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Misfire Question

Tsaxmax

New Member
Joined
Dec 18, 2021
Messages
3
Hello everyone out there is chevy land!

I have recently acquired an 03' 1500 Avalanche z71, with a 5.3L V8 and It has been sitting for awhile. The engine only has 44,000 miles. It was misfiring and running lean codes on bank 2. I replaced the spark plugs, the wires, all 8 coils, all 8 injectors. It now runs quite well, but now it is pulling a single code p0308 (cylinder 8 misfire). I have swapped coils to see if it was a bad coil, no luck. What else can I possibly do to try and resolve this? The injectors are AC Delco injectors, the rest is autozone parts. I am lost here, as this is my first chevy.

I also noticed the check engine light generally comes on when revving to medium RPMs, it will flash, and then turn off after while idling. If left idling for about 10+ minutes, the check engine will come on and stay on until I reset it with my meter.

Please help oh great chevy wizards! :D
 
Compression test the cylinder?

What is your long-term-fuel trim (LTFT) and short-term-fuel-trim (STFT) reading now? Still lean but not enough to set codes? Wondering about a vacuum leak...

Fuel pressure? although I'd imagine it would affect more than the one cylinder, but worth verifying if you're not finding another solution.

good luck tsaxmax, and report back when you find the fix!
 
It might be worth swapping over the spark plug from one cyl to the other too and see if the misfire follows. What kind of plugs did you use?

I believe with some code readers you can do some diagnostics with the individual injectors too.

Next would be a compression & leak down test IMO.
 
These engines are known for leaking intake manifold gaskets that cause misfires.

Sometimes the misfire will go away when the engine heats up and the leaks close up.

If you replace the gaskets, get the Fel-pro gasket set.
 
Thanks for all the help!

I don't have a scanner that can read the fuel trims sadly. The lean code was brief and is long gone, after replacing all those components. One injector in that bank was super dirty, (blackish on the sprayer area). After that, it went straight to cylinder 8, and has the intermittent issue I mentioned.

I was going to try swapping spark plugs as it would be rather easy to deduce at that point as soon as I find some time. If it is a cylinder leak, I will probably have to let her go, although she runs fine otherwise. For her age, and I am the second owner, she starts up just fine, and everything else appears to function fine. Except the silver migration, which will be the final fix.

I will keep you all posted, appreciate the assistance!
 
Is the misfire code real? Can you feel it? Just because the PCM reports a misfire doesn't make it true! Bad harmonics anywhere in the belt drive pulley system, bad flex plates, bad transmissions, bad engine bearings, and jacked-with crank sensors (no subsequent crank sensor relearn) are typical causes for falsely-reported misfires. You have to think about how misfires are detected.

Each crank sensor has an electrical output waveform that is typical for each engine it is installed in. As each cylinder fires, the engine speeds up slightly, and then it slows down in between cylinder firings. The PCM stores the waveform from the last time a crank sensor relean was performed and compares current crank sensor waveform to what is stored. If something doesn't match, then a misfire is flagged. Per factory info, you are not supposed to even remove/reinstall the same crank sensor or otherwise disturb it (much less replace it as so many do).

Get a Vident I-link400 scanner and a fuel pressure gauge. Check fuel trims and fuel pressure per Bain Man. If OK, do a cylinder balance test with the scanner. If OK, clear all codes and immediately do a crank sensor relearn with the scanner (if you wait until a misfire code sets, the relearn will not complete).

It is possible the the PO has been jacking with the crank sensor.

If problem is still there it will be necessary to verify the misfire detection system. I find the easiest way is to display the misfire counters and watch the counts as each injector or coil is disconnected and reconnected. Operation of the misfire detection system will become obvious.
 
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