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2004 Chevy Silverado 2500HD 6.0 4x4 with 225,000 miles.
Issue is cylinder 1 Misfires and a slight misfire on cylinder 8. When the truck is at idle and when you rev it high in park cylinder one misfires bad. At idle it’s like 1-30 times Per cycle. When you rev it high it gets worse like 1-200 times per cycle.
This is the strange part when you brake torque the engine all cylinders including cylinder 1 immediately fall to zero and the engine gets smooth.
I have replaced these items :
New timing chain
New oil pump
New acdelco plugs and wires
New intake gaskets
New knock sensors
New injectors
Installed new bank 2 sensor 1 O2 sensor
I have swapped coil packs and wires and plugs around Cylinder 1 still misfires.
I swapped injectors around. No difference.
Took spark gap from .040 to .030. No difference. Did injector balance test all injectors passed test. Pressure drop was the same and normal. Fuel pump pressure was 62 psi solid the entire time. At idle and at WOT.
Connected amp probe to injector 1 power side and current was higher than others so I suspected stuck open injector. Installed and new injectors and still cylinder 1 Misfires.
Load tested cylinder 1 wiring at connector with the FLUKE 289 load test feature on voltage setting. 14.3 volts solid with engine on and off.
Propane the whole engine while watching o2 voltage to go rich. Never did. So I don’t think I have any vacuum leaks. Propane tested the mass air flow and it read it correctly as rich immediately and ECU adjusted properly.
Did a leak down test on cylinder 1 and I put 80 psi in and it dropped to 74-76 psi. Which is fine in my opinion.
Truck does have a slight rear main seal leak so maybe that’s it.
I’m truly stumped on this one guys. All that is left is looking into wiring from injectors and coil packs to ecu. Maybe looking at injector waveforms and spark waveforms.
I just don’t understand why it would do it at idle and High rev in park. But under load there is no misfires whatsoever.
I would say to put a pressure transducer in cylinder one and record waveform while cranking and while running. the other thing to do would be to look down the intake port with a borescope and check for carbon build up on the valves for those cylinders. the only other thing to do is check cam and crank sensor waveforms for glitches as well as compression test the other cylinders to see if cyl one is within 10% of the highest reading
Thanks! I actually ended up giving in and taking it to a local reputable shop and after one day he called me back and told me cylinder one is the problem. Compression issue. He thinks the cam may be worn and the valves on that cylinder are leaky. He used a tech 2 and watched vacuum on that cylinder and noticed it was lower than the others.
Which isn’t bad news for me really. I was trying to talk the wife into letting me put in a bigger truck cam in it anyways. So now is my chance. The engine is tired. So I’m going to pull it and re-ring and install a melling upgraded cam with springs and new cylinder heads. Should be good as new then.
Upgrade to a hi volume oil pump and replace the oil plug in the back of the motor by oil filter (behind rear engine one piece main seal cover) and replace oil pump pickup tube o-ring with one supplied in kit for your style pickup tube. It's good insurance on a motor with some miles.
I already upgraded the oil pump and timing chain at the same time just to make sure all that was good. I like to see good oil pressure.
Also, I found out what was causing my misfire on cylinder one. Broken valve spring on cylinder 1 intake valve. This is the second time I found a broken spring on a 5.3 or 6.0. I guess it’s a common problem when they have this many miles. I already ordered the new heads and melling cam so I’m just going to go ahead and change it all. Cylinder walls still have cross hatching so I should be good for another 100,000 miles after I do all of this work. Everything on the motor has been changed by now.
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