Right, just picked up my new UK 2000 and a problem...

Driving back, when I clutch down to change gear, the revs rise by about .5k
When sitting in neutral the revs spike to 2k or drop to a very low tickover .5k.
This is a total intermittent issue as it did it around 50% of the time. Now, MAF sensor or dump valve?

Thanks in advance

 

It's a very good question - although it could be a combination of both, or something else entirely (have you checked the inlet tract for air leaks?). You're the only one with the ability to find the answer though.

Is it a 2000 model year car? If so the airflow sensor would be the first thing to look in detail at. Try unplugging the sensor with the engine off and then starting. You will get a CEL (and the car will be in limp mode), but if you also get a rock stable idle, then there would appear to be a significant level of responsibility for the MAF.

 

Many thanks for the advice.