Got the o2 sensor and fuel filter changed today..I hope the nasty filter didnt cause more problems! overall the new stuff seemed to keep the car from running so rough at high mph especially the fuel filter (of course), however it is dragging seriously. No check engine light yet. Still getting a more subtle yet steady whomping sound. I will have to investigate the intercooler piping more still, but I have been looking at the right piping according to the diagram and found nothing.

I figure I will start another thread to handle the turbocharger that gets no boost. thanks for the advice.

 

I apologize but I thought I posted a previous problem I had with my '95 850 R. It was surging while crusing at highway speeds. The tach went from 2200 rpms to 2500 rpms for no apparent reason. After a few months I got a CEL light that was for the rear 02 sensor. I picked one up from the junk yard for $25 and it worked. When doing research about 02 sensors, my mechanic at the junk yard (they service a lot of Volvo's and rebuilds wrecked ones) said that it was not big deal and not to really worry about it but then I found out that it will likely effect gas milage because the sensor helps the computer manage the engine, fuel mix, etc.

So, a bad 02 sensor can effect gas milage & the way the computer manages preformance. After I replaced my sensor the code went away and my car stopped the surging at highway speeds.