Hello, I have been working on "Ben" 1983 300D Turbodiesel and have been having trouble with vacuum.

I bought the car recently and the problems at that time were the door locks and the center vents. There was a golf tee in the yellow line going into the cabin at the firewall so that the engine shut off would work.

I began working on the vacuum system and made it to the trunk and found a small pin hole in the rubber of the trunk actuator so i replaced it and at first it seemed to help. Then I modified the EGR lines by totally removing them. Then for a month I would shut down the car by pressing the STOP lever in the engine bay, but nothing was functioning properly.



Then gradually the car started acting better and would shut down and the door locks started working.

Now this week we drove to the store in town and the car shut right down and the doors locked fast and good. But when we returned to the car unlocked the doors and the car would not start. Vacuum was keeping it from starting so I locked and unlocked the doors until they would not move and the car started right up normally.

Once back home I tested vacuum at the main lines that read 21 and 20 and then the vacuum stopped working, no stop and no door locks.




Question is; why would the vacuum at first not shut the car down or work the locks and then over the space of 2 months time everything worked great until one day it works too well and will not let the car start and then after checking vacuum for two minuets not work at all again?

When the vacuum systems aren't working, what's your vacuum at the takeoff points on the booster check valve? If no vacuum there, you know you either have a serious leak in the brake booster, or the vacuum pump isn't working.

Buy a hand vacuum pump and start pulling vacuum on various circuits in the car until you find the one(s) that don't hold vacuum.

Your vacuum system to the injection pump stop should release vacuum when the key is turned. If it doesn't, you have a problem in the vacuum portion of the ignition switch, or the lines installed backwards at the ignition switch.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

The car was shutting down and locks worked fine for several days and I thought that all of the problems worked themselves out. Then the no start thing started to happen.
No changes have been made other than to check with the gauge once.

Problems rarely, if ever, "fix themselves". I'd be willing to bet either a sticking check valve in the vacuum pump or a tear in the brake booster diaphragm causing the intermittent total loss of vacuum.

As said above, the shutdown vacuum pod not releasing will be in the key switch mechanism. Once the key is "started", it should release vacuum to the shutdown vacuum pod. This is a separate issue from the rest of the system.

To isolate your problem areas, pick up a hand vacuum pump and start pumping up various circuits. You'll quickly find the problem areas which you can then isolate or repair. I have one of the $12 Mity-Vac "clones" from Amazon that works great, so it doesn't need to cost a bunch of money, but can certainly save your hair and sanity!

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

It is highly unlikely that your ignition switch is the problem; they just almost never go bad - at least that part of it. And unless you have fooled with it, there could be no way that the lines are switched. You can work from the dash, prize off the old rubber connectors and replace, but it would probably be a waste of time. I did this on my '81 300SD, against the advice of the forum members, and found that my original 36 year old rubber connectors were still fine - replaced them anyway.

It sounds like one of your rubber connectors under the hood is just not secure, that part of the time it is OK and then it begins to leak some. Or maybe a golf tee in one of the lines. Maybe one is touching something which pushes it back and forth while driving.

Just a guess. If your vac pump is putting out 20-22 ihg, it is working ok. Have never heard of one working intermittently.

I have a vacuum pump and am familiar with the test method and have the service manuals for the car.

The car has never been put into regular service in life, it stayed in a garage and is totally pristine. All rubber goes bad so I have been replacing everything that is not prefect. Last week I pulled the vacuum pump because the gasket was leaking and it looked normal inside and I have not seen or heard any sign of a problem with the pump.

It was very weird to experience the car seemingly fixing itself and my wife was jokingly referring to it as a miracle.

Could the modification made by eliminating the lines to the EGR have any consequences in this regard or in any other function on the car?

As I said the car was rigged to shut off properly (with a tee) when I bought it and I tried connecting everything at first and just lived with the shut off problem, then I removed two offensive lines and after a few weeks the car gradually got better and better, until it was working perfectly for the last week or two and then most recently it is over working.

If you have plugged up (probably with golf tees) the right lines from the EGR, there should be no effect on any other element of the vehicle. If one of those is sporadically leaking, that could have an effect.

A failed check valve at the pump inlet can cause intermittent vacuum loss. The retainer plate rusts through and lets the plunger rattle around in the inlet, where is sometimes causes a blockage and sometimes doesn't.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

The technical term is "erratic problem". Yes it is common, and yes it is hard to trouble-shoot. In my experience, the trunk vacuum lock is the main culprit. I have tried repairing them (pry open, fill flowable silicone). The diaphragm wasn't torn, but appeared to leak at the metal rod or perhaps the periphery where the 2 shells clamp it. Worked for a while, then I replaced w/ a junkyard one which tested OK. Forgot which of my 2 cars, but last time I had a trunk lock issue, I just capped off the 2 tubes to it and haven't looked back. I found my quality of life has not been appreciably degraded from losing the auto-lock trunk option.

But, I have also found shut-down valves which sometimes hold and sometimes leak vacuum. Also, the pump can be erratic from the piston seal being marginal or the check valves sticking. Not much different from many bicycle pumps as they age.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Thank you. I will steady myself and dive into the vacuum swamp again soon and report back.
Today I took old Ben out for a test drive, drove great with new heavy duty Bilstiens, new spring pads, brakes, rebuilt Becker and valve adjustment...
Would not shut down, waited for a couple of minutes and still no shut down just like a month ago before the problem corrected itself.

I should say that the trunk lock that I replaced was from a one year newer 300D in my personal junk yard.

I had an erratic vacuum leak that went away or not depending on what vent setting I used on my climate control. Some bad vacuum pods. The defrost vent pod is fine, so I thought the problem went away for most of a winter - - until it warmed up enough to want to use the center vents.

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1985 300TD, 373,000 mile daily driver