My 2000 Volvo S40 (91,000 miles) has been doing two strange things recently that may or may not be related. First, the RPM level will surge from 2.0 to around 4.0 at highway speeds and from 1.5 to 3.5 at lower speeds. This has the effect of feeling as though the transmission is gearing down. This has occured very infrequently (4 times in the last two months). Sometimes, the Transmission will "shift" and the RPMs will go back to normal. Other times, I have to stop the car and turn if off. When restarted two minutes later, no problems.

Separate from this, I have been noticing rattling from the lower motor and transmission while accelerating. It only occurs when the car is traveling up a steep hill, etc. It sounds similar to a vehicle using low octane gas. However, I am using the same octane that I always have.

The thermostat and coolant sensor were recently replaced. No check engine or other trouble lights have come on. The RPM issue started before the rattling. Other than this, the car runs great and I've had no big problems.

Anyone have any ideas?

Thank you!

 

Listen under the hood for a vaccum leak with the car running.
Also CAREFULLY move the hoses and listen for a leak.

 

Thanks Tech. I listened carefully for a vaccum leak, but couldn't detect any. Assuming there is not a leak (I may take it in for a professional test), do you have any other ideas?

 

Maybe a Mass Flow sensor going bad.

 

Maybe my old favourite - dirty throttle body & idle air control valve causing the surging rpm?

Regarding the knocking, it's worth checking the engine & transmission mounting bushes for wear/damage.

BB

 

Can an intermediate do-it-yourselfer change the max flow sensor? Where is it located? What about the throttle body and idle control valve?

 

Yes the mass flow sensor is pretty easy to replace.
If you can change the Air filter on the car you can do it.
The sensor in screwed into the filter lid.

 

I was having similar problems with RPMS and it turned out to be a bad ECT (Engine Coolant Temperature) sensor. It has been replaced twice now. I have a 2000 S40. If you see my post dated 09/10/06 about my question on "serious decline in gas mileage" I mention some of the possible related problems I had with the ECT. I just don't want to double post as some boards frown on that. :-)

Hope this helps!