1996 850 engine revs without accelerating or shifting gears Topic is solved

[EB53]

Dealing with a mystery and could use some advice please. I have a 1996 850 (non-turbo, automatic transmission) with 131K miles. For past few months the engine occasionally revs up to 3000 RPM soon after I start driving and am still going 15-20 mph. Car won't accelerate or shift to higher gear. If I pull over and turn off engine for a few minutes, it resets and drives/accelerates normally. After some experimenting I found that if I shift to 3rd gear and then back to drive while in motion, the RPMs immediately drop to 2000 and the car shifts and accelerates normally until I stop at a stop sign. When I press on accelerator, engine revs back up to 3000. Check engine light never comes on, and there aren't any codes.After reading through other postings that made it sound like the problem might be a dirty throttle body or faulty throttle position sensor, I cleaned the body and replaced the sensor, but I'm still getting the occasional revving without acceleration or shifting.
I first noticed this problem after I replaced the idle air control valve and mass air flow sensor last year in response to problem with rich idling and stalling. Those replacements did fix the idling and stalling, but then I started to notice the occasional revving without accelerating/shifting. Any thoughts on what else might account for this problem? Local Volvo mechanic speculated it might have something to do with lockplate for gear adjustment not locking appropriately, but was just guessing. Thanks for any suggestions.

[454cid]

.... the engine would occasionally rev up to 3000 RPM soon after I started driving and was still going 15-20 mph...

This sounds like a transmission issue to me, not engine controls, although you do mention the TPS changing the behavior negatively. How did you clean the throttle body and TPS?

Check engine light never comes on, and there aren't any codes.

How are you checking for codes? Could it be your not scanning the computer for the transmission, only the engine?

[EB53]

Thanks for reply. It does feel like a transmission problem. I just took car out for a drive and it drove fine, so maybe the TPS replacement didn't worsen the problem. Mechanic removed the throttle body and cleaned it with throttle body cleaner. I replaced the old TPS sensor with a new Bosch OEM sensor. Mechanic checked for codes but I don't know for a fact that he checked for transmission as well as engine. He suggested that I replace the PNP switch, but I think this is because he couldn't replicate the revving issue and thought the engine was entering limp mode - neither check engine or low gear arrow lights come on when I experience the revving, so it doesn't seem like car enters limp mode.

[Sveedy]

Have you ever drained / flushed the transmission ?

[EB53]

No

[Sveedy]

Well if your planning on keeping the car, then it would be worth doing it. And it might solve your problems as well.
Lots of write-ups on it, and a few ways / options on the best procedure. I did the three drain and re-fill's in my case.

[EB53]

Thanks, hadn't considered this before. I will read up on it.

[abscate]

Clean out the Idle air control valve

Make sure your throttle linkage isn’t binding

[EB53]

Thanks, will check these. The IAC valve was replaced last year.

[454cid]

Thanks, will check these. The IAC valve was replaced last year.

I'd assume there's a passage way and seat where the valve goes in, make sure that's clean.