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2003 6.0 missfire on #1 and #6 need help please

cameronc

By cameronc
in Ask A GM Technician

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cameronc Enthusiast

cameronc

Posted
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2003 2500HD 4x4 6.0 4L80e. Truck has 190K. It runs and drives great. Has great power, no smoke, doesn't use oil,nothing out of ordinary except at high rpm it throws po300 code and misfire shows on #1 and #6. Doesn't miss at idle or under acceleration. Only misses at light load above 2K rpm. I've have it on scanner several times. I've replaced plugs, wires, all 8 injectors, and swapped coil packs around with no change. All the live data looks excellent! O2's and fuel trim is right. All sensors read as they should. I can't even feel the miss when scanner says the cylinders are missing. Any ideas guys? I'm lost, I haven't even heard of cam or lifter wear on these ls engines. And it has 155 psi on #1 and 160 psi on #6 so I don't know what it can be.

Jsdirt Senior Enthusiast

Jsdirt

Posted
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I had these exact same symptoms on a 4.3 V6 - ended up being intake manifold gaskets leaking air, only at high vacuum. Ran great at full throttle. Idled smooth as glass too. Eventually it got to the point where it could be felt - it got worse and worse as time went on, then the check engine light started flashing telling me it was melting down the catalytic converter.

 

I didn't find this out until after I had parted the truck out. I stripped all the electronic garbage off, bolted a carburetor on it, and threw the engine in our '89 S10 Blazer. When I had the intake manifold off, I found the gaskets totally destroyed with evidence of multiple leak paths. Plastic framed garbage gaskets. Never seen any manufacturer use something so substandard in my life.

  • 2 weeks later...
cameronc Enthusiast

cameronc

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What triggers the ecm to recognize a misfire? When the ecm is reading the misfire the engine is running as smooth as silk. It doesn't make sense. If I could figure out what information it uses to see the misfire maybe I could find the issue.

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

From what I understand it's read by the crank position sensor - each tooth on the reluctor is supposed to pass at a certain time/speed, and it will detect the smallest variation, indicating a misfire. It can tell which cylinder by whatever position the crank is in when the fault occurs.

cameronc Enthusiast

cameronc

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Here is link to the video of the live data. I pulled valve covers today and check valve springs, push rods and done compression tests and leak down tests. Everything looks great. The ecm has to be getting false info somehow

 

 

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

It's possible you have a couple bad injectors on the 2 bad cylinders. Only clue I got that the misfire was happening, and it wasn't bad data, was your post-cat sensors were switching like the pre-cat sensors.

 

Once you fix the misfire, you'll probably need cats, depending on where you live.

cameronc Enthusiast

cameronc

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I replaced all 8 injectors with brand new ones, even tried switching with different cylinders thinking I may have got 2 bad ones out of the package, just dreaming I guess

Jsdirt Senior Enthusiast

Jsdirt

Posted
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These days, that wouldn't surprise me at all, unfortunately.

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