Loss of RPMs at speed

[AuburnBC]

Hey guys. Having an issue I can't quite figure out. 95 325i. Once the car is warm and at cruising speeds (ie 55-70mph), when I press the gas 25-75% I lose RPMs and they fluctuate 2-3k RPMs. When I press greater than 75% it acts normal. Sometimes at idle it fluctuates rpms but not much

Any ideas? I've replaced the fuel filter recently (~100miles ago). My guess is either fuel pump, o2 sensor, or maf. It's done this since i bought it a couple months ago.

[JDStrickland]

I'd start by checking the throttle body. Proper movement of the throttle plate. Tight screws that hold it to the manifold. Tight manifold screws.

Have a helper operate the gas pedal as you observe the linkage/stuff that moves under the hood. Check that the nuts and bolts are tight. You're "guessing" at stuff that is normally pretty reliable. And you're guessing at stuff that normally presents itself differently than you are describing.

[tmswell]

Vacuum leaks? check throttle body (clean?), clean out the MAF. (JDStricklands advice -Agreed)

[AuburnBC]

Guessing at stuff that should be replaced anyway. BMW recommends 30k replacement of O2 sensor. They recommend replacing the fuel pump when the fuel filter is replaced. And MAF is a solid guess, clearly the issue is air/gas mixture related.

Already have an o2 sensor so replacing that and cleaning MAF/throttle body. Should hopefully take care of it. Thanks guys.

[JDStrickland]

1.) In cars from '95 models and beyond, the O2 Sensor is not replaced until 100,000 miles, according to the factory. Your comment about 30,000 is not accurate. It's not bad, but it's not accurate and it costs $80-ish for a sensor.
2.) The O2 Sensor is not going to give the symptoms you are fighting. The MAF maybe, but this is a pretty slim chance.

I still think your diagnostic routine needs to have a helper operate the gas pedal with the engine off while you observe the linkages on the engine. If this does not clear things up for you, then a smoke test would be the next thing to do. Open the throttle and blow smoke into the intake and see if it comes out anywhere.