Revs rise when braking FIXED FIXED!

T

Timmo478

Member
 Warwick
Hi all, changed my main belt tensioner today then took her for a drive and now when u press the brake pedal the revs rise from idle 900rpm to 1100rpm.
Wondering if the extra tension is now putting strain on an old alternator and it's picking the revs up to make up for it?
Cheers..... Tim
 
Clutch out buz. She drives perfectly with no other signs or lights on. At first I thought it may have been the rubber servo pipe from bulk head to 1 way valve as it has collapsed. Replaced pipe but no change. Servo is working fine, engine off pedal goes hard after 4 to 5 presses then engine on and drags pedal down as it should. Think I've got a very small vacuum leak somewhere but surely it has to be in the brake vacuum system as my idle is stable and never moves until I press the brake. Cheers for u reply mate.
 
Usually something like that is servo related . If at idle, press the brake pedal a few times and it does rise a few revs but not that much.
 
Yeah would imagine has to be servo related thrusty as nothing else is affecting the idle. Not sure what the symptoms of a bad 1 way valve are. Will remove the whole vacuum line at the weekend and check for splits or holes. All I can think of at the mo.
 
Sounds a wise idea Timmo. The valves are just a one way valve that allow vacuum to the servo and stop positive pressure getting to the servo bellow. So when coming to a stop at speed, the vacuum increases as the throttle butterfly is closed and in theory gives a better assistance to the pedal.
 
Thanks thrusty. Only had her 3 weeks and she drives beautifully apart from that. Not a huge problem but spoiling the drive somewhat.
I'll post again once I've checked things out.
 
I also have/had this problem, however I have recently been working on fixing some of the common issues, such as rocker cover gasket, coolant leak from tank and the latest being the oil filt housing gasket. Whilst I had this stripped down, I also replaced the crank case breather pipe as I noticed this was split when i changed the rocker cover gasket. I haven't taken the car for a long drive, but I have noticed a massive improvement compared to previously where it felt like the revs bounced off a limiter at around 1200 rpm during the short distance I had driven it?? Is this even possible?
 
Newbie
Hi there mate, I get what your saying but I'm thinking if the crank case breather was my issue it would induce an unstable idle and I'd have more generalised idle issues rather than an issue that ONLY occurs when I touch the brake pedal. My engine will idle silky smooth at 900rpm until I touch the brake when it rises to 1200rpm until I release the pedal.
Cheers for your reply mate.
 
There were a couple of cases of this on the forum where replacing the MAF sorted it. I can’t for the life of me understand how the two are connected but there you have it.
 
Yeah I can't think of y it would be maf either but having it occur right after changing my belt tensioner nd removing airbox and maf to gain space, maybe the MAF didn't like it. Thinking of getting a garage quote for a smoke vacuum test just to he sure. If it passes that I'll change the MAF.
 
The idle speed is a touch too high, should be steady at 750, that could be related to your problem.
 
Right guys update. Idle speed is at 750 my fault for reading it wrong🤦‍♂️. Tested servo today, and it holds vacuum fine and also gives me 5 presses with engine off before a hard pedal. Fitted the genuine replacement rubber pipe from 1 way valve to bulk head as the old 1 had collapsed and thought that might be the issue. No change. I did get a nice hiss of air after removing the old 1, so that also confirms that the whole brake vacuum line is fine.
Next step is testing the MAF but as it idles, revs and drives fine I'm not holding out much hope. Can see this being a garsge bill.
 
You could try and get a MAF from bmw for test purposes and then return it. The returns policy at my local stealers have always been really good in that regards....as long as you look after the packaging.
 
Yeah get where u comin from but don't really wanna do that. Would rather waste 10 or 20 quid having it tested.
 
My 2.5 did the same was scratching my head then realised the maf electrical plug wasn't plugged in all the way !
Plugged it in all the way and hey presto sorted.
Check your maf electrical plug may be plugged in correctly but may have a loose or broken connection inside the plug itself.
Hope you get it sorted .
 
Slight update. Made my own YouTube smoke detector and smoke tested her. Only leak I found was out of the centre of the upgraded (which I now regret) disa valve bolt. Which made me realise that if you look at how these upgrade bolts work, there is no seal on the bolt head, thus allowing for a vacuum leak. As i mentioned previous, the only 2 things I've touched are the disa and the MAF removal for the disa. With the car running I put my finger over the disa valve bolt and it gets sucked to it enough to leave a round mark on my finger. However having found this vacuum leak my engine revs still rise on braking when I cover the bolt head with my finger. I read somewhere that if u look at the inside of the intake disa valve area with disa removed its actually shaped to stop the standard pin coming out all the way. So for your disa pin to destroy an engine, the disa valve would have to completely break into pieces to allow the pin out. Looks like I'm going back to a standard disa as I ain't forking out for another, different upgrade kit. Will keep u guys posted.
 
SOLVED..... Was the MAF sensor after all guys. Just changed it out and no more issues. Removed the vacuum pipe at the 1 way valve and blocked both off and it still occurred.
The clue to it being electrical and not vacuum was also if we pressed the brake slowly, the revs went up the instant the brake lights came on.
Cheers for all the advice.
 
You must log in or register to reply here.
Back
Top Bottom