High idle caused by brake servo leak ?

[chriz1]

Hi,

My trusty combo van 1.7 cdti developed an increased idle speed over past few days 950-1000rpm always has been 850rpm
Disconnected maf and idle speed returns to a nice 850 rpm. Tried a spare maf back to 950-1000rpm

So I started spraying carb cleaner to find air leak and sprayed on brake master cylinder where it connects to servo slightly rusty and the carb cleaner was being sucked in can also hear a hiss when engines switched off.
Brakes feel find.
So will a vacuum leak on the servo cause increase in idle speed?

[Happy Jim]

Yes, it’s allowing unmetered air into the intake manifold

Jim

[chriz1]

Sorry forgot to mention that the vacuum for the servo is fed off alternator by a vacuum pump. Would It still do it?

[E-bmw]

Not as a normal servo would as it is "unmetered air" entering the inlet that would cause the issue.

Having said that, if the "vac generator" being in reduced load, then reduced load on the engine could potentially cause an elevated idle.

At the end of the day, if you have a known fault you should fix it first.

[chriz1]

Yes just ordered one now get that sorted being as its brakes related