rwj383

very suddenly i developed some choppy idle yesterday that can be felt when taking off in 1st gear in a normal fashion. before i got home the Check Eng light came on. got home, plugged in laptop and had a "Misfires detected for Cylinder 7" code. ran a Misfire test right there in the garage at idle and got ~200 misfires on 7 in just a couple minutes. pulled plug and wire, ohm'ed the MSD wire to the tune of 28ohms (good), no burns on the plug where it resides close to the FLP headers, the plug looked great, etc. So, i swapped th plug and wires between 7 and 3 to see if problem migrates: it didnt, still getting a lot of misfires on 7 at idle. BUT, when i did the 2nd Misfire test after the swapping, i was getting a lot of misfires on #2, also (which i wasnt before). Wierd thing is that i can only feel the misfire at idle or when just taking off. When i drove around to do the 3rd Misfire test, i was not getting (hardly) any misfire while driving at speeds, only when first start car up at idle and first take off. Wierd. I still havent swapped the coil packs around to see if problem will migrate then. but my questions to those of you that have had similar issues, have you seen the problem reside any further upline than the coil packs? in the wiring or computer? look forward to input. I will be swapping coil packs around to further isolate components. I suppose if the problem still doesnt migrate, then either i have wiring or computer issues, or something going on in the cylinder (busted valve spring?)

rwj383

c'mon...no one has any input here? ttt

Tom Hall

Take a compression test and a leak down test. That will tell you if it is mechanical or electrical problem your looking for.

kp

I have seen the misfires move all around on autotap, its not perfect the way they figure which cylinder is misfiring. I would check all your plugs as a start. I have had a few LS1s short the coil pack harness on the boss on the valve cover, but they usually show a bad coil code. It could be valve spring, rocker arm, pushrod, piston. Its easy enough to get the valve covers off of these so just take a look (and a compression test like Tom suggested if you can).

If you have a dead miss one header tube will be cooler then the rest, thats the easy way to find out which cylinder it really is. You need three things to make an engine work: fuel, spark and compression - you are missing one of those three if a cyliner is misfiring :)

Good luck..


[Modified by kp, 9:11 AM 7/22/2002]


[Modified by kp, 9:16 AM 7/22/2002]

rwj383

i'm having a problem accepting the possibility of a popped valve spring b/c the car runs like a raped ape in the higher rpm's. I will be doing my leakdown tests when i get home this morning, then i will swap the suspect coil with a known good one and see.
KP- i am very familiar with the errant misfires that ramdomly/seldomly come up across the board. what i am getting is a confirmed problem: 200+ misfires on cyl 7 in 2 minutes of idling means something. I will have more to report in the next 2 hrs. Thanks.

kp

No, what I mean is sometimes the cylinder it reports as misfiring isnt the correct cylinder. I have seen the 7 and 2 show misfires when it was another cylinder that was missing. the last car I seen that happen with it showed constant misfires on number 7, some on number two and it was number one that was misfiring.. The code was for a bad coil on number 1, problem was the wiring under the coil bracket was pinched and shorted the coil. Same thing also happened with my own car when it had 500 miles on it exept a different cylinder. the way you describe it I would have to guess bent pushrod, very common, you didnt happen to do a 3rd to 1st shift recently?

SANMAN1

Resolution??

rwj383

well, its a broken piston ring. compression test yielded only 75psi, leakdown revealed bad blowby at the piston. i even took the head off, replaced springs, had head milled, new gasket, checked valves...all good. i didnt see any damage to the top of the piston, or to the shoulder, which would be evident of the top compression ring getting too hot, closing the end gap, then popping and taking some of the piston land with it. all looked good including the cylinder wall. but a repeated leakdown points to broken spring or hole in piston somewhere. i have a new short block on order: a stroker/big bore iron block.