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Discussion Starter #1
Alright, I posted a few weeks ago about an engine miss at idle. Truck was at the dealer for diagnosis. They said it was losing fuel pressure at idle. They did the reflash, the one that adds fuel filter life etc, and then they couldnt reproduce the miss. They also said the fuel filter pulled 5 in. Hg of vaccum and recomended changing it soon, I changed it the next day. That was about two weeks ago, the problem resurfaced today. Lemme give as much background on the miss as possible. It will start to miss after it has been idling for a while (as little as 15 seconds or as much as 10 mins mainly when engine is warmed up). No smoke when it misses, showing to me that it is a lack of fuel issue. I tried pumping the primer while it was missing and it did not change anything. It doesnt miss at higher rpm or have any power loss at higher rpm with one execption. When first starting in the morning (cold start under 32 degrees F) auto fast idle on 1200 rpm, i can here a slight miss, and can feel a slight shudder in the shifter (manual).

Now heres the problem, GM voided my engine warranty (truck has almost 37000 miles on it, however it was brought in when it was still in the 36000 bumper to bumper warranty) when i brought it in for this problem because i have an EGT probe in the exhaust (its the edge egt probe). I am currently talking with GM but they need to talk to the dealer and will call me back tommorow. I plan on fighting this to the fullest because i know that the egt probe or the edge is not the cause of the problem. Edge was not installed when i brought it to the dealer (attitude was in the upper console, its permanently installed) and hasnt been for several weeks now, the miss happens with or without edge.

Now here are my questions, assuming worst case senario, and GM refuses to help me on this problem, what do you guys think it is? Do you think it could be a fuel pump? maybe the bosch fuel control brain mounted on the motor? What tools am i gonna need to diagnose this problem should i have to do it on my own? What should I check for? I dont really know alot about the fuel system in this truck. I dont think it is the fuel wiring harness rubbing through and grounding out problem discussed on here cause it only does it at idle and never has had any other syptoms at rpm (i would think if a wire was shorting out i would have codes and it would have syptoms at any rpm). There are no codes btw. Injector balence rates are all well within the +-4 spec so i dont think its an injector problem. No fuel in oil per last sample.

Any suggestions or help is greatly appreciated, I'm just trying to get ready if I need to diagnose this problem on my own. sorry for the long post, tried to be as through as possible on the problem.
 

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Good luck with this one. I'm afraid there's not much help here for idle problems. I've been fighting this for over 2 mos now. There's just to many things that can cause this, and with no codes, (CEL) dealers "at least in my area" are NO HELP!
 

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Miss!

I had a miss in my '04 duramax a while back, well maybe more of an erratic idle, but it didn't do it when first started. It seem to do it after it had been running for a short time and then would get worse as the day went on. After a couple of trips to the dealership, they finally diagnosed that the fuel pressure regulator was bad. Once they replaced that the problem was cured.

Hope this helps.
 

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Idle problems are hard to diagnose if they don't act up all the time next thing I would ck would be the fuel pressure regulator and you need a scan tool to do this
 

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Discussion Starter #5
can you explain this fuel pressure regulator? where is it located? What should i check for with a scan tool?
 

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Discussion Starter #6
Also, just did some searching, sounds to me that the fuel pressure regulator is most likely the cause. One question though, can someone define lopey idle?

Mine will start to definatly miss which makes sense if the pressure regulator is giving too low a fuel pressure or too much fluctuation in pressure. Seems to fit cause it wont happen til it idles for a while maybe once fuel pressure starts to fluctuate and the dealer did say fuel pressure would drop off. Dont really think its the pump cause i have no problems at higher rpm, other than that one condition i have when its cold which is just off idle (1200 rpm).

What do you guys think?
 

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I'm not sure what they mean about lopey idle but the problem your having sounds exactly like what my problem was and changing the fuel pressure regulator fixed it.

And sorry, but I can't tell you what it looks like or where it's located, the dealership just told me they replaced it and it cured the problem so I was satisfied enough that I didn't even ask to see the old part which I usually do.

Good luck!
 

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You need to live plot or graph actual fuel pressure,desired fuel pressure and engine rpm you are looking for a saw tooth pattern of actual fuel pressure, witch means fuel pressure regulator not good. Fuel pressure regulator mounted on injection pump right side about the size of a roll of half dollars
 

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Discussion Starter #9
truck was it the dealer all day today. they only got it to do it once at the end of the day. here is the exact text from the sheet:

calculated fuel rate: 8-9, fuel rail pressure at idle is 5400, main injection timing 2.0, no unusual change in rpm during injection balance test, #6 injector balancing rate is 0.0 to 4.6, possible #6 injector starting to fail

so they said they think its either the #6 injector or back to the fuel regulator but with this new info they seemed to think #6 injector was the most likely cause. This miss does always feel the same kinda hinting to me that its the some cylinder missing everytime which i wouldnt think would happen with a fuel pressure regulator problem.

Can anyone get anything helpful out of this information? the #6 injector is close to spec of +-4 but should it fluctuate between 0 and 4.6? I know last time it was at the dealer the highest balence rate was 2.3, dunno if that was even #6. Then i guess the next question is how hard is it to change an injector? Which cylinder is #6 (drivers or passenger, how many back from the front of the truck)?

Thanks for any help
 
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