Idle change when applying brake

[jeffc280sl]

What would cause the idle to increase when pumping the brakes in neutral?  Vacuum change effecting distributor retard?  Is this normal?  If not, any ideas how to remedy.  Many thanks.

Jeff C.
1970 280SL 4-speed

[George Davis]

Jeff,

I think it's normal, or at least I hope it is 'cause mine does it too.  I found this on a web site:

"Vacuum enters the booster through a check valve on the booster. The check valve is connected to the engine with a rubber hose and acts as a one-way valve that allows vacuum to enter the booster but does not let it escape. The booster is an empty shell that is divided into two chambers by a rubber diaphragm. There is a valve in the diaphragm that remains open while your foot is off the brake pedal so that vacuum is allowed to fill both chambers.  When you step on the brake pedal, the valve in the diaphragm closes, separating the two chambers and another valve opens to allow air in the chamber on the brake pedal side.  This is what provides the power assist."

Not the greatest description, but basically I think when the pedal is pushed and air is allowed into the one chamber, a bit of that air goes up the booster hose to the manifold, thus supplying a bit more air and speeding up the idle... although it could be the reduction in manifold vacuum allowing a wee bit of vacuum advance (on the vac retard types), too.  Interesting hypothesis!  Anyway, the valve that lets air in must either close quickly or not open very much, because the idle only increases for a second after I push the pedal.

It would be interesting to see if the early 230 SLs with vacuum advance distributors do this.  James?  Other early car owners?

George Davis
'69 280 SL Euro manual

[J. Huber]

Guess I'll have to go check!

Off the cuff, I don't recall any major surge in RPM when breaking - although I have an automatic so I don't visit "O" too often.

James
63 230SL

[rwmastel]

You have a bad brake booster, a bad check valve, or a loose fitting on the brake vacuum hose.  You should not have a raise in RPM when applying the brake out of gear, or in "0" in the case of James and I.  The raise in RPM is from air getting sucked into the intake manifold (by the vacuum) through the brake system, there by increasing the air quantity in the air/fuel mixture.

Check all fittings - maybe use new clamps.  Take the check vavle out and see if air only passes one way.  If the hose looks cracked or brittle, replace it.

My car still had the original 1966 brake booster in it when I bought it in 2000.  Replaced it in 2002.  Car brakes much better now.  Does anyone else have a brake booster with the white date stamped on top?  Mine was 9-8-66.  I don't know what the words in the box above the date are.

Download Attachment: Copy of Factory Options Plate.jpg
54.45 KB

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[jeffc280sl]

Thanks for the help,

The surge is maybe 400 rpms.  I'm thinking along the lines of George.  I have a relatively new booster and the hoses are in good condition.  Will examine the check valve though.



Jeff C.
1970 280SL 4-speed

[Klaus]

Rodd says: "Mine was 9-8-66. I don't know what the words in the box above the date are."
Funktion geprueft - function tested.

Klaus
1969 280 SL

[JamesL]

I sort of have the opposite problem in my auto

Runs like a dream on the go, yet when the engine is warm and I am sitting in traffic (in gear), it idles very rough and sounds like the occasional misfire

Stick in in neutral it's sweet as a nut.

[Benz Dr.]

Vacuum is on the manifold side at all times and no outside air enters this line. The push rod on the brake pedal side has a valve in there that opens and allows a small amount of air to enter which then lets the diaphram move forward. I think almost all vacuum type brake boosters work on a principal similar to this. The parts and design may differ greatly but similar to fuel injection the basiscs stay the same.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org

[glennard]

quote:Originally posted by Lax882@aol.com

What would cause the idle to increase when pumping the brakes in neutral?  Vacuum change effecting distributor retard?  Is this normal?  If not, any ideas how to remedy.  Many thanks.

Jeff C.
1970 280SL 4-speed

Sounds like a leak in the brake system.  Also, if a change in the fuel/air ratio going to the intake causes an increase in rpms, you have a mixture problem.  The fuel charge has not changed. The air charge has.  If air increase caused higher rpms, it was too rich.  If air decrease caused higher rpms, it was too lean. Try adjusting the idle air with no brake interaction. If the rpms had dropped, I would not suspect improper fuel/air mix as much.  Remember the old saw-Nothing happens in a vacuum.

[glennard]

quote:Originally posted by Benz Dr.

Vacuum is on the manifold side at all times and no outside air enters this line. The push rod on the brake pedal side has a valve in there that opens and allows a small amount of air to enter which then lets the diaphram move forward. I think almost all vacuum type brake boosters work on a principal similar to this. The parts and design may differ greatly but similar to fuel injection the basiscs stay the same.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org

Brake rod moves to put a vacuum, replacing atmospheric pressure, on the one side of the diaphram thereby aiding your foot and atmospheric pressure on the other side in putting pressure on the brake fluid, on the pistons, etc, etc.

[glennard]

quote:Originally posted by Tosh

I sort of have the opposite problem in my auto

Runs like a dream on the go, yet when the engine is warm and I am sitting in traffic (in gear), it idles very rough and sounds like the occasional misfire

Stick in in neutral it's sweet as a nut.

Automatic or manual?

[halman2228]

I am a little confused here...

I knew I had a bad mixture (waay strong exhaust smell) and had it adjusted by a knowledgable shop with a "4 gas analyzer". Prior to the adjustment, it did not exhibit an idle surge when applying the brake in neutral... it now does (perhaps I didn't notice it before?)by a minimal but noticeable amount - maybe 75-100 rpm, if that much.

I'll start by looking at all the vac lines involved then the check valve - I am heading in the right direction?

Regards,
Kevin