can't figure out miss on 1997 buick park ave 3800 series II

[bad@ssbuick455]

No check engine codes present, I have replaced new Delco irridium plugs & wires, checked ign. control module & replaced athough it was good, moved coils around while checking fire with timing light... It appears to have a random miss on one cylinder... checked harness to module & looks ok. seems to start the miss when warm & after re starting sometimes it runs rough for a moment then clears out after it revs up on its own. (random miss at idle & stalls when stopping when warm) I had a mechanic put it on a scope & he says the maf sensor was erratic... so I unplug it & still seems to run ok while driving but suffers the same random miss at idle... Dont want to keep forking out money on sensors to have same problem, as it seems to me that without the sensor plugged in it wouldnt be sending a signal to the computer to create the miss.... Has anyone had the same issue? I also cheked compression & it seemed ok with 150psi across the board... My question is why would I have erratic spark on 1 wire from coil/module when I know they are good? Is the computer trying to adjust timing for some reason? Also thinking cat converter but still dont understand erratic spark deal on one cylinder... Its not the plug or plug wire,as Ive triple checked for that issue.

If someone could shed any light on this situation I would greatly appreciate it

Thanks!

Vehicle in question park ave 3800 non supercharged 135k miles, updated intake & gaskets

[bad@ssbuick455]

Also no vacuum leaks & cleaned egr... was good on scope test as well...

[maxwedge]

Do you have it pinned to one cyl. have you check for misfire counts on a good scanner?

[Tech II]

Does this misfire at idle only, or does it misfire under a load when lightly accelerating up a slight grade at 55-60mph?

Maf could be erratic if misfiring.....

Have you checked coil output with a spark tester?

Checked fuel pressure regulator vac line for fuel?

Are you losing coolant?

Since compression is ok, and if coil output is ok, could possibly have an injector problem.....also if car misfires at idle, hole a piece of paper at the tail pipe when it misfires......if the paper sucks into the tail pipe, could have a broken exhaust valve spring....

[bad@ssbuick455]

I do have it pinned to one cylinder, (using a timing light on each plug wire & watching signal, only 1 had a skip which is why I switched coils around & seeing miss stayed at the one cylinder, then changed plug/wires & tested control module & replaced) same miss... I took it to a mechanic with a 10k snap on scope/scanner for diagnostic & he didnt say what cylinder was misfiring or how many times... just said he would start on a maf sensor, because it was erratic & that sometimes caused a miss at idle... well tried that this morning figuring it wouldnt help & it did NOT! same deal... I checked regulator & no sign of fuel while running. It seems to run as it should under a load & down the road.. I did suspect bad gas also & ran tank out then filled with 93 & sea foam... still missing at idle but hasnt stalled so far... I really don't understand this one, was hoping someone had this issue & would say something like yeah theres a ground wire located here that breaks down after time or something & causes the mysterious miss..lol still hoping Injector issue could be a possible, just not a dead miss but very erratic after warming up... tested injectors for air leaks while looking for vacuum leaks.... Also, not losing any coolant & plugs all looked coco brown & pretty good. Im still open to suggestions & thank you for your input! very much appreciated! If I figure it out I will post it on here for future reference.

Thanks again

[HotZ28]

Have you checked the injector on the cylinder that is misfiring with a noid light, or at least checked the resistance? An erratic misfire on ONE cylinder, does not indicate any particular sensor, (including MAF) which usually cause random misfire on all cylinders. BTW, what cylinder is it that is misfiring?

[enslow]

Originally Posted by HotZ28

Have you checked the injector on the cylinder that is misfiring with a noid light, or at least checked the resistance? An erratic misfire on ONE cylinder, does not indicate any particular sensor, (including MAF) which usually cause random misfire on all cylinders. BTW, what cylinder is it that is misfiring?

I would agree you have to look at something specific to the cylinder. I think that mechanic was leading you on the wrong track by suggesting the MAF sensor. Faulty sensors would cause random missfires on all cylinders because the sensors measure air conditions of the air before it gets into specific cylinders.

HotZ28 has you on a good track looking at the injector.

I might also think about the condition of the LIM gasket. If you have a coolant leak through the LIM gasket into that one specific cylinder, that will cause a missfire on that cylinder. Do you have nylon LIM gaskets? Nylon gaskets can fail quite early in less than 100000 miles.
You can put a coolant pressure tester and see how well the system holds pressure on a cold engine. It should not lose pressure overnight.