My '95 850 Turbo randomly drops about 500 rpms when driving. Could be in city or highway. Driving me crazy. It does not do it when idling. Mechanic says no codes popping. He has adjusted the Idle Control Screw several times. Says could need and Idle Control Plate. Any ideas what might be causing this problem?

 

Mine was doing this a little. It wasn't dropping as much, mostly about 200 rpms. It does not do it now and the only change I can remember making is that fact that my rear o2 sensor went out and I have not had any surges since replacing it.

 

Sorry but either you misunderstood what your "mechanic" told you or you need to find a new tech. Not sure what you mean by "Idle Control Plate" but assuming you mean the throttle plate?? If so, unless someone dicked around with it, the throttle plate should not need adjustment, ever. And just how would that cause a misfire (which is what it sounds like you have...) at hwy speeds??? Take it to a good indy volvo tech; first thing he'll have to do is sort out your throttle body adjustments...

 

Thanks for the suggestions. You are right on the Throttle Control Plate. (I am a new Volvo owner so I am still learning all the terminology).

The misfire makes sense, but wouldn't that cause a "code" to show up? There are no "codes" showing up.

Could it be a problem related to a vacuum hose or the fuel pump?

The O2 sensor also makes sense, but again, wouldn't a code show up for that?

 

I forgot to mention that apparently the previous owner did mess with the Throttle Plate