Poor Idle With Foot On Brake..how To Diagnose?

[jrhone]

Before I dive into replacing the brake booster how do I diagnose that it's not something else, like the check valve? Symptoms are the car has a perfect idle when foot is off the brake. Step on the brake and it stumbles and drops rpms. Lift off and it goes right back to perfectly smooth. When driving and braking the car backfires alot. Pedal is stiffer and takes more pressure to stop the car. Ideas?

[MikeWooldridge]

Carb or tii?

[jrhone]

Carb, Weber 38 DGES is the Carb.

[FunkyLaneO]

If you are introducing a vacuum leak every time you step on the brake then all I can think of is the booster, the check valve only maintains vacuum pressure in the booster when you are not generating vacuum so that would affect the brakes but not likely the idle.  You could rule out any weird issue with the hose and check valve by disconnecting it from the booster and plugging it, then see if it runs any differently.  your brakes will be really hard so I wouldn't drive around.

Edited September 10, 2013 by FunkyLaneO

[M3M3M3]

vacuum leak. check hose connections.  other places could be failed internally booster or bad gasket between the MC and the booster.

 

+1 on disconnecting hose from booster.  you MUST fully seal/plug the end.  then do the brake while idling test again.  if problem gone, its booster or MC gasket.

[FunkyLaneO]

^^ oh yeah, forgot MC gasket, try wiggling the MC while it is running and see if it affects idle.  Good call Marshall.

[jrhone]

Ok. Unplugged the vacuum from the brake booster and plugged to. Started it and stepped on the brake and idle never changed. Reattached the vacuum and checked to make sure all was tight. Started it and stepped in the brake and idle stumbled and stalled. I thought maybe the MC was loose maybe so I was just gonna make sure it was tight. I couldn't tighten it at all. It's super tight. No wiggle or anything. Anything else to try?

[Tommy]

Isn't it so that when pedal is pushed it opens a small continuous vacuum leak? Somebody could confirm this.

I'm thinking if idle mixture is just on limits on lean side and then pushing brake leans it more just a bit. Enough to make it run rough. I don't know Weber DGES but if there is idle mixture screw I would try turning it just a bit richer and see what happens.

 

IMO master o-ring would make continuous leak - not just when brake is pushed. I feel that booster diaphragm would do the same. Can't be sure because booster anatomy is not all clear in my mind right now

 

  Tommy

[Tommy]

Thinking more - I guess the vacuum leak should not be continuous when pedal is pushed. But if the internal seal between two sections would be broken it would make it continuous. It could be same if diaphragm is leaking... Booster operation is surprisingly complicated when going into details.

 

  Tommy

[Tommy]

So if there is a mixture screw I would try what it does. But if your braking is hard I guess you should try replace the booster anyway.

 

  Tommy

[M3M3M3]

Ok. Unplugged the vacuum from the brake booster and plugged to. Started it and stepped on the brake and idle never changed. Reattached the vacuum and checked to make sure all was tight. Started it and stepped in the brake and idle stumbled and stalled. I thought maybe the MC was loose maybe so I was just gonna make sure it was tight. I couldn't tighten it at all. It's super tight. No wiggle or anything. Anything else to try?

the only other item would be the fitting on the booster the vacuum line plugs into.  other than that, you are done with trouble shooting...replace the booster and the MC gasket since it will be required to replace the booster.

[Mike Self]

One more thing to check before digging into the booster.  Carefully examine the hose that connects the booster to the manifold.  If a PO replaced it with something other thant thesteel wire reinforced OEM hose, it may well have worn through where it's clipped to the original air cleaner.  The piece that fails first is the short length that connects the check valve to the manifold.  It's always in a bent position, and will eventually crack.  Replace it (anyway) with the same part from an E21 320; it has a bend moulded into the hose and will last much longer.

 

cheers

mike

[LimeySteve]

If the hose was cracked it would be a constant leak , like having the hose removed, it's making the leak when the pedal is depressed, m/c gasket maybe , diaphragm is my bet, replace the booster.Also with enguine off there should be residual vacuum in the booster as you remove the hose at the booster , providing the check valve is working.

[FunkyLaneO]

What you are describing sounds exactly like a bad diaphragm in the booster (or one of the internal valves), the way a booster works is that normally there is a vacuum on both sides of the diaphragm at rest, when you apply the brakes you break the vacuum on one side of the diaphragm  and it pushes the diaphragm to one side (the master cylinder side) and assists with the braking, if your diaphragm is leaky the vacuum will break on both sides and allow air to pass through which will lean out your engine and you will lose some of your assisted braking which are exactly the symptoms you describe.

 

You have done a good job of troubleshooting and unfortunately the diagnosis is the not the one you want but it looks like you need to replace your booster, I feel your pain, those things are a PITA to replace!

.

[jrhone]

Thx all...I initially thought it was the booster and even got a Tii booster to put in since I will be doing DCOEs I thought why not.  I just wanna check ALL bases before replacing it for no good reason.  The hose and check valve were replaced and tested recently.  I also removed the hose at the booster and plugged it.  Then tried the same thing, no problems.  If it were the hose(or check valve) it would be still causing a vacuum leak.  Now to proceed with the booster replacement!