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Duramax Second Generation: 2004.5-2005 (LLY) Discuss the second generation (2004.5-2005 LLY) of the 6.6L Duramax diesel engine & associated components. 2006 LLY engine discussion belongs in Third Generation forum due to the differences of the 2006 LLY and the 2004.5-2005 LLY. Engine related discussion ONLY.

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Old 07-25-2019, 11:21 AM   #1 (permalink)
DownByTheRiver
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Missing at idle except w/load - very strange problem

I replaced my FPR and water pump recently, and have noticed that the truck has a miss after being warmed up. The miss happens in park, and especially when I tip in some throttle. I can immediately end the miss by turning on the A/C or putting it into gear. Clears up right away and runs like a champ through all the gears. I ice picked and then replaced the connectors for 2 and 7, and the FICM harness looks good. I can only read pulse widths, and when it happens, the Ms readings are all over the place on all injectors.

I've probably read 30 threads on this, and none of them come up with anything conclusive. Some threads even end with the OP selling the truck...

This guy explained the problem perfectly, but the thread crapped out like all the others I've been reading:

https://www.dieselplace.com/forum/63...t-surging.html

WTF is going on with this thing?

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Old 07-26-2019, 05:18 PM   #2 (permalink)
Durabiopwr
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What are the balance rates telling you? Is that what you are alluding to with the mS readings?

Might be a grounding issue or a loose crank position gear but should still do it while a/c is on.

Sounds like a misfireing/dirty injector. When in gear or in drive the fuel rate goes up so it can mask some injector issues.

Dump a ton of additive in the fuel and rod the snot out of the truck for awhile. Like pull some heavy stuff for long distances. Thought i needed injectors about 2 months ago and have since done some heavy towing and now balance rates are all good. Just watch for excessive smoke, dont want to burn a hole in a piston if an injector hangs up.

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Old 07-27-2019, 11:41 PM   #3 (permalink)
DownByTheRiver
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Finally got my Chinese tech2 working, so here's some terrible balance rates in neutral. Before I only had pulse width available to read off the ppe tuner. Getting more smoke now and power is obviously severely down. Don't want to buy injectors if it's wiring, so that's tomorrow's project. Tried to run the gm injector cleaner through today but couldn't get the 1/2" line off using the disconnect tools I had. Don't really like the Italian tune up idea because of how its acting right now. I dumped an entire bottle of diesel kleen through it today, didn't help. Did notice that when I commanded the rail pressure up higher, it ran like ****. It's just weird that the famous 2 and 7 are acting up even after I re-did the wiring. Maybe the injectors really are ****ting the bed, but that's a strange coincidence at a minimum.
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Old 07-29-2019, 06:01 PM   #4 (permalink)
DownByTheRiver
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Anything? Return rate test? Where do i go from here? Don't want to throw parts at it.
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Old 07-30-2019, 12:01 PM   #5 (permalink)
DownByTheRiver
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Truck is now missing badly while driving, and producing a fair amount of black smoke when under a load. Still clears right up in park when the AC is turned on though.

If I do a return rate test, how can I be sure the wiring in some way isn't the culprit and that it's actually bad injectors?
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Old 07-31-2019, 11:23 AM   #6 (permalink)
Durabiopwr
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Balance rates look fine but they are not the end all of diagnosis.

Yeah would keep chasing the wiring thing maybe you caused an issue when you moved everything for the FPR change? Can you do a power balance/cylinder contribution test while running with your tech 2? That might tell you better which holes are having issues.

Maybe some dirt got in while the FPR change? Just tossing ideas.
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Old 07-31-2019, 02:18 PM   #7 (permalink)
DownByTheRiver
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My understanding is that at idle, injectors shouldn't be more than 4.0 difference from each other. I'm seeing 5.9 from inj 2 to inj 7
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Old 07-31-2019, 02:53 PM   #8 (permalink)
Durabiopwr
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4 is the limit per injector. its not a difference between. and 2 and 7 are not 5.9 apart anyways. They are both negative and in spec???
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Old 07-31-2019, 03:30 PM   #9 (permalink)
DownByTheRiver
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I'm sorry, I meant #6. Thanks for clarifying, I thought these things were totally hosed because of the difference between 1 and 6 was 5.9.

So my next question, if I run a balance test, could it be clouded by potentially bad wiring?
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Old 07-31-2019, 06:06 PM   #10 (permalink)
DownByTheRiver
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Went back to stock just now and ran the balance test again. Also plotted the desired vs actual rail pressure. Most waves match, some are delayed or sawtooth

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