Help With Dreaded Random Misfire Please

[Fuzzy Nuts]

I have a 2004 GMC Savana 3500 with the 6.0. I have been having a random misfire problem with only the p0300 code. At first I didn't notice any rough running, but now it is noticeable at idle. The light only flashes at cruising speed.

I logged misfire and cyl 5 and 8 are bad. I have run the cylinder balance test and the results were 99-100 on all cylinders. I replaced the plugs and wires today and nothing changed. I think it might have a small exhaust leak on the right side that I'm going to look into...don't know if that would cause this?

Any ideas or help would be greatly appreciated.

[10_SS]

I've seen two cars now with these symptoms, and both had leaky fuel regulators... the leak was internal and allowed fuel to enter from the regulator vacuum line, into the intake, and create excess fuel to be burned. This was causing fuel trims to run max or very negative (pulling fuel), and run rough at idle and erratic at cruise speed also. Only a few cyl's were affected by the excess fuel but the entire bank would run lean trying to compensate.

We could see it just by pulling the vacuum line off the regulator with engine running... it would dump fuel from the regulator.

it's sure is easy enough to test this, give it a shot.

BTW, what are your fuel trims saying?

[Fuzzy Nuts]

10 SS, thanks for the idea. I will check it out tomorrow. All the plugs did look very equal?

I haven't logged the trims yet, but will try that tomorrow also. I think I did a while back and they were pretty far off.

[Fuzzy Nuts]

Ok, I looked for the fuel regulator and it appears to be a non return fuel system. I swapped coils and the same cylinders misfire. Checked a ground on the back of the block and fuse box.

I have LTFT turned off and the STFT are running 2-10% range. I'm not sure if it doesn't log misfire at idle or not with the standard misfire config, but the counts go nuts right at 1500 rpm.

What to check next, compression, injectors? Open to ideas. Thanks.

[Fuzzy Nuts]

Did compression on the 2 bad cylinders and 1 good. Tested with rest of the plugs in and throttle open, all checked about 180 lb. I did notice that it started to crank kind of slow, but I think a weak battery would not show problems on just 2 cyl?

Firing order: 1-8-7-2-6-5-4-3 Another thing I found is that 8 and 5 use the same reluctor wheel position? This has me a little concerned.

[Fuzzy Nuts]

Switched cyl 8 injector to cyl 4 and 8 is still missing. Going to check cats next or light van on fire.

van misfire injector switch.hpl

[Fuzzy Nuts]

Got case learn to work...problem fixed.