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01 3500HD 8.1 Running Problem



Hi guys,


Im new here so bear with me....


I have a 2001 3500HD dump truck with an 8.1 and auto trans in it. It just turned 125,000 miles.


It has been running bad and I hadnt had time to work on it until it broke down on me. When it stopped, it still ran, it just had absolutely no power. To the point I had to have it towed home.


I replaced the spark plugs first. Most of them had the electrode burned all the way out of them.


I also replaced the wires.


I scanned it with a Snap On Solus and it also had a 5 & 7 misfire, so I the coils on 5 & 7. That got rid of the misfire on the scanner but it still ran like crap.


So I check fuel pressure and barely had 40lbs. I replaced the pump and filter and ended up with almost 70lbs. Runs better, but not right.


While watching engine data on the Solus, the spark advance seems to drop when you rev it up or power brake it (sometimes as far as -5 degrees) and advances as far up as 39 degrees on decel. Im not sure if the scanner is getting that data later than the engine is adjusting it or if that lies my problem.


The knock retard stays at zero on the scanner. I did get -1 out of it once, but thats the only time Ive seen it move off zero.


The cam and crank sensor counts are in sync on the scanner.


Basically, heres what its doing now. I can start it up and it will idle rough and basically sits there and misses. When you rev it up slowly, itll go all the way to the rev limiter and not miss a beat. If you let it idle and tromp on it, it just sits and stumbles and backfires while slowly climbing to the rev limiter. Then it will clear up and rev perfectly as it should for just a minute before going back to running bad.


No check engine lite at all. No codes present since I replaced the coils and cleared the codes. Im thinking of replacing the knock sensors but Im really surprised theres no codes for that. Any advice?


Thanks a bunch!

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