2003 Gmc 2500 6.6L smokes only at idle.

lmorris · August 26, 2016

A tech here owns this truck. It smokes a lot at idle, fine off idle. Slight stumble at idle but is smooth when driving. Our generic scan tool won't let me monitor power balance. All I can do is disable injectors one at a time and monitor RPM. When doing this, the smoke never stops and all cylinders are pretty even in RPM drop. We cleaned the MAF sensor and no change. Any ideas on where I go with this one?

JoeR · August 27, 2016

White smoke? If I was guessing, and I am, I'd say it was injectors. I'd want to start the diagnosis with a better scanner though. Check the injector balance rates to see if they are even. Try changing the pressure regulator to the extremes of the adjustment. If the smoke clears up when you drop it, replace the injectors.

Joe

jimmy57 · August 27, 2016

You need a scan tool that will let you read out balance numbers to get the best evidence. Your symptoms are spot on for failed injectors on that LB7 that has the trouble-prone injectors. The balancing strategy on that motor is like the 6.0's. It will cover the bad one up the point it reaches limit. Cylinder power balance won't show it unless the balance feature is cancelled but read out of balance numbers will already have told you it is pooched. Rarely is it one so the smoking is from 4 or 5 of the 8 most likely.

I have never had a trusted source explanation of the failure that leads to smoking but it seems there is a valve in the injector that regulates body pressure and this sets the "rattle" of the nozzle and thus the atomization of fuel when operating. Large droplets make for smoke and eventually more noise. The control valve failure causes more return line flow and the balancing number being high is the FICM lengthening the duration for that cylinder's injector to get crankshaft rotation deviation back in tolerance. The control valve failure increasing return fuel flow rate decreases injected fuel quantity. I have never seen this myself but there is supposedly a risk that the injector with tthis issue could hang open with bad result (DUH!).

dieseldoc · August 31, 2016

Sure sounds like classic LB7 Injectors worn out to me as well. Like they said above check your balance rates and they will likely tell the story. A snapon solus is capable of reading balance rates if you know anybody with one I believe my autoenginuity can as well but don't quote me there its been a long time since I have had a duramax in my shop. Don't screw around with only a few injectors it needs a set or it will only be back in the not so distant future. Everyone I have talked to about this issue including my local pump and injector guys say not to drive it and definitely don't pull anything until it is fixed or it will take the cylinders with it. They tell me they have seen burnt pistons and scored cylinders from driving or pulling with this condition for too long. No personal experience here just going by what they told me. Also don't be surprised if the injector cup pulls out with an injector or two this is also common on these engines, try to pull the injector as straight out of the bore as possible. According my local fuel shop this is a leading cause to the cups coming with the injector, they make a tool to pull them so it comes straight out but even than some times the cup still comes with. Overall the job is not the worst on the planet.

jimmy57 · September 1, 2016

I made a removal tool using a cheaper medium size blade screw driver. I heat the tip and tapped it over just a tad less than 90 degrees. There is a gap between injector body and the sleeve. If you put the tip in that gap and lever screwdriver left-right it push the injector up and break the carbon that sticks them.

Remember that is an Isuzu engine put in a truck that had to have a body lift done to hold it. If 12 bolts would have done it then they use 22. If it was easy to access they moved it.