Cylinder 5 misfire

Yeah, check the coil. Can swap them around to see if problem moves too.

Yeah I meant to say that I checked the coil. I turned the engine over with the cyl 5 coil exposed and it was arcing to through the air to ground and it was loud and bright. I could check it again but I'm pretty sure that's not it. If you swap coil to cylinder positions won't that mess up the timing or create more misfires? My Impala is not COP, rather is has one clump of coils on a common rail.

Compression at 100 per cylinder is pretty low. Did you do a wet compression test also? Does this engine make any unusual noises such as ticking or pecking?

Hmm....no I just did a try comp test. I don't hear anything unusual other than a pulley squeal and the putter from the exhaust pipe sucking air back in.

The interesting thing is that the misfire is felt only at idle. Once I throttle up a few hundred RPM it's very smooth. Acceleration is always good as it's always been and I never have starting problems, just the misfire and rough idle.

If you have an exhaust leak near the converters it is possible that may be causing part of the issue. Exhaust leaks interfere with O2 sensors and the readings they report to the ECU.

What do you recommend my next action should be?

I’m having that same exact issue. Im an ASE certified Technician with over 24 years experience. This one has my head shaking. I checked the coil pack. It was faulty, I replaced it. Car ran fine for about five miles then it started idling really rough. I did some further testing. I checked the plugs, they were fine. I swapped the plugs to another cylinder to see if maybe I missed an issue. Same problem. I swapped the wires with another cylinder same problem with only cylinder five. I checked the Injector and I went and replaced the injector for five cause it was getting a weak signal. Same problem and code after new injector. I checked compression and did a leak down everything passed. I’m thinking of taking off the valve cover just to check the valve springs should they be damaged on cylinder 5 cause I’m out of other options. Ohh by The way I replaced the Booster grommet cause I was getting a pretty bad leak. For now I will tell you what I find should I find anything. And yes I replaced the upper intake manifold gasket...any tips on what I can be missing if I’m even missing anything. Thanks, Tony, Covina, CA

Been a while and still haven't fixed this yet. Was able to get a friend to connect the alltell to the one and we watched the live Scan of cylinder 5 and holy smokes, just like I thought, it's only misfiring at idle! Wow! So what would cause a cyclinder to have low compression but pass leak down test and only misfire at idle? I had an epiphany the other day. I had an exhaust rattle that was never fixed, I think it's the resonator. It had a specific rpm rattle and now it doesn't. I think maybe it is falling apart internally and perhaps causing excessive back pressure at idle causing a misfire??? That or maybe an intake leak???

Yeah this is a weird one. But I’ll be at it again today. Will keep you posted...

My 2011 Tahoe is having this exact same issue. We’ve done everything you guys have and it keeps happening but ONLY at idle... we’ve stumped 4 mechanics with over 100 years combined experience. Let me know if you figure this out!

Has anyone got any updates? I’ve been pounding fuel and oil additives down this things throat with fresh oil changes to see if maybe there is some kind of debris fouling up the valves or lifters, nothing positive yet. Oh also it recently developed a fuel trim lean code on bank 1 I think it was. I’m leaning towards head gasket perhaps? I am just so lost with this whole thing! There’s seemingly no consistency.

Has anyone fixed this problem??

I took my car to 3 “good” mechanics. 2 of them been mechanics for 30+ years and the other was 20+ GM certified mechanic. All said “it shouldn’t be doing that”. I ended up trading it.

Gents! I think I may be on to something. I noticed that the vehicle only misfires when it's cold but as soon as hit hits operating temp, the idle drops and then it starts throwing the code. Now that I've learned enough about reading the obd2 scanner and checking live data I realized that when you first start a vehicle and it's cold the fuel system runs in open loop, which is a preset rich fuel trim that ignores data from the upstream O2 sensor until the engine is hot. Once the engine is hot and the O2 sensor heater has done its job the coolant temp sensor gives data to the ECU and the fuel system clicks into closed loop where it now adjusts the fuel trim mainly based off the readings the upstream O2 sensor gives. I checked my O2 and the milivolt readings look good, however there is alot of time delay when the exhaust experiences changes to when the computer is receiving that data. So after much googling of a closed loop only misfire I've determined it must be the upstream O2. So soon I hope to install this and confirm whether or not this resolves the issue.

Having similar problem in 2011 impala... Changed spark plug wires and ingnition coil twice...

Ok so I disconnected the battery, installed the new pre cat upstream O2 sensor, connected the battery, fired it up and.....after it warmed up it still started missing, but when you hit the throttle a little it smooths out, so again the problem only happens in closed loop and only while idling. Any thoughts guys?! Please I have been reeling with this for 2 years off and on now. It’s exhausting. My final thought is, I had an exhaust rattle, in the resonator I think, that I never repaired and I haven’t heard that rattle for a while. Do you think that less than proper exhaust flow would cause bad readings with the O2’s and create this very slight specific misfire?

A leaking intake valve will allow a cylinder miss at idle but run smooth at higher rpm. (Velocity) To verify, inject regulated compressed air through the spark plug hole while on the power stroke of the cylinder in question. You will have to put a wrench on the crankshaft to keep it from reaching bottom dead center. Listen for air leak in the intake manifold.

Well after a valve job and a replacement ECM programmed to my VIN the cylinder 5 misfire is still here. Anyone else have any choice ideas? I’m so lost here. Plugs, wires, MAF sensor, upstream O2, ECM. I had checked the coils but maybe I missed something. Maybe I’ll just replace that and see if by some miracle that’s it.

There has been some question concerning the lifters in gm cars, that is an item that certainly needs to be checked. You didn’t say if the injector in cylinder #5 had been verified to be good. I also noticed the intake gasket had been sucking crankcase fumes when I removed the intake manifold

Does the problem go away for a little while if you unhook the battery for a few hours

I found my problem with mine. The previous owners zip tied the fuel injector wiring harness to the motor mount on the front and it chief the wire and was grounding out My symptoms was cylinder, five misfire, open circuit and traction, control, light and service stabilizer the one wire fixed all my problems

Good job! You have been chasing this problem for a while now but you got it. Dogged determination!

Well good news gents, I discovered that I had a collapsed or plugged failed lifter on intake valve. I sadly the chipped roller of the lifter did start wearing the peak of the cam lobe about 0.010” but let me tell you I did replace both lifters and pushrods and I can’t believe it but it fixed it completely. No more rough idle, no more miss and I passed my state inspection and been running great for two months now.

Here is the worn lobe, see how long it hood up. Next time I’ll need new camshaft.