Revs Increase when Braking on cold startup

F

FunkyGBrown

Member
 Kent
Hi,

Just about to book the Zed in for MOT but while moving it on the drive I noticed the revs Increase when pressing the brake pedal.

Ive checked the air intake and it had separated from a rubber sleeve at the front. Secured it back in and hoped that would cure it, but it's still happening.

Last year I replaced the lower intake book as it was split. The car has run fine since, but I felt it was the first place to look. All looked ok, and no visible tears in any of the rubber tubing leading to the brake system.

Anyone experiencing something similar?
 
I haven't had anything like this recently, but I replaced a faulty brake booster on a SAAB 9000 ages ago. A leaking booster can cause your symptoms under some conditions.

Hate to quote Google AI, but they give a concise summary: "A brake booster causing RPM to increase when pressing the brake at idle, typically indicates a vacuum leak, most commonly a faulty brake booster diaphragm or a cracked check valve hose. When the pedal is pressed, the faulty booster allows excess air into the engine, causing it to idle higher. [1, 2, 3]"
 
That’s a symptom of a faulty MAF sensor especially if on cold startup.
 
When I disconnected my maf sensor trying to trace engine lights the revs on my car increased whenever I braked so I worked out it might be a faulty valve going into the servo or a split on the hard plastic line from the valve into the intake boot. Replaced but to no effect, maf revs still increased when disconnecting mag and engine lights still coming on after resetting with code reader. Still haven't worked out what's wrong as car off the road over winter .
 
I’m not sure disconnecting ting the MAF is a sure fire test. I’ve had one that did it regardless. The only thing that fixed it was a replacement known good MAF.

That’s said it’s only one possibility. The other worth checking it whether you have a torn diaphragm in the brake booster. (Big circular thing Right side of the engine bay at the back). Disconnect the hose and block it temporarily. Now apply your brakes and test. If it still revs on brakes it’s not the brake booster.
 
Thanks street, thats worth a try. Would a dodgy maf throw codes on both post cat airflow sensors which clears for about 30 miles before coming back when reset
 
I had erratic hunting and increased revs on braking.
We'd done some maintenance under the bonnet so retraced steps to ensure everything was put back together correctly.

It was at this point I found I'd not plugged the MAF back in. 🥴
 
Mike6 said:
Thanks street, thats worth a try. Would a dodgy maf throw codes on both post cat airflow sensors which clears for about 30 miles before coming back when reset
Click to expand...
Pre Cat O2 sensors measure the exhaust gases and trigger dash lights if it’s not burning properly. That could quite well be because the air/fuel ratio is wrong if the MAF is giving wrong data.

Post cat O2 sensors measure the effectiveness of the cats and don’t throw dash lights. It could be linked if you really mean these O2 sensors but I’d expect to see the pre cat O2 sensors throw dash warning s first.
 
Pre cat sensor never had a problem. One of my post cat sensors was throwing codes for a faulty heater element so got my garage to replace both as I could not get them undone. Had to be burnt out. Almost immediately I was getting codes for oxygen enrichment on both post 02 sensors. Suspected that there might be an air leak on the post 02 sensors so garage rechecked everything and are adamant no air is getting in around the sensors not was the exhaust joint leaking air.
I have been fiddling around ever since checking intake side for any air leaks around boot etc, disconnecting maf and noticing revs increase when braking but not when reconnecting maf connector. I also replaced the check valve to servo and hard pipe going into boot but no different.
After resetting codes they stay off for about 20 miles and then reappear, they are both for post cat 02 sensors and last time I used car before it got tucked away for winter car went into limp mode during a spirited drive.
I want to find cause myself but garage says they can do a deep scan and possibly a smoke test.
Thanks for help . Anything I could check would be gratefully received
 
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