cranks but no spark - tough problem for you

[blabla914]

Ok guys, I'm new here but I'm not new to chevy trucks. I've got a tough problem. If you can solve this one, you'll have a good story to tell.

Me: I've got lots of experience working on older cars, but no so much with this many control units. I also have the full shop manual, yes the tan one that is 5 volumes. I'm communicating using an ELM327 and DashCommand with extended GM codes.

What I've got: 2004 2500HD with the 6.0L. I bought this truck brand new. It's got 182k on it and it's been well used, but well taken care of too. Not a crappy rot box. About 7 months ago the motor was replaced with a used one with 75k on it by a pro shop. I had the dreaded oil pickup problem and didn't catch in time. Thought it was a sender issue. I was wrong .

This is where the fun begins.........

About a month after the motor install I get a flashing CEL follwed by steady CEL that occasionally starts flashing again. I pull the codes and I see P0300 misfire and P0327 Knock Sensor front. I talk to the mechanic that did the motor install and we figure used motor has a bad knock sensor. That would be my luck. I decide to keep driving it and do knock sensor when it gets warmer out. It was Feb. I clear the codes occasionally, but CEL comes back. However, now all I get is P0300. Knock sensor seems ok now and I have never seen that code again.

Fast forward to June. I'm getting ready to tear into this to get rid of the CEL that has been on and off, flashing, steady, you name. it. I pull the codes for the nth time and now I see P0300 and U1000 Class 2 Data Link Malfunction. So I start pouring over the manual to figure out what the heck U1000 is.

A couple weeks later I'm driving the truck. The CEL happens to be off today and it dies on the highway. Seems like it wants to re-start, but won't. It gets dragged home and I confirm I have fuel, but no spark. I can still see the P0300 and the U1000 in the stored codes.

After dinking around for a while I have it towed to the mechanic that did the motor install. Crank sensor and cam sensor are confirmed good, as I thought. He follows the flow chart for no start on page 6-3150 and says let's do the ECU. Gets new ECU and has the guy come to program it, won't program. Gets a second ECU and still won't program. At this point he says going to have to go to the dealer. To his credit, he didn't charge me for the diagnostics. The guy is legit.

I get it towed home to figure out my next move. I get a refurb ECU from flagship one pre-programmed. I do the manual program, and BOOM! starts right up! I actually drive it around the neighborhood. Then sitting in the driveway it starts to run real rough and dies. No start, no spark, no codes.

After some considerable dinking around checking connections, fuses, grounds, etc it mysteriously starts! I get a U1000 data link malfunction, a U1026 TCM communications, and a P0449 EVAP solenoid contol circuit code. Jackpot! It again seems to run fine and then will mysteriously start to run rough. Sometimes it will smooth right out and sometimes it will die. Sometimes it will restart, other times no so much. After clearing the codes I'm still getting the U1000 Class 2 data link malfunction, but the TCM and EVAP codes have stayed clear.

It seems to me I've clearly got some kind of wire harness issue. I've been all up and down the harness from the fuse block all the way down to the trans. Everything looks clean and tight with no damaged wires. I'm including an underhood shot just so you know I'm serious that this is a well maintained high mileage truck.

Let's hear your theories!

TIA - Kelly