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Increased revs on braking
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- Newbie
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Increased revs on braking
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?
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
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Re: Increased revs on braking
I've had the same thing for about 8 months! I've been thinking it was a vachuum leak in the break servo
- DrNick
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Re: Increased revs on braking
I think something to do with an anti-stall mechanism...
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Re: Increased revs on braking
Perhaps it is to make the car sound nice at the lights ?
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Re: Increased revs on braking
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.
But vacum leak and anti-stall mech I'll keep in mind while I try to figure this out.
- AlanJ
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Re: Increased revs on braking
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
Not sure I undertand your point about the brake fluid...... check the reservoir
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- Dubbedown
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Re: Increased revs on braking
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.Dubbedown wrote:Misfire on cylinder?
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. :\
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- Lifer
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Re: Increased revs on braking
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.Nova2k7 wrote:I've had the same thing for about 8 months! I've been thinking it was a vachuum leak in the break servo
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Re: Increased revs on braking
Yeah, this is starting to sound more and more likely to me.Maniac wrote: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.Nova2k7 wrote:I've had the same thing for about 8 months! I've been thinking it was a vachuum leak in the break servo
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!
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Re: Increased revs on braking
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.
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.
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- Nova2k7
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Re: Increased revs on braking
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
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Re: Increased revs on braking
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.
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.
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- Nova2k7
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Re: Increased revs on braking
ill give that a go! thanks!cj10jeeper wrote: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.
The BMW guys seems to think its electrical!?? are there any sensors other than the MAF that could cause this?