as fuel trims are negative to zero, but it's possible there's one that affects mainly cylinder 4 from the LIM. If, when I do the LIM job, I discover a crack in the EGR tube, what would you recommend I do?

Thanks!

[BillBoost37]

fix it.....

At least that's how I handle broken things. I always have a spare on hand.

[komrad36]

fix it.....

At least that's how I handle broken things. I always have a spare on hand.

Haha! Of course, but... may I ask how? I looked in the part numbers list but didn't see an ACDelco part number for it. Junkyard? Aftermarket?

Also, my vac at idle was 19 in/Hg. If memory serves from other threads, that's normal, right?

Thanks,
Komrad36

[Turbocharged400sbc]

remove it and rtv a penny to it and the lim for an easy bypass till you find a good one in the junkyard