Cylinder #7 misfire?

NVLTGO

MEMBER
2009 CHEVROLET AVALANCHE
5.3L
V8
4WD
AUTOMATIC
99,000 MILES

Motor has the AFM system. Has a misfire on cylinder #7. Have replaced the lifters, coil, inj, plug and plug wire. Engine goes in and out of V4 mode with no problem and passes all wiring tests when it does so. The misfire only happens at lower speeds and seems to happen more after making a left hand turn. Not sure that has any relevance or not. Once the misfire starts it will continue even after stopping and putting it into park. If I rev the engine up to 3500RPM the misfire will stop until either I let the RPM drop below 3500 or I cycle the key. After cycling the key it will run good anywhere from a few minutes to a few days then act up again. The spark plugs center electrode was very worn compared to the other plugs but did not have a lot of carbon buildup or oil sludge on it. It was wet and had the smell of fuel. I air blew the cylinder out and did have fuel blow out of plug hole. This led me to think I had a bad coil so I swapped it out but it did not change anything. I then thought maybe the ECM is dropping the signal to that coil but why would hitting 3500RPM then start the ECM to work right and then stop working again after dropping below 3500RPM. I have had this problem before with weak valve springs but it wasn't just one cylinder and it happened all the time. With the head off the top of all pistons looked the same. It does not burn oil nor do you see any blue smoke etc. I am a retired Chrysler Mechanic and have never run into such a problem. Any advice from you all would be great. Dave

7 Replies

HMAC300

MECHANIC
48,601 POSTS

Misfires are very hard to diagnose, have a pro scan it as he can do some tests with his scanner. It may be a maf sensor or any other sensor as well as a vacuum leak causing this. Even low fuel pressure. Which should be 50-60 psi should not go down more than 5 psi in a minute auto parts rent gauge. If you have a scanner capable of freeze frame do that and check your sensors.

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Thursday, May 21st, 2015 AT 4:27 PM

NVLTGO

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3 POSTS

Maybe you didn't read my entire post.I am a retired mechanic. I have checked all the basics. You mention maf sensor. First the only cylinder that is misfiring is #7. I mentioned I had replaced the lifters for the AFM system so all gaskets are new. No vacuum leaks.I have a scanner and as far as letting a pro scan it; well I was in the business for 30 years, own a scanner, has freeze frame etc. Still have all ASE certs including L1 am a three time master with Chrysler etc. Etc. So thanks for the slap in the face. Now I gave a lot of information in my post about RPM's etc. If you are not familiar with the 5.3L engine with VLOM from GM please allow someone else to respond as for what you gave me is far from worth 25 bucks.

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Thursday, May 21st, 2015 AT 5:51 PM

HMAC300

MECHANIC
48,601 POSTS

I have handed this off to another person to answer. I meant no disrespect in my answer but fuel pressure should also be checked as I said in first reply as it can cause this as well. Sorry about the response.

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Friday, May 22nd, 2015 AT 7:41 AM

JDL

MECHANIC
16,098 POSTS

I'm not trying to argue. I don't doubt your expertise. But w/o being able to look at the vehicle, trying to answer questions over the internet, it can be tough.

Can you feel the misfire? You get a po307 code? Does the freeze frame data on your scanner show anything, useful? Can you check fuel trims?

If the computer is disabling the cylinder for some reason, I was hoping your freeze frame data would show something.

If you losing spark, I'd have to check coil wiring circuit when problem is ongoing. I did check service bulletins, nothing caught my attention for a p0307. I did see an issue with cylinder number 1 and 7, plugs becoming oil fouled. That may not be your issue?

Just curious, any vehicle vibration that may be interpreted as a misfire by the crank sensor?

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Saturday, May 23rd, 2015 AT 1:34 PM

NVLTGO

MEMBER
3 POSTS

Yes the misfire can be felt. When it starts doing it is always at slower speeds and when the AFM is inactive.

Freeze frame doesn't show anything making a sudden change.

Yes I get P0300 and P0307.

Fuel pressures good as are all sensors, at least according to scanner.

I read those bulletins also and do not think my issue is related as I do not go through oil and the plugs are not oil soaked or carbon and sludge buildup anywhere.

My biggest question is why does just cycling the key fix the issue for a while and why if in park and feeling the misfire I increase the RPM to 3500 does the misfire end and then start back up as soon as the RPM drops to 3200? I can sit there and watch the misfire counter go up then stop, then go up then stop as I play with the RPM at and below 3500.

My thinking was coil wiring also but doesn't fit with being able to stop the misfire at 3500RPM. I get the fuel pressure being important as well as all sensors but I would it only affect #7? I thought maybe weak springs or burnt valve and that's why the higher RPM would fix it but that doesn't add up either when I can go a couple days without it misfiring.

Was really hoping to come across someone who had also ran into this issue.