LLY Engine Miss commanded by A/C Circuit (Duramax Gurus Needed)

CumminNotRunnin

CumminNotRunnin

Internet Mechanic
Been fighting with a truck for a few weeks now on and off.

The engine will stumble with the A/C off. Soon as you command A/C on the engine smooths right out.

Balance rates are ok and FRP is spot on between actual and desired. This seems to be electrical because it happens almost to the degree of 170* it will start acting up.

Also noted with A/C off and engine stumbling the balance rates all go to Zero and CMS on cylinder 7 and 8 drop from and average of .45-.63 To a spread of .00- .63. Just on cylinders 7&8.

Soon as you kick the A/C on and put load on the engine the Balance rate come back and the CMS smooth out to an average of .43-.55.

The two major determining factors that are exact every time is the temp of 175* it start doing it immediately. Meaning it will not do it when cold. The other factor is the A/C command will every single time turn the Skip on and off.

Parts, we have changed or replaced so far: 8 bosch injectors, FICM, Crank and Cam Sensor, Performed ICE pick test, Checked the FICM control harness for rubs on the alternator. Non found. There is also no diagnostic codes logged.

Truck has EFI Live tuning. Still does the same thing on stock tunes.

Please feel free to comment below with any information or you can call me at the shop the discuss more detail.


Thanks in advance
Jake 973-668-9242
 
Balance rates aren't monitored if the RPM is fluctuating.
Any smoke when it's happening?
How much is the RPM fluctuating?
Which cylinder or cylinders are showing the miss?
Have you tried killing each cylinder to narrow down what's going on?
Does the tune that's in it, not monitor some codes?
Compression good?
Damper tight, and no broken rubber?
I had a Allison cause something similar many years ago.
 
smokinVE said:
Balance rates aren't monitored if the RPM is fluctuating.
Any smoke when it's happening? Non
How much is the RPM fluctuating? About 80 rpm and 28 rpm when not acting up
Which cylinder or cylinders are showing the miss? Cylinders on CMS 1,2,5,7,8
Have you tried killing each cylinder to narrow down what's going on? Yes injector kill test does not change anything because it is like rolling miss.
Multiple cylinders but never at the same time.
Does the tune that's in it, not monitor some codes? Egr delete tuning only
Compression good? Have not performed an compression test because I believe its electrical. And how perfectly it is duplicated with the A/C but under load drive drive it still does it.
Damper tight, and no broken rubber?Have not checked into these.
I had a Allison cause something similar many years ago.
Click to expand...
What about the Allison caused it?
 
I assume it was torque converter related but the powers that be wanted a whole unit installed so they could tear down the old one and find the issue. I never heard back from our engineering...

Any chance there is a suction side leak? They typically only show up when cold but I've had them act up hot too.

Really silly question but does it have a diesel fuel cap and not a non vented gas one?
 
There is diesel cap on it yes.

Did not check for bubbles on suction side yet. If the issues was fuel related and not electrical, when I apply a load such as drive to the engine it should smooth out as well. Not only the A/C compressor.

I'm not quite ready to throw a trans at it yet.
 
I'm not suggesting throwing a trans at it, it'll be a little work but you could disconnect the converter and run it.
 
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