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  #1  
Old 01-09-2010, 03:07 PM
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Engine shuts down when brake depressed

I have an interesting one, at least to me... When I shut off the key the only engine shuts off immediately sometimes. However at times it shuts of slow and then not at all. I have found that if I depress the brake pedal with the key off, it will shut down right away. I am not sure if it has been doing this since (I bought the car about 4 months ago) as I generally have my foot on the brake when I shut it down. Where could be some possible things to check for? Prefer easier and cheaper the better, lol. I am assuming that there is not sufficient vacuum to shut down the engine and the surge from the booster when the pedal is applied forces the shut off. Any ideas or others experience this issue? Thanks.

1983 300 SD
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  #2  
Old 01-09-2010, 03:56 PM
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Posts: 18,353
I'd suspect that activating the brake booster reduces the volume of vacuum behind the diaphragm, allowing the system to compensate for a slight leak in the brake booster and permitting the shut off valve to engage. You can test it by pulling a vacuum on the brake booster with a MityVac and seeing if it holds. It will take a lot of pumps on the MityVac to produce a vacuum in the booster.
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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Old 01-19-2010, 02:51 PM
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I just noticed a few moments ago when pulling into the driveway...I shut off the key after applying the parking brake and putting shifter into park, released my foot off the brake then shut off the key and the engine didn't shut off like it sometimes doesn't. I then went to turn off the climate control as I try to remember to do so I have full power to the gp's next time I go to restart (So the blower fan doesn't some on). And guess what?? The engine shut down, without my foot on the brake pedal. I tried this again by restarting the engine with the climate control on econ, turning off the key, motor still was running, then hit the defrost button, it shut down again! My center vents do not work as many others do not, but was wondering if this is a related issue? Also now that on topic, the blower fan sometimes cuts out and I have to press random buttons on the CCU to get it to come back on again. it is intermittant. It doens't really bother me as it will still come on after a few buttons pressed, just a minor annoyence really, but now am wondering if they are all tied together. Per the previous owner's records the CCU was replaced back in 1991. I have not tried the mighty vac yet as Kerry suggested, as I need the funds to purchase one, unless somebody in Central California has one for use????? I have many automotive skills for trade.
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Old 01-19-2010, 03:09 PM
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If you think of vacuum as a sort of refillable commodity -- like a fuel tank with a constant trickle running into the top and a trickle running out the bottom whenever it's consumed -- there are several things on the car that "Consume" vacuum. Normally, it's just transferred from place to place, not necessarily wasted.

1. Climate control uses vacuum to open and close vent doors with each mode change.
2. Brakes use the single most massive quantity of vacuum to help make the brake pedal moody and sad. (Easily depressed)
3. The shut-off system uses vacuum to mechanically depress the lever that stops fuel flow, thus shutting the car off.

The car's vacuum pump constantly produces vacuum, which is blown off as waste if all the systems are at zero consumption, or is used to put vacuum force on any given system when it calls for it. (I'm 90% sure about what I'm saying. My point remains if my tech details are lacking.) If any of those systems is leaking, it causes the vacuum supply to be consumed at a greater rate than the car can instantaneously replenish. If the leak is major enough (brake booster total failure) then the car may not be able to produce sufficient vacuum to operate all systems. If it's just a partial slow leak, then vacuum operated things will work -- but they'll work more slowly because for a given percentage of vacuum force, some of it is used where it should be but some is lost through a leak.

The fact that changing climate control settings affects engine shutoff ***STRONGLY*** suggests to me that one or more of your vacuum "pods" in the climate control system -- little vacuum-driven arms that open and close vent doors -- has completely failed, causing a large leak. Thus, when you have the buttons pressed that activate that particular pod, the vacuum supply all "leaks out" constantly through that leak faster than it can be replenished. As a result, the shut-off mechanism lacks sufficient vacuum force to activate, and so the car doesn't shut off.

When you change the button selection, to a mode that does not attempt to move the failed pod, the leak is "cut off" from the system -- and so the car replenishes its vacuum supply and the shut off valve has enough force to activate.

The brakes are the single greatest consumer. In my cars, which don't have any vacuum leaks at present, everything works instantly. However, to have fun -- I can pull into the yard. Put it in park. Pump the brakes repeatedly. Doing so consumes ALL the vacuum that the pump has produced, because the brakes can consume in one push what the vacuum pump requires 5 seconds to produce. Once the system is drained that way, if I turn the switch off -- the car keeps right on idling as if the key was on. I can keep it running without the key indefinitely by continuing to pump the pedal. When I stop pumping, the car shuts off about 3-4 seconds later as the supply replenishes and the valve actuates.

The point of that lengthy and wordy illustration was to demonstrate that a large enough "leak" -- in my case, a deliberate brake pedal leak -- can cause the system to lack sufficient vacuum to shut off the engine. The change of behavior with climate control strongly suggests a leak in the climate control system somewhere of large proportions.
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