Hey all,

About a year ago, out of the blue, my brake booster stopped giving power to the brakes, the engine refused to shut off, and the entire car basically lost vacuum. I had thought it was the pump, so I went out and bought a Mityvac to do some testing. Luckilly, I was wrong: the pump was putting out a solid 24hg of vacuum. I shrugged it off as being the main vacuum line, replaced it, and things seemed to work from there on out.

Fast forward a month or so later, and it was doing it again. Huh, guess it wasn't the main booster line. I had found that "fiddling" with the hoses under the hood tended to fix the problem for a day or so (sometimes less, sometimes more), with the problem becoming a constant headache of, "Oh s**t, manual brakes!". It was about this time that the summer came around, and the problem went away. Well, here I am in the thick of winter again, and it has reared its ugly head once more. I replaced the check valve (yellow) under the hood just for the heck of it, but that didn't seem to fix the issue, either.

Anyway, I'll cut to the chase here. Today, after standing out in the rain, cursing the idea that I'll never have working vacuum power, something became apparent. Even when the "fiddling" doesn't produce any results, idling the car in park seems to allow the car to build up enough vacuum to shut the thing off, and allows the bladder to build up enough to lock my doors. Correct me if I'm wrong, but is this indicative of a faulty vacuum amplifier on top of my injection pump? I'm at my wits end, here.

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]

Fiddling rarely produces good results.

So, are you going to break out the Mityvac and find the GD vacuum leak or are you just going to whine about it on here..............

Heh, the problem is, I can't find any article on what vacuum levels to look for. The booster holds 20hg for hours upon end with little to no loss, so I ruled that out. The interior dashpots (when they get vacuum) seem to work just fine, and even if they didn't, I couldn't see them draining enough vacuum to knock out the whole system. Locking system is tight, as well.

That just leaves "stuff under the hood". Where should I test first, and what should / shouldn't hold vacuum?

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]

The vacuum levels vary day to day with the barometric pressure.

If the subject vehicle is so equipped, the EGR control valves on the valve cover are notorious for leaking. You might disconnect and plug the supply line and see if that makes a difference.

I have the EGR plugged (at the EGR) with a plastic BB, but you make a good point, here. Lucky for me, my company is really big into golf outings, and has a supply of ~1000 golf tees in the basement. I'll try going for it at lunch. Nothing that comes off that black box is important to normal functionality of the car, correct?

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]

Do yourself a favor and follow this set of instructions to the letter:

3 vacuum lines 5 holes, help or picture please

Will do. I'll try and brave the rain at lunch, and yank the unneeded tubes. Hopefully this gremlin is being caused by the EGR, and not by something more difficult / expensive. =)

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]

I wouldn't rush the job in the rain...........it'll just lead to errors and more problems...........the vehicle waited 25 years..........it can wait another few days.

That won't have any affect on leaking control valves.

No.

For troubleshooting purposes, all you need to do is disconnect and plug the vacuum supply line that tees off the main vacuum line to the 3/2 valves. Don't worry about the black vent line.

How have you determined that your door lock system is leak free? Will your door locks cycle 4-6 times after the car has been shut down overnight?

I didn't have the time (nor the desire to get soaked) to actually remove the lines, but I did "disable" it by removing the plastic lines, and jamming golf tees into the 3-way splitters, where the lines were pulled from.

So far, so good.. but that's the story of this problem. It'll fool me into thinking I nailed it, only to figure out that I didn't do a single thing. My mechanic seems to think that the vacuum pump is the culprit, although from what I've seen, that sucker puts out 24" without a hitch. I suppose it *could* be having problems, but there's not a drop of oil in my vacuum lines. Can an intermittent leak elsewhere in the system cause me to lose all vacuum power?

Disconnecting the EGR at least made me realize my air cleaner had become unseated (a project for the spring), and the breather tube had spewed oil all over my valve cover (it's never sprayed in the past). If it's not one thing, it's another.

Hopefully it was just the EGR that was causing this headache, and I won't have to deal with it ever again. Not keeping my fingers crossed, though.

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]

If 'fiddling' helped, you may have some cracks/slow leaks in the rubber vacuum hoses. You may want to consider checking those too.

What has me the most concerned, is the fact that, well, it doesn't make much sense to me. Would a slow / small leak (such as those caused by a cracked rubber hose / leaky check valve / etc) really cause such a massive vacuum loss, that it would knock out the whole system? It just seems kind of strange to me.

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_______________________________
(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]

This may be of interest:

http://www.peachparts.com/Wikka/TransVacTune

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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A likely culprit with intermittent vacuum system "failures" is the check valve that is inside the nipple where the metal line attaches to the vacuum pump. The retainer has a habit of breaking after 25 years or so. This allows the check valve components to rattle around and intermittently block the air flow into the vacuum pump. The nipple can be easily removed and inspected with the pump still on the engine. If it is broken, it will be necessary to fish the components out of the pump inlet.