I'm trying to solve the age old quest to smooth out my rough idle on my 1997 900 SE Turbo Automatic Transmission. After reading a bunch of old threads I checked off what seams like the most common causes.

What should I try next?


  • Rough idle, vibration, rumble. Especially when car is in Drive. Worse when A/C is on.
  • Idle sits right at about 910-920 RPMs when A/C is on in Park, and 900-935 RPMs when A/C is off in Park. The idle feels rougher, but RPMs are actually more stable in Drive when A/C is on.
  • No codes.

What I've done:

  • Tested and cleaned IAC (per this thread - IAC Bad Maybe)
  • Tested EVAP Purge Valve (per the following thread - Major Evap Leak -No Diagnosis -What Next & Ticking purge valve?)
  • Clean Throttle Body it's super clean.
  • O2 Sensors - Replaced both less than a year ago with new Bosch.
  • Vacuum Leaks - I've got no codes, and I've checked most of the hoses and connections. Everything appears to be in good shape.
  • Plugs & DIC - Plugs have less than 5,000 miles on them NGK BCPR7ES gapped to spec, DIC is relatively new.

Ideas that I've seen:
- ECU Temperature Sensor - "And I learned something about the "ECU temperature sensor", that it can affect the idle when it is not responding to the coolant heat..It would also raise havoc with the MPG, creating a run rich condition." I'm not sure what this means. Seems to imply that rough idle can be caused by the coolant temp sensor sending a wrong reading?

- Failing Engine Mounts - Visual inspection of mine appears like they are good. I replaced the left and right about 100,000 miles ago. And I don't believe I've ever replaced the rear mount.