what causes the rpm to go up in idle when pushing the brakes

[pdc]

When engine runs in idle the RPMs go up when pressing the brakes, I assume is not normal and wonder what could cause this?
Couldn't find anything on this using search, hopefully someone has some idea.

thanks

[ctaylor738]

Probably a leaky brake booster allowing extra air into the intake manifold when the brakes are applied.

[kampala]

Agree ... had the same issue ... could be brake booster.  But do check the gasket between the master cylinder and booster as this can also cause the same symptoms. 

[Lou F]

I have noticed the RPM go up when I pump the brakes.  But only when the car is cold.  I no not noticed it when it is warmed-up.  Is this normal or do I also have a problem?

[pdc]

Chuck and Oz, thanks, will check this out and also the hoses for a leak. Hopefully it isn't the booster that needs to be replaced.

@ Lou, I  don't think you should have the rpm go up at all when pushing the brakes, warm or cold.

[ejboyd5]

When you depress the brake pedal, some of the engine vacuum is bled off to power the brake booster.  The engine senses this decrease in vacuum the same as it would a decrease in vacuum caused by stepping on the accelerator pedal and increases the flow of fuel through the injection system in response.  With more fuel being supplied, the engine speeds up.

[garymand]

.02$ more  
You can think of it as air sitting in the booster.  Pressing the brakes allows that air to 'leave' the booster and travel to the intake manifold.  If there is no leak in the booster, not much air is sucked out.  If you have a leak, depending on the size of the leak, air continues to enter the manifold as long as you apply the brakes.  

There is a one way valve in the hose between the booster and the manifold that maintains the vacuum.  It stops the air from going back into the booster, but only from the manaifold, when the pedal is released.  If the vacuum in the booster is held, next time the pedal is pressed, and the oneway valve is working, not much additional air is sucked out unless there is a leak in the booster to air.

That was fun.  I couldn't answer that question 20 years ago.

[glenn]

If the brake(breakened) booster leaks air into the manifold:  (at idle)When additional air enters the manifold, the engine can either speed up or slow down.  If it speeds up, it is running rich.  If it slows, it is lean.  Yes/No??  Most engines are running rich at idle -  due to multiple adjustments to the FIP knob, idle air screw, WRD, barometric compensator, linkage, CSS, CSV, relays, FIP stops, modulator, etc. by control experts(sic).    What's a poor MB fuel management system to do??

[jpinet]

.02$ more 
You can think of it as air sitting in the booster.  Pressing the brakes allows that air to 'leave' the booster and travel to the intake manifold.  If there is no leak in the booster, not much air is sucked out.  If you have a leak, depending on the size of the leak, air continues to enter the manifold as long as you apply the brakes. 

There is a one way valve in the hose between the booster and the manifold that maintains the vacuum.  It stops the air from going back into the booster when the pedal is released.  If the vacuum is held, next time the pedal is pressed, not much additional air is sucked out unless there is a leak in the booster to air.

That was fun.  I couldn't answer that question 20 years ago.

Are you saying it's normal for rpm to go up when depressing brake pedal? I'm at wits end here trying to figure out what leaks. I have a remanufactured booster from a very reputable company. The tests on my booster show it's good. But idle goes up appx. 800 rpm when I depress the pedal. I have checked all my connections and the one way valve and I see no vacuum leak anywhere. I have even replaced the vacuum hose with a new one. If my booster is good and there is no vacuum leak, why would my idle go up as long as I hold the brake pedal depressed?

[George Des]

Disconnect your brake booster and plug the vacuum hose. Step on the brake and hold. If there is no change in rpm, it is your brake booster. I too had the same problem and assumed just because my booster was rebuilt that it wasn't the booster. I tried another booster and the problem was solved. The rebuilt booster went back to the rebuilder since it was still under warranty and they fixed it. You probably used the same rebuilder I did.  I wouldn't assume just because they did a rebuild on it that that isn't the source of your problem. Hope this helps.

Georg

[garymand]

I'm with Georges.  No, the RPMs should not go up when pressing the brake pedal.  We were just explaining why a leak in the Booster results in higher RPMs.  Or as Glenn says depending on where the FIP and air knob is set.

[jpinet]

Disconnect your brake booster and plug the vacuum hose. Step on the brake and hold. If there is no change in rpm, it is your brake booster. I too had the same problem and assumed just because my booster was rebuilt that it wasn't the booster. I tried another booster and the problem was solved. The rebuilt booster went back to the rebuilder since it was still under warranty and they fixed it. You probably used the same rebuilder I did.  I wouldn't assume just because they did a rebuild on it that that isn't the source of your problem. Hope this helps.

Georg

George and Gary,

Thanks for clearing that up.
I had my booster rebuilt by John Stuart in Ontario.
Brent assures me they check every booster before they go out.
Every symptom pointed to my rebuilt booster, but I wanted to eliminate all possibilities.

[glenn]

Plug the line to the manifold------rpm should return to previous??     Step on the brake and suck on the line to the booster???

[jackhughes81]

Georg,
where would I get my booster rebuilt? I am located on very tip of NW Washington..  thanks- jack

[George Des]

I used Power Brake Exchange in California. They have a website and many club members have used them but there are others as well.

[pagoden]

At the recent mid-atlantic area spring gathering of Pagodas, I was told by a German-trained Meister of many decades experience and considerable repute that pressing the brake pedal on a 113 in proper tune would result in a temporary increase of ~500 rpm in idle speed.  [I believe a visitor here, "ejboyd5" supplied the correct analysis of this phenomenon in his/her post of May 22nd.]  Greater &/or longer changes definitely do get you over into vacuum-leak-somewhere-in-the-brake-booster-system territory as covered above as well -- and as applies to any other vehicle with a vacuum-boosted brake system -- but that little fillip is just one more quirk of the fascinating IP on ours.  And no, I had no inkling of that little factoid until recently, and wasn't even close to sussing it out for myself; does make sense, though.       And then I start to wonder if it shows up in any other injection systems.       etc.               Wishing you a healthy booster diaphragm,   Denny

[GGR]

When you depress the brake pedal, some of the engine vacuum is bled off to power the brake booster.  The engine senses this decrease in vacuum the same as it would a decrease in vacuum caused by stepping on the accelerator pedal and increases the flow of fuel through the injection system in response.  With more fuel being supplied, the engine speeds up.

That would be true on an EFI system as indeed there is a vacuum sensor. But is that the case on these MFI systems?

[stickandrudderman]

No, it is not the case on mechanical injection.
That would require a MAP sensor (Manifold Absolute Pressure).

A brake booster converts pressure to mechanical work and if the booster is in good order then it should make no change to the fuel mixture.

If your idle speed increases when the brakes are applied then the booster is allowing some of the manifold pressure (vacuum) to escape.

A failed booster hose would allow pressure to escape regardless of brake pedal activation.

[garymand]

Yeah, I agree with Stick.  With no doubts at all and lots of experience to say otherwise.  This is a fun topic!  None of my 250SE, 250SL, 3x 280SE's, or 250SL w/280SL motor change rpms with the brake application.  IF any of them did, I would emmediately pull the hose from the booster and see if I can suck air through the one-way valve.    of course, with the motor off, so maybe not so immediately.  If the valve is good then air can't get into the booster with the motor off (with no manifold vacuum, the manifold goes up to air pressure and air can leak past the valve into the booster, allowing the booster to also loose its vacuum.)  Next, plug the hose at the booster end and start the motor.  Now press the brake and you will get no change in rpm.  If you do  we can start a thread on that one.  That is how the motor should react if the booster is a closed system.

We all make mistakes/slip of the tongues/mis interpretations of what a mechanic is really saying.  (I think that covers the gamut)  (hmm must be a German word)  Sometimes the most intelligent of us misinterpret the most, because of the communication level differential.

Anyway, You can only vacuum so much air out of the booster.  Actually the motor vacuums the booster down to what ever the lowest vacuum in the maniifold and the valve holds it at that low pressure until youl you press the pedal:  Now if the manifold is at a lower vacuum that the booster, the rpms might go up while the pressure equalizes, but I wouldn't expect thaty to be noticeable unless you put a vacuum gauge on the manifold and tried to do it on purpose. 

Think of the booster as having an inflated balloon ball in a can:  You can only suck so much air out.   You just can't suck anymore air out of it unless it has a leak in the ballon or in the can.  The booster has a few places for air to get in if it fails.  If no outside air gets in, you can press the break pedal all you want and the rpms will not change, unless some air is leaking in.  I could go on, but I have no idea why a good mechanic would tell you it is normal for a booster to leak.