a friend's car is misfiring from cylinder #5 as much as 150 misfires...i scanned it with the gm tech 2 and it'll go from 0-150 misfires in a matter of seconds, go back to 0 and keep misfiring, we've switched out plugs, wires, coils, checked the injector balance on the tech 2, checked the pushrod, and the springs, also changed the knock sensors and it still misfires...the front o2s are giving me a reading of 300-400 mVs and keeps going in and out of closed loop...anyone know what could be going on? any help is appreciated.

 

Did you do a compression check on that cylinder? Could have a cracked ringland.

 

Im betting on the cracked ringland also same thing happened to me. Checked all of the above and still missfire did a compression check and it was like 30 compared to like 150 on the surrounding cylinders. Mine was also #5 anyone have any problems specific to that cylinder. Also that bank O2 was reading real lean that might help. Cant remember what the O2 is supposed to read but my LTFTs were way off on that bank.

 

it is my car he is talking about, what exactly is a cracked ringland? also the car misfires when its idling, normal driving, and during deceleration.. on wot the car will run fine with no misfires

 

Ringland is a piston ring. You need thoes to hold compression. Mine ran fine at wot or at least seemed like it. Matter of fact it ran 14.0 @ 98 so not way way off stock

 

Do a compression test you can rent the tool from autozone. Worst comes to worst pulling the motor really isnt that bad.

 

swap coil packs with another cylinder... Plugs, wires, coil packs (check the connectors too) are easy to do .... if no results there ... then do a compression test (pain in the *** .. at least with what I used the last time I did it.... greasy hands + trying to turn a rubber hose to tighten in the spark plug hole... ugh)

 

Since he said he checked everything Thats why I said to do a compression test. I had the same probelm with a car I was working on at the shop. Took it apart and I was ring on. Ringland was broke.

 

Yea he already checked the coils and number 5 cylinder isnt that bad to get the compression test tool into. Its actually the easiest to get to IMO. Then check the two next to it. With my reading 30 psi I only checked one more cylinder before I realized what was wrong. With as many missfires as your getting I bet you will will find the same result if that is the problem. If there is no or little compression wouldnt hurt pulling that cylinder head to check for a bent valve. Although I doubt a valve would have bent without bending the stock pushrods.

 

i'll see if i can rent a tool from autozone this afternoon and hopefully see whats wrong with it

 

Just wondering if you did the test and what the compression read.

 

Just to let you know that i had a missfire on piston 3 code P0353, changed plug coil injector to move problem then changed hydraulic lifters & put a new head (less than 2000 miles) mine only done 25000 from new, still same problem, got an electronic mate to do some checks & after 3 hours found there was no spark on piston 3 tried different coils plugs etc still no spark yet put them on piston 1 & spark. Checked wiring for continuity & it was all fine up to the plug into the PCM so i have just had to order new PCM as my mate checked for dry joints but found nothing so now waiting for new PCM then i gotta get it re-programmed
PS, sorry this was on my 2001 SS Camero

 

I had a similar problem on my 01 SS. Look very closely at the wiring harness where it is bolted down near the valve cover. The head of the bolt had pierced the insulation of
the wire on my car. I loosened the bolt and pulled the harness slightly back and retightened the bolt...problem solved. By the way, this is the only problem that I have ever had with my car. It came from the factory with this problem.
Good Luck.

 

thanks for the replies guys, they've been very helpful...as of right now i really havent had a chance to do a compression test and after that probably look into the pcm or the wiring...i'll let ya'll know asap

 

Don't rule out a bad injector.

In my father's car, a Corsica (V6), he was getting constant misfires on only one cylinder and the car was down on power. We ran an injector balance test on a tech 2 and it passed. After lots of troubleshooting, we decided to slowly pull the motor apart. When I removed the injector for that cyclinder, you were able to see some rust on the injector nozzle. Replacing the injector fixed the misfires. I don't understand why the injector balance test passed.

 

Well an update guys the new computer installed & still missfire.Technician checked all possibilities & continuity,fine yet no spark, after 3 hours couldnt find anything causing it & i just tugged on the harness,guess what it sparked. Been ok ever since & cant make the problem re appear.Electrician thinks when checking continuity its fine but when was under load ie, 12 volts giong to coil was causing problem.At least we know if it happens again i gotta fish a new light blue wire from computer to coil, but this has been an expensive problem that like SUNSET SS mentioned must have come from manufacturer.We live & learn & at least i know i can strip my car & get 1 head off in an hour.

 

A key fits in a keyway. A ring (piston ring) fits in a ringland, not a ringway. If you look at a piston from the side you see grooves that the compression and oil rings ride in. The surfaces that make up the groove that the piston rings ride in are called lands. If the land for the compression ring fails your compression is shot. A reading of 30 when it should be 150 spells big trouble. Get out the wrenches and brake cleaner.

 

I have at least four bad 99+ coil harnesses laying around here. They either short out near the boss on the valve cover or on the coild pack bracket. Have seen it happen with cars the coils have never been off, its hard to spot sometimes but it will wear through that silver tape and insulation. Here is a pic of a couple of them, I dont know why I save them One was wear from the valve cover boss and one was from the coil bracket. Sometimes the missfire is reported on the wrong cylinder so that really confuses things then. Just something to check, its a lot more common problem then most think and will cause some major headaches.

 

Does that harness chafe the insulation away enough that the bare wire touches ground (bare metal)? If so the harness isn't secured properly. Strain relief is in order here to elevate the harness up and out of harms way so that chaffing is no longer a problem. I didn't see any exposed stranded wire in the pictures which would indicate chaffing sufficient to cause a direct short to ground.

 

It may be hard to see but on both the blue wire is down to copper and was shorting directly to the valve cover and to the coil pack bracket on the other. The harness is secured very tight, the boss on the valve cover pinches them and on the second pic the wires were up against the coil bracket. Both those harnesses came off crs that had very bad misfires, one even had the engine replaced by GM under warranty because the dealer couldnt figure out the problem