3 replies to this topic

[coopertrooper2001]

I have noticed that when stationary at traffic lights etc and I pump the brake pedal my revs increase 200-400 RPM.

Any reason for this?

I am running a 1293 MPi (Supercharged) with a standard servo setup.

[lrostoke]

Not sure on later minis, but I know on some cars they have compensaters built in that increase the revs when more load is being asked of the alternator, which when you pump the brake it will do.. although that does sound a bit sophisticated for a mini
what happens when you turn the lights on ??

[Aus]

I dont know the science behind it but I believe its something to do with how the servo works. I think its that since the servo works under a vacuum that when you pump the pedal repeatedly you loose the vacuum. Since the vacuum is got by the air been drawn out of the servo throught the intake you are effectively adding extra air downstream of the butterfly as you purge the servo (works a bit like an airleak) thus your revs will rise momentarily until the vaccum is restored.

As I said I'm not 100% on the science side of this but I'm sure someone will correct me if I got it wrong.

[Guess-Works.com]

The servo makes use of the vacuum in the inlet manifold... when you press the brake pedal the servo is opened to atmosphere and therefore the vacuum is partially destroyed. There is a diaphragm in the servo which acts as a barrier between atmosphere and the vacuum... there may be a 'leak' in this which is allowing air through and thus changing the air/fuel ratio and causing the rise in idle speed.

To see if your servo is functioning, switch the car off... and then pump the brake pedal until hard... then with your foot on the pedal, turn the car on... the brake pedal should decend. If not your servo's not working and probably leaking.