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  #1  
Old 03-11-2016, 05:14 AM
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Engine Will Not Shut Down with Key- 300SD

I took my car to a Indy shop to replace brakes on all 4 wheels and master cylinder since it had been acting up.

Prior to taking to shop it always shut down with key.

After shop replaced master cylinder it would no longer shut down at shop on key.

The fellow unscrewed the fitting at vacuum booster and put gauge there and checked vacuum source at this location and it was good and he said at 20 inches + and with this hose plugged and not connected to vacuum assist unit, big black thing, the engine shut down with key so vacuum lost at vacuum assist unit.

He and I looked for any jarred loose hoses and found none. We looked 6 times easy.

He and supervisor mentioned that this can happen but is rare.

The supervisor asked me if the one on there had been changed since it had an ATE tag on it, I said no. I see ATE is OEM so what is on there looks original and could be on there since 1982. He asked in case it had warranty is all but it did look OEM to me for sure.

So they ordered and new or rebuilt one for $187. not sure of brand but I think NAPA sourced, didn't ask.

Anyway I asked if the gasket at base of master cylinder could cause leak they said no because vacuum is sealed before getting to this gasket.

I'd like to know what you all thought about this and had any input.

Car is in shop till next Tuesday when part gets in then it will be installed.

I have had no complaints with work done there before just asking here for some help that the mechanic and I over looked.

Steve
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  #2  
Old 03-11-2016, 07:44 AM
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most common issue is the brown line got disconnected when the parts were moved around.

post a picture of your vacuum connections and the IP, and I bet we spot the mistake.
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Old 03-11-2016, 08:18 AM
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Thank you

So you think in the area of the injection pump a brown hose got disconnect VStech?

I can have them take a pic sure, good idea and email it to me and I can upload it, however I have not done that yet.

Steve

Be sweet if it was just a hose.
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Old 03-11-2016, 08:34 AM
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Oh forgot most important thing about this:

With new master cylinder installed and all hooked hydraulically up pretty much no power brake assist at all! But when I drove it brakes worn but ok.

Could hardly handle braking in parking lot. So left at shop.

So there is more wrong than engine not shutting off.

I have no power brakes now and u can't drive care without assist I learned.

Doesn't this point more to the power assist unit having a leak. Maybe a seal was ready do go bad and went bad when new master cylinder was put on?

O'ring? brittle, etc?

Unless I'm over looking something I sort of think there is major vacuum leak inside vacuum module as mechanic says and I agree too.

I have FSM and I'm going to look for Brown hose. Thanks.

Steve
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Old 03-11-2016, 09:33 AM
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If the car shuts down fine with the booster disconnected and the hose plugged, but doesn't when the booster is connected - that's a pretty good clue that your booster has a vacuum leak - though there may be a valve in the vacuum line that could be bad, i'm not sure.

I seem to remember there being a rubber o-ring (or possibly a special seal) that went between the master cylinder and brake booster on my 300D. You may be able to back out the two nuts that hold the MC to the booster to see if that seal is there.

When I replaced my master cylinder, I found quite a bit of nasty old brake fluid inside the booster. One would hope that your mechanic would have pointed that out, but it's something to check (A good clue would be stripped paint under the cylinder on the booster)

It's not a very bad job to change out the booster, though I think I wound up disconnecting the cylinder to do so on my 123 chassis. (Something like four nuts that need to be accessed from under the dash, plus disconnecting the brake pedal linkage - and possibly replacing a bushing)
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Old 03-11-2016, 10:02 AM
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Thanks,

I did not see any other vacuum connections on the booster other than the big one that is threaded on that he took reading on and plugged which make shut off work.

I'd say 4 mechanics and myself looked carefully and could not find anything loose or disconnected.

Sort of points to Booster crapped out when Master Cylinder was put on.

Just due to go. Better for it failing in shop.

Need to get FSM out and find Brown hose still on dwg.
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Old 03-11-2016, 01:29 PM
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I recently replaced the brake master on my w123 with a reman master and had same issue afterwards.

Turned out they hadn't supplied the o ring that goes between master and booster. I didn't notice because I'd given my old master (and o ring) as a core.

Try blowing smoke around where the master meets the booster with engine running and see if it sucks it in.
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Old 03-11-2016, 03:28 PM
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I would definitely check for an o-ring/gasket before you worry about replacing the whole booster. imryanmoriarty's suggestion for a test procedure makes a lot of sense to me.
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Old 03-11-2016, 03:39 PM
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Great,

I had asked them if it mattered there.

They both say no.

oops.

I did see the O-ring on a dwg.

I bet that is what it is.

Let you know if I guy remembers he should.

Steve
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  #10  
Old 03-11-2016, 03:46 PM
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Mechanic says mine had gasket and they used spray to check for leaks, no leaks. I mentioned I thought O-ring he said mine had gasket. Anyway. He is confident it is bad but appreciated input. Wish I was there but wasn't.
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  #11  
Old 03-11-2016, 04:07 PM
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Part number is 004-997-32-45 in case you need one (used up to end of MY85).

Edit: I originally thought it had the square cross section o-ring. Turns out it is round, square one is on the newer cars.

Last edited by Rick76; 03-11-2016 at 04:41 PM. Reason: Correction
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  #12  
Old 03-11-2016, 04:21 PM
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Definitely need a good seal from that square O-ring between MC & booster. Not sure how "spray to check for leaks" was done. There are ultrasonic microphones one can move around to try to "hear" a leak. If not from that joint, the booster itself may be leaking (big rubber diaphragm inside). Did they "tee" in a vac gage to measure what vacuum you have with the booster connected?

I don't know what the "vacuum assist unit" is. My 300D's don't have that. Mine do have the "blue saucer" (vac amplifier), but that is for the transmission modulator signal.
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  #13  
Old 03-11-2016, 04:32 PM
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I guess if it sucks it goes in.

Better if it "blew".

My car is in stand by there now awaiting for new power assist unit.

See how it goes next week I guess.

They are 2 mechanics down and I can't drive it home.

Just so they get it fixed with rebuilt one is all I care about now,

Can't do much at all.
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  #14  
Old 03-11-2016, 04:35 PM
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I don't think they did a Tee test but I drove it hooked up.

ZERO power brakes, so major vacuum leak.

Hoping diaphragm. But weird timing or good timing I guess.
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  #15  
Old 03-18-2016, 10:12 AM
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Diaphram "Vac-Booster" came in rebuilt by Cardone via NAPA, $171 my cost. Mechanic installed it but had to pull master cylinder of course to do it. Very pleased.
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