What can cause bucking under deceleration

[joeym4130]

This is another of my barrage of problems with the newly acquired Tsi. I love trying to figure it out haha.......but havent yet. Ok I can be driving in any gear, and let off the gas. I feel the car cut its fuel like its supposed to when you let off the throttle to help decellerate the car, but when it drops to around 2000rpms, it starts to buck, turning the fuel on and off as if someone was tapping the throttle, its trying to accelerate, so it bucks. And sometimes it doesnt go into decelration mode at all, it just kind of coasts as if you had your foot on the gas steady. Its not the TPS (replaced it in case), but what other things could be a culprit? If the TB plate was stuck could this cause it? Any other ideas?? Thanks guys... this is my first intro to the world of the DSM and i like the car immensly...just these little thingswrong.


oh yeah, btw the car runs fine every other aspect except this problem and idle, which i KNOW is tied in with this.....

 

[joeym4130]

Anyone?? i have tomorrow to work on it, then I have a busy schedule and wont be able to attend to the DSM...i'll take any suggestions, even theories!! ...anything to check, even just to eliminate as a cause!

 

[joeym4130]

ok if anyone wants to know .... the throttle cable has slack in it..so its not holding it open... Someone mentioned cruise control, but i thought its mechanical on 1g's so the cable would be tight if that was the culprit wouldnt it? A vac leak couldnt make the car accelerate on its own could it??

 

[Newlogics]

most likely your throttle cable is over-tensioned, or your TB need cleaning/adjusting.

What's happening, is that your throttle plate doesn't close fully when you let off the throttle (or there is a leak past TB that gets axtra air in).

When you're decelerating from higher RPMs, the inertial force created by the car is powerful enough to overwhelm any engine power fluctuations, forcing the engine to rotate along with the wheels.

When you get to about 2000rpms, inertial force of the car drops and about equals to the force produced by the engine using that air that your TB leaks. At that point any engine power output changes will be enough to overpower inertial force, thus jerking car and bucking.

In other words, you're experiencing an idle surge, when coasting at slow enouh engine speeds in gear.


P.S. Remember, that your thorttle cable may not look-feel over-tensioned when the car is cold. Let the car warm-up completely, and then check agin. As throttle cable insulation expands from heat, it will put tension on the cable that wasn't here when car was cold.


If you have a logger, check your TPS position when off-the throttle while cold, and then while hot. See if there is a difference.

 

[joeym4130]

Ok, after a day of looking around and testing....

My throttle cable has been messed with, I put it at a little slack, no difference it still accelerate on its own. My TPS is ok, i even adjusted it again. I did a leak test, and it held 15psi with no leaks on any vac hoses. Tried 2 ECUs also (quality of them could be questionable of course, im sure mine could be bad, and the one i bought cheap too haha). Im gonna junk the car in a minute here...haha.... any other ideas?

 

[Zenja]

I'm having a similar problem. Well, different symptoms but same problem. I don't know what's causing it but I know what's happening. Maybe try cleaning out your throttle body.

It seems like for some reason the car is trying to keep it from stalling but it's doing it at too high of a point. Once the car drops to idle RPM it will give it gas to keep the RPM up.

I think your idle speed controller is bad, especially since you're getting a jumpy idle.

 

[Newlogics]

Originally posted by joeym4130
Ok, after a day of looking around and testing....

My throttle cable has been messed with, I put it at a little slack, no difference it still accelerate on its own. My TPS is ok, i even adjusted it again. I did a leak test, and it held 15psi with no leaks on any vac hoses. Tried 2 ECUs also (quality of them could be questionable of course, im sure mine could be bad, and the one i bought cheap too haha). Im gonna junk the car in a minute here...haha.... any other ideas?

How does your throttle butterfly look? Does it close completely?

Think of it this way:

there is no way for an engine to rev up without air, so something lets more air in then normal. Have you tested your idle control motor?
(vfaq.com has detailed instructions on doing that under idle-surge section)

 

[joeym4130]

Yeah my TB butterfly is the next thing im gonna work on as soon as i have time to take the TB off and stuff. I suspect (and hope) its the prob...cause everything else i could think of i checked. (ISC tests good, adjusted w/ new TPS, MAFS, no leaks, cables. So maybe it needs a little smack or a new TB. thanks guys ill post what happens when i get to it.