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

[AussieinNC]

Easy peasy my young friend....

Under the drivers door, on the chassis, are two ground points fr circuit looms...

Remove the two bolts,,,,clean the chassis with rough sandpaper, cut off each end connector fitting and replace with SOLDERED ends to the original wiring...

Then under the hood, at passenger side rear of engine, there is a ground strap between engine and cab...replace it....

Then trace battery ground from battery down to engine then chassis...renew both leads with factory leads...

Now, under the main fuse panel remove the loom plugs and clean them with electronic contact spray....

Finally, fuel tank filler ground wire to tank then to chassis...make sure its in excellent condition....

These trucks are renowned for BAD GROUND circuits....

Have fun...

[blabla914]

Thanks for the reply!

Yes, I'm on board with a potential grounding issue.

I've already done the two under the drivers door, but I did not replace the terminals. Can do.

I'm pretty sure the ground strap from engine to cab is on driver side on my truck. I will order one.

I don't know why I didn't think to check the ground lead from the battery. I'll figure out where that terminates to the chassis. Thanks.

Fuse block loom plugs. You mean the individual connectors that connect the loom to the fuse block from the bottom? I tried messing with those and they don't come out so easily. Will have to consult manual.

I also didn't know there was a ground strap back by the fuel tank. Will check that too.

Thanks again for the reply

Kelly

[95 S_Trucker]

On the drivers side of the engine, between the oil pan and exhaust manifold, behind the power steering pump, there is a ground. Its only job is to act as a ground for all 8 ignition coils. The wire was corroded about 2" into the harness. I've spent 12 hours on a truck until I found that one. It would start up, and run for a few minutes, then sputter and shut off. It would start after it sits for 45 min.

I've since ran into that same problem on 2 other trucks in my shop, but I've been able to find them quicker now.

[blabla914]

Wow, thank you. Have to say I've never seen that ground before. I'll definitely check that out. Maybe tomorrow after work......it's only supposed to be 98F.....should be fun...

Kelly

[95 S_Trucker]

Number 2 in this picture:

[AussieinNC]

Yep, the corrosion going back up that loom piece is well known among us old guys....sorry if I forgot to mention it...

Seriously, the quality of the wiring looms from manufacturers globally over the past ten or so years is a disgrace.

You have crappy "copper" mix core wiring, soy based insulation that the critters just love, especially mice and squirrels...

I work a lot of 67-72 Chevy and GMC pickups and the wiring in these is generally in better condition than some in vehicles that are less than 5 years old....

Sadly, its all about the all mighty dollar and the disposable attitude of consumers.

[blabla914]

Just wanted to say thank you to both of you.

Been working at replacing terminals. The one behind the powersteering pump was barely hanging on by a thread.....and green where the wire connects to the terminal. Just soldering on a new connector here got it running!

I still have the U1000 class 2 data link malfunction and now I have P0449 EVAP vent solenoid control circuit codes, so a bit more work to do, but running is good.

Thanks again,

Kelly

[95 S_Trucker]

I'm glad that worked.

The vent solenoid is a common problem on these trucks. It most likely needs replaced.

[AussieinNC]

The two codes are linked....the P0449 code is usually the Evap valve failure, but in your case, it is most likely the electrical circuitry...check connectors etc and damaged wiring...

Replace the valve and also the vent hose itself...hose failures have caught many a good mechanic out...

Have fun...Just imagine paying for someone to do the repairs for you !

[blabla914]

one more, potentially last update.......

Got my hands on a new AC Delco ground strap that runs from the cab to the back of the engine block. Made sure the other ground wire that connects with it was in good shape and cleaned up the back of the block nice and shiny.

Started up and both the codes cleared! let it run in the driveway for a bit and then started it up and shut it off a few times in the driveway and so far so good. Going to do some test driving today and see how that goes.

Yeah, I try hard to only hire out stuff I'm either not equipped to handle, or stuff I'm 90% certain I will never have to do more than once.

Thanks for all your help. Very appreciated!

Kelly

[blabla914]

so..........

Unfortunately not the end of the story. I've been travelling a bit for work, so I haven't had huge amounts of time to work on it. After getting the new ground strap in and the grounds under the drivers door soldered on I took it for a ride around town.

About 5-6 miles in I get an ABS light and a needs brake service message where the odometer usually is. Check the codes and it has lost communication with the ECU. A few min later I get the ding ding ding warning. I look down and see the battery light is on and the oil pressure, fuel level gauge, and the water temp gauge are all on the pin like the ignition is off. A few seconds later and they jump back to life.

Pretty sure this is why the mechanic couldn't get the ECU to program. Clearly I've got more wiring issues to sort out.

Kelly

[AussieinNC]

There is a ground between the dash module and the cab...it sounds like its loose or about to break....

The dash ground is actually attached to the right side engine head at the back of the head...feel around ...there are two black wires with eyelets,,,engine movement over time hardens the wires and they break or get loose...just find the wires and ground them to a convenient location...