Ok, so I've been having some weird problems recently. Both these problems aren't constant, and seem to have no real pattern of what causes them

#1 My brakes. When I start the car I can always press the brake pedal like normal, but sometimes, after I shift into reverse or drive to pull out and then I go to press the brakes again it becomes impossibly hard to press. It feels worse than the power brakes just failing, but maybe that's it. Then after driving for a while (carefully) they come back, just like normal. Sometimes I'll be at a light waiting for it to turn green and (pressing very hard to just stay put) then I'll feel the brakes "turn back on" and I can press them with normal pressure.
Also, once the brakes start working normally again they never have become difficult to press. Until the next time I use the car.

#2 Runaway diesel. I've had my engine continue to run after I've turned off the car a total of 4 times now. It appears that the engine only continues to run if and only if I'm having the problem with the brakes at the time I turn off the car.

Maybe these problems are related. Maybe not. But does anyone have any ideas as to what the heck could be going on?

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1992 300D OM602

Ben

sounds like vacuum issues. both the power assist brakes and the shutoff are vacuum controlled. if its intermittent like that, could be a dying/on the way out vacuum pump? since the brakes should be on their own circuit and more than one system is affected. maybe also the check valve on the vac pump is broken and intermittently blocking the vacuum.

by the way, you don't have a true runaway. you just have a problem with the shutoff. a true runaway would spin up to max RPMs uncontrollably!

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1981 NA 300D 310k miles

X2
You have a vacuum problem.
Either a bad leak or a bad vac pump.

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Grumpy Old Diesel Owners Club group

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1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
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1968 Ford F750 truck. 6-354 diesel conversion.
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A runaway diesel draws in fuel from an outside source, such as fumes in the crankcase or leaking turbo seals. Diesel engines, unlike gas engines, are controlled by regulating the flow of fuel into the motor. When the motor finds another source of fuel, it will run off of that, launching into a death spiral wherein the motor sucks in more fuel causing it to build up speed, causing it suck in more fuel, causing it to build up speed, etc. until either the foreign source of fuel runs out or the motor dies. The motor generally dies from either oil starvation (if oil was the "foreign" fuel), or the normal damage of running beyond the redline. The motor usually dies in at most 20 seconds. A runaway can be stopped in one of three ways: by blocking off the intake with something such as a 2x4 (NOT your hand, you will get injured), emptying a halon (I believe it is halon, am I wrong here? Is it CO2?) fire extinguisher into the intake, or, in standards, flooring the brake, shifting into high gear, and popping the clutch.

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1982 300SD -- 211k, Texas car, tranny issues ____ 1979 240D 4-speed 234k -- turbo and tuned IP, third world taxi hot rod

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Ah, vacuum pump makes sense. Good to know the difference between actual runaway and just-not-shutting-off problems. I'm glad it's not an actual runaway, that sounds very scary.

BTW, could a poor vacuum pump like this cause all sorts of problems with the tranny? Such as not wanting to shift gears when it should?

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1992 300D OM602

Ben

yes yes. the tranny is partially controlled by vacuum!

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1982 300sd from craigslist $800 greased on one tank with NO CONVERSION in the Hot Texas Sun. (currently dead & awaiting engine damage investigation and/or longblock swap)
new daily: '03 vw 5speed jetta tdi wagon. bagged&chipped

You do NOT want to wait to find out what this problem is!!!

My SD started doing this, and a few days later the engine shattered from a broken timing chain when the vacuum pump fell apart...

DO NOT DRIVE THE CAR until you have found and fixed the problem, and start by removing the pump and visually inspecting it!!!

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My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Not only is the car not developing vacuum fast. Even after running with the marginal shut off situation it sounds like it is not even generating or maintaining adequate vacuum.

Either the vacuum pump is badly worn or you have a large vacuum loss occurring in the system. This would make it slow to develop vacuum and perhaps keep what is developed very substandard. Personally I would read the vacuum at the pump on startup.

If it were there in say a few seconds and at normal readings I would go for leaks in the system. It probably is the pump though but I try to always check things out first. The only absolutely certain thing is that you have a vacuum problem.

So i have a might vac, can i use that to check the pump itself? Our does sound like it's the actual pump. It'd just be nice to be able to check before i tear it out of there and replace it.

What would be the best way to check the pump?

Thanks for the replies

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1992 300D OM602

Ben

Ok, well I did a couple of quick tests. I don't know if it's anything conclusive, but I disconnected the vacuum line from the pump where it splits at the 4 way connector. I put my mighty vac on the line connected to the pump and started the car. The gauge went up to about 20 and stayed there. Then I shut off the car and connected the mighty vac to the 4 way connector. I could pump it and get it up to about 15, but it was always falling and would soon reach 0. I don't know what is normal. It could be that the pump was just working correctly at the time I tested it and is still going bad.

Any ideas on how to test for certain?

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1992 300D OM602

Ben

sounds like your vac pump might be fine but you've got a real serious leak upstream. i dont know anything about the vacuum routing on a 92 though...

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1981 NA 300D 310k miles

Could be a bad brake booster.

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1977 300d 70k--sold 08
1985 300TD 185k+
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1984 300SD 315k--daughter's car
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1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Divide you vacuum system up to locate the leak or leaks. The duty cycle of the vacuum pump is either constant or far greater than required with leaks in the system.

It may be helpful to have a vacuum diagram for your model of car. Usually very cheap to seal up an old system requiring more time than money.

Plus some vacuum devices will either be substandard in operation or not operate at all basically with leaks.. I do not think the intention of the pumps designers was for the vacuum pump to see a constant or almost constant duty cycle either.

Members should check out their actual vacuum systems from time to time. Just because it works is not really enough.

Well I think I just got very lucky with this one. I opened up the hood and the first thing I checked was the big line that went directly to the brake booster, kind of. There was a red vacuum line that came off of it that was a normal size and I put the mighty vac on that and what do you know, it controls the stop lever! So I switched the mighty vac to the other side (the sided that goes into the big brake line) and the rubber connector snapped. It was very brittle. So I cut a new piece and slapped it on there and wa-la. No more brake problems (at least so far). And the car shut off normally.

Now I definitely want to look into this vacuum issue more. I've been having problems with my tranny. It doesn't seem to want to shift when it should. It'll stay in one gear when it should up shift (especially in cold weather). Also when I'm coasting to a stop and then put on the gas it will rev a bit before it gets into gear, causing a bit of a lurch.

Anyway, I started investigating this on the vacuum side of things. Went below the car and found the line that goes into the tranny. It had bubbles of transmission fluid in it! Is this normal? I would think not.
So I took it off, blew air through it to clean it out, plugged it back into the transmission and hooked up the mighty vac on the other side. It sucked up more transmission fluid! Also, it didn't hold the vacuum. Don't know if it should, but it doesn't.

Any thoughts? If you think I should start a new thread about this I can.
Thanks in advance!

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1992 300D OM602

Ben

Well, it appears that my brake problem was not fixed after all. I drove it two or three times without any problems, and then they came back. I guess I just got lucky those two or three times.

Now, I've been trying to go through the vacuum system, but honestly I don't know what I'm doing. Lots of things don't seem to hold air, but I don't know if that's because they're not really suppose to or if they're broken or if I'm doing it wrong. I'd be willing to bet it's the most latter option.

Should every working component in the vacuum system hold air if it's working correctly?

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1992 300D OM602

Ben