I have a 2005 2500HD DMax with ~38k on it (100% stock). A little more than year ago (May/07) it started what I can best describe as a "rough idle" problem. What I'm calling a "rough idle" is this: Sitting in PARK, the engine is running real rough - as if a plug wire or two is missing from a gas engine. The truck is physically shaking/rocking from side to side. To visualize, the tip of the radio antenna is moving back/forth six to eight inches. If you go back to the exhaust, it sounds like it's under water (those of you who visit boat ramps will know the sound - glug-blub-glug-blub - it's not how a normal running engine sounds). No obvious black/blue smoke.

Conditions:

1. Cold start: sometimes the truck would start/run fine on cold startup, other times it would start, but idle real rough (as above).

2. After driving at hwy speeds (~60mph) when pulling my travel trailer (~6k lbs). I would pull off the interstate and once I stopped at the ramp the truck would start jumping/bumping as above.

These two conditions started off as rarely happening, progressing to darn near all the time by Nov/07. The dealer first replaced the fuel filter (at my cost), when that didn't fix it flush injectors (at my cost), then they did some wiring harnesses work, egr swapped, new flash to the truck computer, injectors swapped, etc (all warranty). None of this fixed it. Note that all this time misfire codes were being thrown.

By Sept/Oct 07 the truck would fall into this rough idle problem after normal in-town driving with no load. Drive around until it warmed up, pull up to a stop sign/light and hang on - it felt like you were on a bucking bronco. One issue that has compounded this is that (of course) the truck never exhibited this problem at the dealer. I do understand that it's darn near impossible to diagnose a problem you can't witness...

By Nov/07 I had found a condition that I could repeat at will: drive around a little until the truck warmed up and it would develop a very slight rough idle when at a stop light or in park. If I "gunned" the engine at that time, it would kick the truck into the violent shaking/jumping as described above. With this new found knowledge I went to the dealer, sat in the service bay and repeat the problem. The darn truck was about to walk away sitting in park it was running so rough.

I grabbed the (one) diesel tech they had and brought him out to the truck (while in the rough idle state). To paraphrase, he didn't know what the problem was, he'd already changed out whatever parts GM would allow him to, and further speculated that it was my motor mounts. I've seen broken motor mounts - no freaking way. He wouldn't hook any device to the truck to help determine what was going on - come back next week I was told. Having purchased three trucks and a Tahoe from this dealer, I was very disappointed that I was turned away with an obvious and serious problem.

Later that day I'm driving through a parking lot and the truck shuts down. DEAD. It'll crank, but no start. Fuel is pouring on the ground. I'm not a mechanic, but even I know that's not good. I have the truck towed to the nearest dealer (not the one I've been going to) and they find that the fuel filter gasket has been blown out. They put on a new filter, replace a water in fuel sensor, and send me on my way.

The truck runs fine for about a week, then the faint rumblings of that rough idle start to show up. Very slight and rare at first, but it has progressed along the same lines as above. It's getting worse, and more frequent. I very much suspect that whatever blew that fuel filter out last time is coming back at me and I can't afford to have it happen when I'm 300 miles from help.

Someone else tells me that the water separator is leaking air, the leak got so bad that it finally blew the fuel filter out. But it's all greek to me...

I could use help with 1) what the heck this problem might be, but more importantly, 2) a diesel tech in the Tampa/Brandon FL area that I can take this creature to. I've lost faith in the local dealer, and need a mechanic that will do more than plug up a computer to my truck. I need a mechanic that will go looking for a vacuum leak, or a manifold leak, or whatever the heck is going on here.

tia,
Chris Curran

This is one of those that you will have to look at to get a good diag. My only suggestions would be to search 'icepick' for the #2 and #7 injector harnesses, then check the wireing around the alternator for rubbing, but if you are sucking air into the fuel system, you'll need to find the source. *some* reports of the fuel filter housing being cracked has been noted to happening, but that is an expensive guess.

Vacumme and pressure command tests need to be performed, balance rates need to be looked at, basicaly this may be a wierd one and trained eyes need to look at it.

My truck has the exact same shake problem, only it is intermittent. If I give it enough throttle to bump up the rpm's by 50 or so, it stops shaking and runs smoothly. I'm thinking that it is an injector not working properly at times.

Do the #2 & #7 Injector Harness repair with an icepick (as outlined in DIY section). Let us know if that helps.

basicaly this may be a wierd one and trained eyes need to look at it.

Yea - I agree, and why I'm looking for a referral to a good shop in my area...

Do the #2 & #7 Injector Harness repair with an icepick (as outlined in DIY section). Let us know if that helps.

Thanks, but the #2/#7 wiring harness replacement was already done by the dealer last year to no avail. Besides, sticking an icepick into wires is not my cup of tea. :)

I need a good diesel mechanic (in the Tampa/Brandon FL area) and I don't mind paying for it. All I want is my truck fixed...

The GM fix is not a fix all-- they still mess up! :eek: Just try the ice pick fix-- All you are doing is raising a small dimple inside of the terminal so the injector terminal will have better contact. I say hundreds of LLYs have been fixed this way and no more problems.

Check the harness to the passenger side of the alternator-- the sharp corner cuts into the wiring and will cause problems. You have to raise the harness up to inspect it. It is tied down with a tie strap.

You can move the connector at the injector while the engine is shaking to see if it clears it up. That will tell you where the problem is. I doubt it is your injectors- these last a long time.

The GM fix is not a fix all-- they still mess up! :eek: Just try the ice pick fix-- All you are doing is raising a small dimple inside of the terminal so the injector terminal will have better contact. I say hundreds of LLYs have been fixed this way and no more problems.

Check the harness to the passenger side of the alternator-- the sharp corner cuts into the wiring and will cause problems. You have to raise the harness up to inspect it. It is tied down with a tie strap.

You can move the connector at the injector while the engine is shaking to see if it clears it up. That will tell you where the problem is. I doubt it is your injectors- these last a long time.

X2

wasnt a couple memebers here having problems with the dampner also?

<sigh>.... No change.

The GM fix is not a fix all-- they still mess up! :eek: Just try the ice pick fix-- All you are doing is raising a small dimple inside of the terminal so the injector terminal will have better contact. I say hundreds of LLYs have been fixed this way and no more problems.

Check the harness to the passenger side of the alternator-- the sharp corner cuts into the wiring and will cause problems. You have to raise the harness up to inspect it. It is tied down with a tie strap.

You can move the connector at the injector while the engine is shaking to see if it clears it up. That will tell you where the problem is. I doubt it is your injectors- these last a long time.

This is one of those that you will have to look at to get a good diag. My only suggestions would be to search 'icepick' for the #2 and #7 injector harnesses, then check the wireing around the alternator for rubbing, but if you are sucking air into the fuel system, you'll need to find the source. *some* reports of the fuel filter housing being cracked has been noted to happening, but that is an expensive guess.

Vacumme and pressure command tests need to be performed, balance rates need to be looked at, basicaly this may be a wierd one and trained eyes need to look at it.

X2

If you did the icepick fix and there are no rubbing wires, i would definietly find someone with EFILive or something that can scan the truck to check the balancing rates of the injectors at idle when its doing it to you. Along with the commanded fuel rail pressure and actual.

Correct me if im wrong but based on what you said earlier after they changed the fuel filter ect. the problem went away for a while. You could have a hairline crack or something to that effect, that when the fuel filter is clean its not trying to hard to suck air, because fuel is flowing so freely. (if there was a hairline crack in the filter?).

Just a thought, but as mentioned above, its an expensive guess. I've read of one guy on here bypassing the filtration system to verify air wasnt coming in through the housing, but i wouldnt recommend it without another filtration system of some kind in place.

More extreme things that could be causing it might be a head gasket, cracked piston or bent rod, but i wouldnt even venture down that road yet considering your miles and it being stock.

I'm no diesel mechanic :smashfrea but thats my .02! good Luck, maybe someone on here can chime in who has EFILive close to you.

mines at the dealer now, for some warranty repair a lot to list....., it does seem to idle surge(like my blower jet boat) when sitting at a light for a few minuates, ive been told maybe a fuel pressure regulator is blow'n out..we will wait and see how it goes..

The idle surge your describing is most likely a worn fuel pressure regulator valve, that can no longer accurately hold exacting pressure in the rail causing a random fluctuation in rail pressure resulting in the surge/stumbling, most noticable at idle while in gear.

I know that fuel pressure will cause erratic idle, and sometimes its enough that you need to have your weight into the brake pedal to stop the truck from moving.

I agree with TNT... you've probably got issues with the fuel pressure regulator valve on the CP3 pump. Don't mistake it with the fuel pressure relief valve on the back of the rail. I think the only fix for a faulty unit is replacing it. It shouldn't be much of a cost, but its a fair bit of labor to get to it. So if your paying somebody to do it, it could get pricey. Hopefully its under warranty.

Did you check for the harness rub problem? I would have thought that the ice pick on #2 and #7 would have fixed it. Maybe it's the harness. I guess it could be the fuel filter but I doubt it.

x2 on the fuel pressure regulator, what you need to do is find someone with efi live and check your commanded fuel pressure vs your actual fuel pressure while the truck is idiling all gypsy and you'll be able to tell right quick what the problem is. could also be a crack in the fuel filter housing but i think thats a long shot, if you could pick up a houseing from a wrecker cheap maybe try that.

I've come across an interesting new "condition": the clock is resetting to 12:00 when I start the truck....

This has been happening intermittently for a few months, progressively getting more and more frequent. One thing I couldn't remember to do at startup was to turn the key on, check the clock, then start the truck - I always did it in one motion so I never kinew if the clock was buggers (again) or is the voltage dropping due to the engine crank.

Well, I remembered this morning. Turned the key to "on" and the clock correctly read 8:29. Crank the engine and the clock resets to 12:00.

I'm guessing that I have a weak (or bad) battery or two. I'll have them checked later today. The question is this: Could a weak battery cause the rough idle problems I've detailed above? I know that in a gas engine a weak electrical system can cause spark problems, but I got no spark, so.... but could low voltage cause other problems in other subsystems of this hitech complicated diesel beast? The rough idle problem has been getting worse - is there a correlation here, or two separate problems?

Have you checked the voltage that the alternator is producing when the truck is running?

Have you checked the voltage that the alternator is producing when the truck is running?

Yea - it's over 14vdc.

I've come across an interesting new "condition": the clock is resetting to 12:00 when I start the truck....

This has been happening intermittently for a few months, progressively getting more and more frequent. One thing I couldn't remember to do at startup was to turn the key on, check the clock, then start the truck - I always did it in one motion so I never kinew if the clock was buggers (again) or is the voltage dropping due to the engine crank.

Well, I remembered this morning. Turned the key to "on" and the clock correctly read 8:29. Crank the engine and the clock resets to 12:00.

I'm guessing that I have a weak (or bad) battery or two. I'll have them checked later today. The question is this: Could a weak battery cause the rough idle problems I've detailed above? I know that in a gas engine a weak electrical system can cause spark problems, but I got no spark, so.... but could low voltage cause other problems in other subsystems of this hitech complicated diesel beast? The rough idle problem has been getting worse - is there a correlation here, or two separate problems?

I had the same problem going on with mine for about 2 months. My batteries were junk. The only reason i let it go on for so long was because the crank sounded strong most of the time and i didnt want to drop 200 on new batteries. But reprogramming the clock all the time was a pain in the ass.

That has nothing to do with your rough idle, espically if your running a good voltage when your engine is running.:)

fuel pressure regulator valve. there is a very specific diag for it and it is very clear as to defining if it is bad or not.

I had the same problem going on with mine for about 2 months. My batteries were junk. The only reason i let it go on for so long was because the crank sounded strong most of the time and i didnt want to drop 200 on new batteries. But reprogramming the clock all the time was a pain in the ass.

That has nothing to do with your rough idle, espically if your running a good voltage when your engine is running.:)


Yea, thanks. A load test showed one of the batts as bad. The dealer wanted $168 for one new batt (installed), so I declined and will pick up an Optima at my local Costco for much less...

fuel pressure regulator valve. there is a very specific diag for it and it is very clear as to defining if it is bad or not.


Thanks (and you too TNT et al). I just got back from dealer and they did not touch the FPR or the valve - they said no codes so no work. They DID witness the rough idle problem, but no parameters are out of range, so they say there's nothing they can do.... They claim GM will not let them do any work without codes being thrown. Forget that I've bough three top end new trucks, two new Tahoa LT's and a used 1500 since 2000. I guess GM wants me to spend my money elsewhere...

I really, really need to find a *real* mechanic in the Tampa Bay area. Someone who will do more than plug a computer into my harness.....

depressed in Brandon,
Chris

that is bs. the regulator usually does not set a code. it is a computer based diag. go to a differant dealer. they need to do a graph check in the tech 2 and see the regulator graph.

that is bs. the regulator usually does not set a code. it is a computer based diag. go to a differant dealer. they need to do a graph check in the tech 2 and see the regulator graph.

Thanks. I've made an appt with another dealer in the area...

UPDATE: Dealer was no help (surprise, surprise). However, I replaced both batteries a week or so ago and it's been running GREAT since.