By Simon928 - Wed Apr 28, 2021 1:20 pm
- Wed Apr 28, 2021 1:20 pm
#76435
Hi everyone! Since doing a complete intake refresh on my '90 S4, I have been experiencing an issue with RPM surging that only manifests itself on deceleration. The car idles fine, but when I'm driving 40+ mph and take my foot off the throttle the RPMs drop to 1100 and then bounce up a few hundred RPMs, then drop down to 1100, then the process repeats itself until I slow down enough to break through that 1100 RPM barrier.
Things I have tested:
Vacuum System
I have done multiple smoke tests on the intake with nary a wisp of smoke escaping anywhere. I have also checked the vacuum at the front damper when the engine is running with my MityVac and it is around 16" Hg. I have read before that it should be around 18" Hg--is that 2" difference enough that I should be looking into it or could it just be variance in measuring devices?
Fuel System
My FPR and rear damper didn't seem to be leaking fuel but they didn't hold vacuum either so I replaced them. No effect on the RPM issue.
TPS
I confirmed that the computer is getting the signal from the TPS by testing with my multimeter at the EZK as well as by confirming that my digidash showed 90 MPG when I took my foot off of the gas after driving on the highway.
Is there anywhere else that I should be looking that could be causing this RPM issue? O2 sensor? MAF? LH? I have seen some mentions of a rich mixture causing surging but I was wondering if the fact that this only happens on deceleration isolates things or not.
Things I have tested:
Vacuum System
I have done multiple smoke tests on the intake with nary a wisp of smoke escaping anywhere. I have also checked the vacuum at the front damper when the engine is running with my MityVac and it is around 16" Hg. I have read before that it should be around 18" Hg--is that 2" difference enough that I should be looking into it or could it just be variance in measuring devices?
Fuel System
My FPR and rear damper didn't seem to be leaking fuel but they didn't hold vacuum either so I replaced them. No effect on the RPM issue.
TPS
I confirmed that the computer is getting the signal from the TPS by testing with my multimeter at the EZK as well as by confirming that my digidash showed 90 MPG when I took my foot off of the gas after driving on the highway.
Is there anywhere else that I should be looking that could be causing this RPM issue? O2 sensor? MAF? LH? I have seen some mentions of a rich mixture causing surging but I was wondering if the fact that this only happens on deceleration isolates things or not.
1990 S4 Automatic