revs up and down up and down

My car has the most annoying problem, when hot or cold it revs up then down up then down etc etc and as the revs drop you get what sounds like bottom end rattle then it revs up again. Fuel economy is great and it pulls like a train. Please help!!!!

Digifant would do this if the idle stabilizing valve packed up, which fuel system have you got?

It has the bosch system that has the weighing flap above the air filter.

Hi

I had a similar problem for about 3 years. I had it to a bosch garage and changed the pump, accumilator, distributor, injectors, air flow meter and even swapped the engine. Some time it would idle okish and the next minute up and down untill it started to stall (seemed like it was hunting), but it always went well when i floored it…….found the problem a month ago and it was the fuel pump relay at ?35!

The connection inside the relay must of been worn or rusted so the when the car idled at 900rpm the voltage from the battery (12v) was dropping to lets say 11v and causing the fuel pump to drop voltage. The pump voltage would drop meaning less fuel and the car would then try and stop itself from stalling. When i floored it the voltage reached its maximum so increased the voltage traveling success fully through the relay thus running well!

One thing i noticed was it was made worse by switching the lights on as it took extra voltage again.

Hope this helps as it annoyed the hell out of me for years!!!

Dan