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Old 10-15-2021, 04:42 PM   #11
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Issue Update

Since last post I had an auto electric place go through grounds and wiring. Nothing found and they couldn't replicate the problem. Then to the Chevy Dealer. They checked Compression - all even, did a fuel injector balance test - all injectors flowing equally and drove it 20 miles on highway with no sign of problem.

I picked it up this morning. It ran great for about 8 miles and once warmed up it started running very rough when throttle was neutral but accelerated no problem. As soon as I would get to cruising speed on freeway it would start up again - accelerate or let up on throttle all is good. This is no longer an intermittent issue.

The one thing I did notice is temp gauge never got over 140 degrees the entire 50 miles home. I'm thinking there could be a correlation between a stuck open thermostat (cold running engine) and what the ECU thinks the fuel needs are once the engine has warmed up some. That might explain why earlier in the Summer it was only intermittent since ambient temp was quite a bit warmer??

I've dug into the internet for answers but coming up empty. Any and all thoughts appreciated.
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Old 10-16-2021, 07:59 AM   #12
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Agree with your comment about temps and ECU. Does sound like it isn’t getting up to normal operating temp.

Could be a thermostat stuck open.

I once bought a 1989 GMC and when the ambient temps started cooling off I had a similar problem of running too cold especially on the highway. Loss of power, smoke, etc. Turned out there wasn’t a thermostat, the PO had removed it. Installed one and fixed the problem.
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Old 10-16-2021, 08:09 AM   #13
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Definitely replace the t stat if it’s an issue. Maybe that could be related. If not, at least you ruled out another obvious problem.
I would agree with earlier posts, and you should personally look at the engine and transmission wire harnesses very carefully. Everywhere it’s close to contacting anything. Brackets, frame, metal clips etc. The way you describe it happening over bumps, accelerating, (engine moving/twisting), it sounds like a signal is getting disrupted and causing the misfire. This would only take one strand of a chafed wire rubbing on any ground point.
Not discounting the shops you have had it into, but you can do a more thorough job than they probably will.
Also check all of the drivetrain fuses. Pull them out and look carefully at them.
Even check connectors on all the components. Look for pins in them that may be pinched or opened to far/loose. You can do a “pin drag test” if you get the proper sized test probes. It checks the tension on the connection between the male and female pins. Unlikely, but it does happen. Look for any corrosion in these areas.

I’m not familiar with this drivetrain. Does it have any additional type of ignition module besides the main ecu that was already replaced?
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Old 10-16-2021, 02:12 PM   #14
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I'll do the t stat first. I've read it could lead to this issue and the fact that Chris experienced it gives me hope.
Good suggestions on electrical. My first post lists all the parts the previous owner had replaced however none of those shops ever experienced the problem. I had the fuel tank dropped to put in a new fuel pump as the gauge was wonky and led us to believe it could be related (it wasn't). The fact that the problem is now quite evident at highway speeds while keeping a constant pace but goes away on acceleration leads me to thinking it's a fuel delivery problem of some sort.
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Old 10-16-2021, 08:03 PM   #15
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I had an intermittent "rough cough" on my Ford 6.0 diesel for months. Turns out it was a wiring harness issue. It had a broken wire but not broken through all the way so the jacket looked mostly OK. The wire was between the speed sensor on the trans and the ECU so when it would lose contact, it lost info to the ECU and the ECU would not know what to do and start to shut down, just for a split second. This happened dozens of times as I was driving it and it felt like it was about to downshift but it didn't. Just a tiny surge. Took months for my shop to figure it out. All the codes said it was the speed sensor, which makes sense as the ECU was not getting constant info from the speed sensor. Shop finally found a tiny bad spot in the trans wiring harness and tried fixing/splicing the wiring. After several attempts at this then sent it to a trans shop and the trans shop swapped the trans wiring harness and it fixed all the issues.

One last thing, this only happened when the van was warmed up and running on the freeway. Somehow this affected that broken wire and it never did it while cold or driving around town.

All that to say, it sounds like a wiring issue to me. Good luck.
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Old 10-17-2021, 09:41 AM   #16
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It could well be an electrical issue. That 140 degree operating temp though is not right and should be addressed. I would replace the thermostat either way and see what happens.

Shouldn’t get the same Ford diesel 6.0 wiring issues, but it is a great example of wiring issue impact. Grounds are known trouble spots over time.

On the same 89 I mentioned above I was getting a no start issue that turned out to be a wiring harness/plug issue at the ECU.
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