Brake boost problem (mainly on track)

E

exocet

Member
 Near Geneva & Green Hell
Hi all,

Since I bought my Z4M, I often have a problem on track, when I quickly brake after a long acceleration. The brake pedal is very firm, same as if there was no (or very few) brake boost. That's quite frightening...
It happened with original brakes, but after I replaced them by AP-Racing (front & rear) too. So it's not related to pads, fluid, calipers or discs.
I did a simple test : engine on, when I push the throttle pedal and pump 2 or 3 times on the brakes at the same time, after then there is no boost at all. Does it happen on your car too ?
I checked on the ETK and the "air shut-off valve" reference number has changed in 2007 : http://bmwfans.info/parts-catalog/E86/Europe/Z4_M3.2-S54/L-N/browse/engine/vacuum_control_engine/ . Do you know if my problem will be solved by replacing mine with that "new-referenced" part (my Z production year is 2006) ?
If you do the previous test I did, can you please tell me the result and the production year of your car too ?

Many thanks in advance for your help !
 
It's a safety feature - if the car sees you suddenly jump off the accelerator, it will prime the braking system so it's easier to do an emergency stop. Not sure if the feature can be disabled though.
 
That's strange because the result is that I can't brake as hard as I'd like to, even if I strongly push the pedal. If it is an emergency feature, it has the opposite effect as it should : braking distance is longer.
And that does not explain why there is no boost anymore after pushing the throttle pedal and pumping brakes at the same time (engine on, car stopped, gearbox in neutral).
 
I made an INPA diag, no codes :( .
Vacuum leak, maybe but why does it happen only immediately after (or during) an acceleration ?

I'm wondering if it's normal or not. Can anyone do the test I described previously please ?
 
What do you want testing? Do you mean at a standstill when pumping the brakes the pedal doesn't stiffen up?
 
Sorry for my english (I'm french) :oops: .

Car stopped but engine on, push the throttle pedal a bit with your right foot, then with your left foot, push the brake pedal 2 or 3 times. Is it clear ?
On my E36 Compact 323ti, the brake pedal firmness is still the same. But on the Z4, it becomes very firm, the same symptom happening when I very quickly brake hard after an acceleration.

Thanks :wink: !
 
Looks like your brake booster is loosing it's vacuum storage.
Normally there should be a one way valve in or before the brakebooster, so that when the engine doesn't make any vacuum (full throttle, or braking with engine off etc), there is still vacuum left to assist the brakes.

Maybe this valve is situated in the suction jet pump (a small device that makes vacuum by venturi effect, situated in the line between brake booster and throttlebodies) or the last elbow that connects to the brake booster.
So check this part too, besides that air shutoff valve.
 
Give me an hour or so and I'll check it, not sure why the throttle's necessary?! (sorry no expert in the braking system!). Isn't it normal for any car when at a standstill and pressing the brake pedal for the pedal to get firmer to a point where you can barely move it at all?
I do know you can test the suction jet pump and the one way valve in DIS for operation, so I suspect if they weren't working correctly they would throw a fault code...?
 
As said above ..sounds like a leak in the brake servo or check valve , the throttle position you are setting is just creating less vacuum, so the servo vacuum air is not replenished as quick !
 
AndyBeech said:
Give me an hour or so and I'll check it, not sure why the throttle's necessary?! (sorry no expert in the braking system!). Isn't it normal for any car when at a standstill and pressing the brake pedal for the pedal to get firmer to a point where you can barely move it at all?
Click to expand...
Your brake pedal should be 100% firm in the 2nd press I think.
1st press is clamping the brake and compressing the pads. 2nd press: there's nothing to compress anymore (so no movement after the slack in the pedal/piston construction.
I do know you can test the suction jet pump and the one way valve in DIS for operation, so I suspect if they weren't working correctly they would throw a fault code...?
Click to expand...
Are you sure? the jet suction pump is a passive part. It has no electrics or so. I also think there's no vacuum sensor.
Only the air shutoff valve is electric and shuts off airflow to the suction pump. I think the air shutoff valve is used (opened) to create vacuum on full throttle, and closes when throttle lift off; then there is vacuum behind the throttlebody that sucks directly air from the brakebooster (so through the jet suction pump etc et).
I think that is the way it operates. But maybe I'm missing something :)
 
Yup my bad, the suction pump is a passive part, I was thinking of just the shutoff valve. I'll boot up DIS in a min and have a look at what options there are to test....

Also, going back to original post, it is strange that the shut off valve part changed in 2007...wonder why....? Not like it's a complicated part?!
 
Ok so with no throttle at standstill pressing the brake pedal repeatedly results in the same firmness however many times pressed, it's quite soft as well not that firm.

Raising the throttle slightly whilst pressing the brake pedal results in the brake pedal being a bit firmer to press after 1st or 2nd press but not overly firm, still a fair bit of movement...

Is that correct?
 
Thanks for the report ;) .

Mine is getting very firm :( .
What is your Z production date ?
 
2006.

Still not sure I'm seeing what the problem is really, as above its a built in feature from what I've read. I tested it earlier myself and after a bit of hard acceleration and then onto the brakes the pedal has a lot less travel than normal but braking power is just as good....
 
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