Does your DSM jerk forward when switching between boost/vac?

[eboy0]

This is just a thing I've been wanting to know, but have no one to really ask around here who know anything about turbos.
When I'm driving and when I start boosting and whether I'm about to shift into a higher/lower gear, or just let off the gas at like 15psi, vac falls to 21, exhaust sounds 100% more bassy and the car nose "falls" forward hard enough if I wasn't leaning back, my face would be in the steering wheel.... Is this normal? I'm not sure if this is something mechanically wrong...
Any info/help would be appreciated.

 

[romeen]

I haven't checked to see if the compressor spins freely since this started. But the turbo is very new, less than 2K miles on it. It pulls hard and smooth after the initial abrupt jolt. I feel the problem is unlikely to be from the turbo. However it is easy enough to pull the intake and check. I will do a boost leak test while there...it won't hurt. I'll post findings tomorrow.

 

[95autogst]

huh... Well it looks like my theory just got shot down, at least for most of you. I think for older turbos it might still be a possibility though. In my mind it would have the same effect as a diesel engine brake when you let off the gas.

 

[Deadly BlaZe]

I highly doubt it has anything to do with the turbo. The only reason the car jerks harder under boost is because there is a bigger load on the motor. I'm thinking it could have something to do with either the brakes or the transmission.

 

[romeen]

It feels like it could very well be related to the transmission.

 

[romeen]

Possible diagnosis made!

I did the boost leak test. In addition to my leaking TB shaft seals (which I have had since before this problem started) I found a small leak in the vacuum hose that runs from the J-pipe to the solenoid for my EBC. I corrected that leak and test drove the car. The problem doesn't exist now. I can't say with absolute certainty that this was the cause as it could be a coincidence. I only drove a few miles after the fix but it is clearly not doing it now.

I have been trying to work this through and it seems reasonable that a leak in this location could cause erratic and abrupt opening/closing of the wastegate possibly leading to the sudden acceleration/deceleration.

I have said in several posts before today that boost leak tests are part of my "routine maintenance". This is a good illustration of why.

Good luck guys. Let me know if this helps.

 

[eboy0]

Maybe it can be something with "leak in this location could cause erratic and abrupt opening/closing of the wastegate possibly leading to the sudden acceleration/deceleration." I do have like 2 T splits in that general area....
Although I have absolutely no boost leaks, I may have exhaust leaks before the turbo..
Another issue which is somewhat similar to this, when I'm really getting the car moving fast, like 5-6k when I'm about to shift, the car continues to rev for a split second before I get it into a higher gear. During that split second is when the "bucking" occurs as well.

 

[RMeadows]

In my experience(which is limited) this is usually associated with a boost leak. I had a major one which was making my car run rich all the time and it had these exact symptoms. Fixed the leak, no more problem.

 

[romeen]

eboy0, the leak that I mentioned was actually where the vacuum hose attached to the nipple on the solenoid box for the boost controller. The vacuum hose was just a tad too big. I switched to a smaller diameter vacuum hose and the problem disappeared. Since I can't give a clear reason why this may have solved the issue I have to admit that it could be a coincidence. Having said that I did not do anything else at the time so go figure.

Good luck trying to figure it out, I know how annoying it can be.

 

[Laysher91]

Check to see if your tps is working correctly throughout the entire movement range of the throttle plate. I've had a bad tps without throwing a CEL and one hell of a buckin' dsm. Just grab the steering wheel and try to hold on