Help please - bought the truck this way years ago but finally trying to chase down the problem. Check engine light, and OBDII reader shows codes P0351-P0358... thats right, all 8 cylinders read misfire codes but the truck runs great and always has! Tried replacing the ground strap, the only change was that my cab light doesn't come on when you open the door anymore... great another problem. What could be a resolution to ALL the cylinders showing a misfire code but no actual misfired occuring!?!

2004 2500hd with Blackbear tune.

reason for wanting to fix it now is that my shop will no longer inspect it with these coded present.

 

Those aren’t misfire codes. They are codes indicating problems with the ignition coil circuits. Possible causes are weak grounds on the front left/right rear engine block (ground points for each bank), low supply voltage, damaged wiring, loose/corroded connectors, faulty PCM, severely worn spark plugs, coils or boots, or the tuner.

Besides basic inspection, I’d recommend disconnecting the tuner for a few days to make sure that’s not causing it. To have the codes set for every coil rules out anything that can affect just one or two coils or a single bank.

 

More Info - I bought the truck with this problem, PO had recently installed new coils and plugs to try and remedy the situation but that didnt work. I had it tuned a year later with issue present, and it's an ECM programmed tune for the trans, not a plug-in tuner. Also, recently replaced the ground straps. What else does that narrow it down to?

 

I've heard a crank re-learn could be the fix since all the coils and plugs are in good condition. That would re-flash my ECM apparently and erase my tune, then I would have to ship it out to be re-tuned. But is the re-learn a likely solution? all 8 cylinders showing the same code...

 

If you suspect ecm, you can have a new one programmed to your vin, and replace it.

 

If you mean the ground straps between the engine and frame/body, it’s good to replace those as needed, but they’re not direct grounds for the coils. All the ignition coil primary circuits are spliced together on the same ground for each bank, and those circuits ground to the cylinder heads/engine block.

Bank 1 (driver side) grounds at the front of the cylinder head or block and bank 2 (passenger side) grounds at the rear. Actual location of cylinder head or block seems to vary - I’ve seen them grounded on both and I don’t know if it’s a factory thing or from previous work. Just trace the harness from the coils and you should find them - any corrosion is cause for removal, cleaning and repair as needed.

The most likely cause is wiring, but other causes such as the PCM can’t be ruled out until a methodical diagnosis leads you there. If the engine isn’t misfiring at all, this suggests the PCM and secondary circuits of the coils are functional and supply voltage to the coils is sufficient, which is why I recommend checking the grounds first. If they look OK, use an ohm meter between the black wire of each coil and a good engine ground. Anything over .5 ohm means the wiring harness is suspect.

Not true at all. Besides doing ignition tune-ups all the time at work and not doing a cam-crank relearn and not having problems, it’s not recommended in GM’s service procedure. The cam-crank pattern is there so the PCM can recognize what a normal relationship between them looks like. The only reasons a cam-crank relearn would be necessary is if one of the sensors were replaced or a mechanical repair affecting the sensor signals is performed (timing chains or cam replacement, for example), and even then, the PCM is usually capable of learning the pattern rather quickly. Replacing plugs and coils would not affect the mechanical relationship between the crank and cam, therefore, it wouldn’t be necessary.

 

Not sure which engine you have, but a quick look at rockato.com reveals that there are 2 types of ignition coils for virtually all of the V8 engines - a square coil and a round coil. Are you sure that the correct coils are installed? The round coils are for a Delphi ignition system and the square coils are for a Mitsubishi ignition system.

 

The coils look different externally, but the only difference between them is how they mount to the bracket. The coils work the same and use the same connector - you could swap them out and not notice a difference in how the engine runs.