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LH6 5.3 misfiring, need some help

 
Old 03-03-2019, 06:19 PM
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Default LH6 5.3 misfiring, need some help

ok guys so I have LH6 5.3 in an 07 envoy Denali with DOD.


So the the other day out of the blue car started mid firing BAD. Plugged it in and cylinder 1 and 8 are misfiring.
cylinder 1 is a DOD cylinder 8 is not.

Ive done all the normal stuff plugs, wires , coils, vacuum leak, pulled the fuse block check that all out. I did a compression test on it, and itís got 175 or better across the board.

I plugged my buddys snap on scanner in and its really odd bc it misfires like crazy at idle. 700 plus mid fire between those 2 cylinder. All the others are fine. But if you bring the Rpmís up to say 3000 it goes almost away. Less then 3 misfires for those 2 cylinders. I havenít done a leak down on it yet Iím going to try that this week some time. Few freinds keep saying burnt valve. But I find that hard to believe. Itís 100% stock motor. I also check the fuel trims those are all in check as well. Any insight will be much appreciated
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Old 03-05-2019, 04:13 PM
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Originally Posted by 02LS1ram View Post
ok guys so I have LH6 5.3 in an 07 envoy Denali with DOD.


So the the other day out of the blue car started mid firing BAD. Plugged it in and cylinder 1 and 8 are misfiring.
cylinder 1 is a DOD cylinder 8 is not.

Ive done all the normal stuff plugs, wires , coils, vacuum leak, pulled the fuse block check that all out. I did a compression test on it, and it’s got 175 or better across the board.

I plugged my buddys snap on scanner in and its really odd bc it misfires like crazy at idle. 700 plus mid fire between those 2 cylinder. All the others are fine. But if you bring the Rpm’s up to say 3000 it goes almost away. Less then 3 misfires for those 2 cylinders. I haven’t done a leak down on it yet I’m going to try that this week some time. Few freinds keep saying burnt valve. But I find that hard to believe. It’s 100% stock motor. I also check the fuel trims those are all in check as well. Any insight will be much appreciated
Move the coils and injectors on those cylinders. See if the misfire follows.

If it has 175 psi + it doesn't have a burned valve or leaking valve. Not bad enough to be concerned with anyway because 175 is plenty for a thorough burn.

A vacuum leak will cause your issue but fuel trims should be off, so that doesn't add up.

A misfire at idle only is usually a bad ignition component, vacuum leak, or low compression.

How does it do under load? What about hard throttle?

Any other codes?
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Old 03-11-2019, 02:27 PM
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Originally Posted by 00pooterSS View Post
Move the coils and injectors on those cylinders. See if the misfire follows.

If it has 175 psi + it doesn't have a burned valve or leaking valve. Not bad enough to be concerned with anyway because 175 is plenty for a thorough burn.

A vacuum leak will cause your issue but fuel trims should be off, so that doesn't add up.

A misfire at idle only is usually a bad ignition component, vacuum leak, or low compression.

How does it do under load? What about hard throttle?

Any other codes?
ok so a little up date On the car.

replaced the valley cover DOD BS with a new one with gaskets the whole nine yards. Changed the oil pressure sensor. Also threw new oil in it. It started up and ran fine for about 10 seconds then boom back to the way it ran before. Number 8 and 1 did look crappy around the gasket area.







Last edited by 02LS1ram; 03-11-2019 at 05:45 PM.
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Old 03-11-2019, 02:32 PM
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Checked fuel trims there all in check. The only thing Iím also noticing that O2 on bank number 2 is bouncing around a good bit. So I may change that one out and see if anything changes.
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Old 03-11-2019, 04:25 PM
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Check ALL your connections. Sounds like an intermittent short or bad ground.
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Old 03-11-2019, 05:13 PM
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Check the valve springs too on those cylinders, look for broken springs or anything funny about the valve or spring.
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Old 03-11-2019, 05:48 PM
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Originally Posted by G Atsma View Post
Check ALL your connections. Sounds like an intermittent short or bad ground.

I did try that, put an ohm meter and went threw everything I could. I saw nothing out of the norm. That was one of the first things I thought myself. The 06-07 envoy Denaliís are very known for electrical problems. I will say this Iíve owned it 8yrs now and Iíve yet to have a single problem electric wise with it.
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Old 03-11-2019, 05:51 PM
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Originally Posted by 00pooterSS View Post
Check the valve springs too on those cylinders, look for broken springs or anything funny about the valve or spring.
i pulled cylinder 1 springs out they are good. I have a set of stock 28lbs injectors I may throw in the car see if it makes a diffence or not. As it an ez swap. Will keep updated
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