RPM bouncing when tapping gas

MARKUS NORREBJöRK

MEMBER
2008 BMW 335
3.0L
6 CYL
TURBO
2WD
AUTOMATIC
8,000 MILES

Hello, I do not know if this is a problem but I am thinking about it much because I never felt this in my manual car that I had before. When I cruise about sixty mph and let go off the gas. Then tap the gas really fast and let go directly I then can see the rpm go up and down. It first goes up then goes down and then it gets to the middle where it should be. Is this something that has to do with torque converter lockup?
I am also feeling a clunk from the drive train or underneath me when I let of the gas really quick after the acceleration. Could this be the guide or bushings or is it transmission related?

1 Reply

STEVE W.

MECHANIC
13,452 POSTS

The rpm fluctuation is normal. Most newer vehicles will shift into overdrive and lockup at cruising speed and when you tap on the throttle the converter will unlock and the trans will drop out of overdrive. That is because the car thinks you want to accelerate and if it did not downshift you would have very little power. The initial upswing is the throttle and transmission response. The drop is from both of those systems returning to normal and then the lockup occurs which drops it into the middle.

The cluck can be from multiple things. All drive-lines will have tolerances for each part to enable them to move and be lubricated. If you add them all up it may take 1" of travel to take up all the slack. Normally you would not notice this but if you snap the throttle you are taking up all that in an instant. It is also very easy to break parts doing that.

Both of these issues are compounded by the fact that you have a turbocharger. That has to spool up when you snap the throttle and release it. The sudden increase and decreases in power hit the parts even harder.