What are the symtoms? The truck still has power and I don't see black smoke under hard acceleration. I'm asking because the truck at idle runs great but when I place it into drive I hear a thumping sound towords the front of the engine. Put it into neutral or park and no sound. I'm going camping in the morning with the family a 3.5 hour drive. The truck is all packed and ready to go but this sound bothers me. It's not a loud sound but I know it 's there.
The latest mods were gutting Inlet Intake and a K&N air filter modified like most of the guys have done.

This sound you are talking about doesn't sound like a vacuum pump. Sounds to me like it could be your harmonic balancer pully. Does the truck make a clunk when it is shut off? While the motor is shut down see if you can move the belt, your belt should have very little movement. If you can move is an inch or two side to side then I would replace the balancer pully. The balancer pully has a rubber bushing in it that gets old and cracks, when this happens it allows slop in the belt and other pully's making that type of sound. This problem will not affect your drive for the trip but I would check it out. Hope this helps.

Thanks for the Info. I check it out and let you know.

Checked it out and the belt is tight and the truck shuts down properly.

maybe you need to change your spark plugs lol. man when my vp went out i had motor oil going through the rear seal and into the power strearing resivore. not sure about duramax though. good luck on your trip though

maybe you need to change your spark plugs lol. man when my vp went out i had motor oil going through the rear seal and into the power strearing resivore. not sure about duramax though. good luck on your trip though


Umm since he is asking in the 6.5 forum, guessing at a DMAX issue is a moot point,

No vacuum is it's indication it is going out, I don't recall of a partially bad one, low vac from tubing leaks but from all I ever recall is either you have main vac or you don't.

Clunks could be crank pulley or bad motor mount how many miles on the engine, how about when going into reverse, possibly a bad U joint ???

It's weird I hear from the front when I place into drive. It's not a clunking noise. I'll take off the accessory belt and see if the nise goes away. When I place in neutral or park the sound is barely heard.
I haven't used the truck since I came back from my trip. It ran great and was happy with the cleaned out the upper intake and K&N air filter. It ran better than usual.
I changed the oil cooler hoses and the seal at the end of the transmission due to leaks last week. Tonight I looked underneath and now the fuel is leaking on the driver side behind the seat. I was pissed. I guess this what you get when you keep an old truck. Hopefully it won't be the fuel tank. Now it makes it two problems.

Maybe belt tensioner getting skechy?......or Pully getting hung up?

Louis

check the tranny and motor mounts-that's my first guess based on your putting it into drive statement. Open teh hood and try to observe if the engine shifts up when you put it into drive (this works on some other cars-not sure about the 6.5td).