Increased revs on braking

[ulfurk]

I finally got my Z4 through the MOT (Icelandic equivalent) and on the road after, more or less, a year and a half.
After standing for so long there where a number of issues that needed fixing before it would pass inspection.
Amongst other things I had to replace the battery. I'm still having an issue that where the radiator light comes and goes and I suspect it's a wonky water pump.
But that's neither here nor there.

The issue I'm curious about is this: When ever the car idles at full stop and I step on the brake (at a red light for instance) the car will rev up a little.
It's not much, it goes from maybe about 800 rpm to about 1200 rpm. Sometimes it will rise to about 1500 and it sounds like it's hitting a limiter (to 1500, back to 1200 fast then back to 1500 over and over again). I'm not sure if this is a real concern or not but it's quite noticeable in engine sound and it bothers me.

I'm thinking I could be low on brake fluid or that it might be a computer thing, although I'm fairly sure they reset the computer like I asked them at the garage when they replaced the battery. Any ideas what might cause something like this?

[Nova2k7]

I've had the same thing for about 8 months! I've been thinking it was a vachuum leak in the break servo

[DrNick]

I think something to do with an anti-stall mechanism...

[redhotwheels]

Perhaps it is to make the car sound nice at the lights ?

[ulfurk]

I don't think the sound is by design because a) I'm pretty sure it wasn't there before and b) I don't think it sounds all that nice.

But vacum leak and anti-stall mech I'll keep in mind while I try to figure this out.

[AlanJ]

Never noticed the 'rev' issue on mine and 'anti stall' is new to me
Not sure I undertand your point about the brake fluid...... check the reservoir

[Dubbedown]

Misfire on cylinder?

[ulfurk]

Misfire on cylinder?

Doubtful, doesn't sound like one and they checked both the computer for things like that and vacum tested all the cylinders at the garage.

As for my thought about low brake fluid it went something like this: Low on fluid, less pressure, pump works harder, takes more power, engine revs to provide it.
It's really far fetched and probably quite implausible now that I think about it but it leaves me completely clueless as to what the actual problem might be. :\

[Maniac]

I've had the same thing for about 8 months! I've been thinking it was a vachuum leak in the break servo

I'd be thinking this ^^ a vacuum leak in/to the servo could do this. rpms will rise to provide more assistance to servo if its leaking or not working correctly. Have a garage check for vacuum leaks or issues with the braking system such as failing seals in the master/slave cyls.

[ulfurk]

I've had the same thing for about 8 months! I've been thinking it was a vachuum leak in the break servo

I'd be thinking this ^^ a vacuum leak in/to the servo could do this. rpms will rise to provide more assistance to servo if its leaking or not working correctly. Have a garage check for vacuum leaks or issues with the braking system such as failing seals in the master/slave cyls.

Yeah, this is starting to sound more and more likely to me.
Although this is starting to become just one of many problems that seem to be plaguing my car :\

Word of advice, never let your car stand and deteriorate for months!

[domsz4]

a commen problem for the v6 mondeo was lamda sensors failing causign rising/falling revs. for me it would happen as you braked to a standstill. the car would rev up by 1000rpm and then settle down.

it was linked into what could be mistaken as an antistall system. i cant remember the exact way things worked now but it was something to do with a stopping unburnt fuel dropping into the exhaust system or something like that. ( i really cant remember) anyway the problem manifested as revving after braking then dropping. it did this for a year or so till the sensor failed much worse and showed the more obvious signs of a lamda failure.

i would go with what the others said first but if you get it checked and its ok then lamda checks are easy to do with a sensitive voltmeter or oscilloscope.

[Nova2k7]

hmm any update on this at all, still having the problem, ive had a BMW guy take a look and even he doesnt know! thinks it might be the MAF?

[cj10jeeper]

Not sure about MAF sensor

Easy way to test the servo and eliminate the servo. Run the engine with car static, switch off wait a few moments and there should be enough residual vacuum to assist in applying the brakes a few times. After that the pedal will feel heavier.
If there's a leak it will be heavy straight away.

[mjgerrard]

KERS?

[Nova2k7]

Not sure about MAF sensor

Easy way to test the servo and eliminate the servo. Run the engine with car static, switch off wait a few moments and there should be enough residual vacuum to assist in applying the brakes a few times. After that the pedal will feel heavier.
If there's a leak it will be heavy straight away.

ill give that a go! thanks!

The BMW guys seems to think its electrical!?? are there any sensors other than the MAF that could cause this?