Intermittent Jackhammer Idle Mystery

[eb2143]

Resolved: It was a boost leak! Intake manifold...bolts fell out. I seem to have fixed my VC and vacuum pump seal leak with gray gasket sealer.

I have an interesting one for you all. Car is in signature. I know I need to get under the car to attempt to diagnose, but time is scarce at the moment and I wanted to see if this could be a DMF failing.

For about two weeks it has been intermittently loud at idle. Otherwise running like a top. The sound is difficult to describe, but it's not a tick, rattle, screech, or metallic exhaust rubbing sound. It's similar in pitch to normal diesel clatter, but it's LOUD for a TDI, it sounds like a Cummins 12 valve from inside. There is no vibration in the cabin. The rpms are holding steady -- it's not a rough idle in that sense. The engine sounds great going down the road and accelerates smoothly. It makes no difference whether the clutch is in or out. But, it goes away as soon as I increase the rpms. Again, it's ONLY at idle and if I raise the rpms to 1000 the engine actually gets quieter and sounds exactly like it always has at 1000 rpms.

The loud idle comes and goes. It's never been present when I first start the car up (so far only noticed on warm engine, but not reliably so). Last night a normal idle actually devolved over the course of 5 seconds into this "jackhammer" idle as I was sitting at a stop light.

The strangest thing is that when this loud idle is occurring, if I step outside the car and pop the hood, the engine sounds totally normal!

I'm learning toward a motor mount issue especially because of the last point that the engine sounds normal when you have your ear to it. Maybe there is some sort of resonance being transmitted to the car's body thru a mount?

I'm suspicious because everything MEYLE I've ever installed on this car has failed prematurely, and I have a MEYLE dogbone with 50+k installed. On the other hand, the clutch is original and the takeup is rough (lots of feathering needed with the throttle). I know it's getting close to time for a clutch and flywheel -- could this be the first signs of DMF disintegration? Could be some injector issue?

TIA.

 

[turbocharged798]

Sounds like the DMF. Better fix it before it explodes.

 

[eb2143]

Sounds like the DMF. Better fix it before it explodes.

Thanks.

Given the expense of a new clutch kit, I'd like to get a few other opinions before making a repair on a hunch, which I try to avoid except when catastrophic consequences of delaying are real (like in this case).

Is there anything I can do to eliminate the DMF or at least make that cause less likely?

I'm really thrown off that I can only hear the difference when I'm sitting inside the car.

 

[eb2143]

This has progressed over the last few weeks. I'm afraid I agree with Turbocharged now. I am now getting an odd sound at WOT or close it it that I cannot describe well. It's not quite high pitched enough to be a screech, but I'm sure it's either the turbo or the DMF. The best way I can describe it is if you were to take a spring wind-up toy and then let it go...it sounds like a spring releasing its built up energy. I've gotten some other intermittent bad noises. Still no relation to whether the clutch pedal is in or out. No pulsation in clutch pedal, but rough take-up.

I'm trying to schedule an appointment with local guru and feel that I'm driving a ticking time bomb. I'm driving around at 1/2 WOT, being very mindful of lugging the engine (1/4 WOT below 2k) and keeping rpms down. Unfortunately have no other vehicle. I will be going with the old Sachs G60 SMF kit, hopefully soon. Proven and cheap...

 

[eb2143]

Update: The bulk of symptoms described in this thread were indeed caused by a failing DMF. Though post-mortem did not indicate any imminent catastrophic failure, I am glad I changed it out for drivability alone. New clutch: http://forums.tdiclub.com/showthread.php?p=4378800#post4378800

Unfortunately the sound at WOT only remains. It's a higher pitched sound overlaying the normal engine sound; sounds like a bearing, but quite subtle still. Can only be reproduced under load; revving in neutral doesn't reproduce it It never seems to end...

 

[Lensdude_com]

is the high pitched sound sound like gas escaping? Like a boost leak?

 

[eb2143]

^I could make myself believe that it was escaping gas, but it's not the first thing that came to my mind (more bearing-ish). The next chance I get to be under the car I am going to look at intake plumbing, especially around intercooler and EGR piping for any signs of leaks. I also have a leaky valve cover gasket (leaking a small amount of oil down back of engine). I want to fix that on the off chance that under increased pressure of WOT it is making noise.

 

[CopaMundial]

Regarding your oil leak. You can't get just the gasket, it's part of the valve cover.
So there are a few real easy fix spots to check for that oil leak before you get into the valve cover.

My EGR valve and CCV breather tube both had tiny 'spray' type oil leaks that put oil down the back side of my engine. In both cases the spot where the oil accumulated far enough from the real source of the leak that it made it look exactly like the valve cover was leaking.

 

[eb2143]

I was going to start with cleaning the two VC surfaces very well also replace the vacuum pump seal; then I could move up to the CCV if still getting leak. I doubt the EGR only because I bought brand new OE EGR not too long ago.

EDIT: It was a boost leak! Intake manifold...bolts fell out. I seem to have fixed my VC and vacuum pump seal leak with gray gasket sealer.