Vibration under acceleration...bucking bronco!
 

I've developed some severe vibration (full seat-of-the-pants massage chair style) during acceleration and I'd appreciate any expertise anyone can offer. I realize this subject is well-traveled territory, but I figured I'd submit my particular case.

2001 6MT allroad, 173,000 miles, stage 1 chip.

Pertinent service history: both front CV axles replaced within the last 25,000 miles, new ignition coils and plugs in the last 15,000miles, both ICUs replaced 2000miles ago, driver's side outer tie rod end 4000miles ago.

I'm not experiencing engine misfiring issues, and I sincerely believe this is a drivetrain issue. The vibration does not really start bucking while the car is cold, but after about 10-15 miles it starts kicking in. It's only under acceleration and completely smooth when letting off the throttle.

I've pulled n' prodded on all control arm components n' such and I'm not feeling excessive play anywhere. I tried my best to get a look at the driveshaft hanger bushing with the exhaust on, and it still looks to be intact, but I can't really tell.

I've read a lot of posts blaming the front cv axles for this vibration, but I do not believe that to be the case here. I've also heard tales of unbalanced tires, but I don't believe that's the case here either (the cold vs hot phenomenon).

Last bit of pertinent info: I believe the clutch is original, which seems like a helluva long run; it would stand to reason that she's done for.

My main questions: Could a failing clutch cause this type of vibration under load? (I've never experienced any jumps in RPM from clutch slippage). Could rear differential have anything to do with this? Is changing the rear diff fluid a good idea? How about front tranny fluid? Is there any chance wheel hubs/bearings could manifest vibration in this way?

Thanks folks!

Do you have any lights?
Does the vibration happen if you rev the engine, clutch disengaged?
Does it still go through the gears smoothly?
Do you get a vibration while cruising at a steady speed?

No lights on the display
 

No MIL, no other lights, no significant vibration with clutch disengaged (although I have not yet had a helper rev while looking at the engine to diagnose engine mount issues, which I am sure could use replacing), still shifts through all gears pretty smoothly.

I just took her out for another 50 mile drive and the first 15-20 miles were smooth, then the vibration became more and more pronounced under acceleration and intensity increases and decreases with how much I step on the gas. What I noticed this time is that she also shudders a bit when decelerating down an incline in gear (I tested this at 50-65mph in both 5th and 6th gear). On flat road with minimal gas needed to maintain speed, the vibration is more subdued, but still present.

I tested acceleration with traction control on and off: no difference. Changing ride height didn't really seem to effect the severity either.

1st gear acceleration doesn't seem to shudder as badly, but anything from 2nd on up vibrates under acceleration.

I think for starters I'm going to change the rear diff fluid...seems like an easy place to start...

Who did your stage 1? I have a stage 1 tune, and it can confuse the trans at certain rpm's under acceleration due to the torque settings. Although, mine is not only when warm.

I definitely would not rule out motor mounts. Being so close to the turbos/exhaust, they could definitely soften after warm up.

AMD in Woodinville, WA
 

My software is from Addict Motorsport Design (fastaudi.com). I've had it tuned for over a year and never experienced any issues.

I am planning on doing motor mounts and transmission mounts as soon as possible for sure. I just don't understand why the vibration wouldn't manifest regardless of cold or hot if mounts were the issue.

I'm still wondering if the clutch could be the primary cause of the vibration.

Yesterday I changed my rear diff fluid for the hell of it, and that didn't change anything. If I had to guess, I'd reckon the vibration was coming from the front half of the car anyway. Should I try changing my transmission fluid next? I can find in the previous owner's records that it has ever been done.

The wild goose chase of causation continues!

If you think of the material the mounts are made of, it makes sense. Rubber softens when warm, so if they are already worn, they will loosen up more when heated by the exhaust, turbos, and engine. You have to remember how tightly
these engines are packed into the bay.

valid point
 

I do think they're pretty shot. How much of a bear is it to do the job? On that note, how about trans mounts?

Trans mounts aren't bad, engine mounts...not so easy. Best way is to put the nose in service position, and hang the engine from a hoist. do one at a time, so you can tilt the motor one way or the other to give you more room.

quick question: at speed and the vibration happening if you disengage the clutch does the vibration stop? wondering if the clutch is heating up with use and starting to shudder/slip. Another test when the vibration is happening, accelerate; do the revs creep up before the car catches up with it?

Disengaging clutch does stop vibration,
 

But I'm not really noticing any rev creep. I have also wondered if the clutch is the culprit, but everyone I speak with seems to think that the slipping would be more noticeable than I am experiencing.