No brakes and bad idle?

[Jonny-Jimbo]

Hi all,

Just a tech question about my 'modern', 'reliable' Volvo 480 Turbo Intercooled Injected beast thing.

Usually it is an electrical failure... not this time!

Anyway, it has always run a bit poorly, particularly idling, but then sometimes 'judders' when accellerating from a partial throttle opening to full throttle. It doesn't ALWAYS do this, only maybe one day of the week for example = or even just for a few minutes of a 3 hours journey.

I've always been able to ignore this as uncomfortable as it is sometimes.

When it hunts at idle, it can tick-over anywhere between 500rpm, then shoot up to 1500, 2000, even 2500 revs, then drop back to 500 etc.

I've always assumed this is a porrible leak in the intake manifold, but as a turbocharged car there are pipes everywhere, some of which are impossible to inspect at all.

HOWEVER, when I left work today I tried to stop at the end of the drive, but nearly carried on into the road! I had to stamp the pedal again to get brake pressure up. The engine had already ran for about a minute or two before hand.

Also, sometimes if light pressure is applied to the brake (ie, resting a foot on it) when at idle, and it does the hunting thing, the pedal can gradually move towards the floor. Lifting off, and hitting it again makes it rock solid.

SO - I'm assuming it is a vacuum leak somewhere in the system, but as it effects the brakes in this way could it also be a knackered servo?

I also assume it won't be doing my fuel economy any good with a leaking vacuum?

Ta for any pointers.

[Alex]

Sounds like it could be 2 issues.
Theres every chance the seals have gone in your brake master cylinder......

[Lord Sorbington]

A dirty throttle body can cause poor idle as can intake air leaks, easy to clean and check.

Sometimes a worn throttle position sensor can cause hesitation when you accelerate as the ECU gets confusing signals from the faulty sensor.

Try to see if there are any fault codes stored in the ECU as they'll point you in the right direction.

Check the hose to the servo too, brake fluid level, any leaks at the calipers, air in the system etc?

[thebrookster]

Or try MAF sensor. Good luck with finding the problem, this is why I hate modern cars!!

[Oobyscoot]

air bypass valve on the intake.

Had a few of these fail, one on a ford, one on a citroen and one on a pug, all started with intermittent idling problems.

[Jonny-Jimbo]

Okay thanks all.

I'll try to have a look at it at the weekend if I can. Brake fluid level is fine, no leaks that I can detect at all either.

I've had it at a Volvo dealership, and they said they couldn't find any fault whatsoever and I was making it up! Needless to say they didn't get any buisness again after that.

[Gwaredd]

I'd go for a split pipe from the servo to the inlet manifold. This would cause both the problems you mention.

[Lord Sorbington]

My brother had random brake loss on his old GTI.  All had been OK but after stopping for 10 mins, he drove off again, went to stop at a T junction, no brakes!!  Hit the pedal again and they were fine, only ever happened once.

The brakes were fine when checked over although the front discs and pads were well worn.  Maybe a combination of disc expansion / contraction led to a situation similar to that of having just fitted new brake pads?

Could also be the brake pads sticking a bit in the calipers, not returning fully due to the slideways being rusted up  so first time you press the pedal the calliper just moves out to meet the pads?  It’d be worth dismantling the calipers and cleaning  / lubricating all the sliding surfaces.

Or it could be a fault with the ABS pump, they often do a self test the first time you reach 10mph or so after driving off.

On another car I knew of, the brake would sometimes go right to the floor then  be fine next time and for weeks afterwards.  That turned out to be a pinhole in a flexi hose, I suppose certain ways the hose was stretched it let air in?

[MURPHIT RITESPIF]

Change the vacuum pipe from the servo to the inlet manifold and fit a non-return valve near the manifold,in the pipe. It will stop air being taken into the inlet manifold and affecting ilde revs and will also cut down on petrol vapours getting to the servo and perishing its seals. non return valves are quite cheap and can be sourced from motor factors or deisel factors ie. truck repairers

[Jonny-Jimbo]

Thanks Murphit, I might give that a try...

Thing is nothing happened today at all. Runs fine, no brake problems...

[jms700]

not sure if this helps, but when the wheel bearings went on my front wheels, i had to pump the brakes after going round a tight corner as the free movement in the wheel pushed the caliper pistons in.