Vacuum problem 85 300TD won't shut off - PeachParts Mercedes-Benz Forum

Richard Eldridge · #1 01-13-2009, 06:07 PM

Prior to doing a purge on my 85 3oTD, the car shut off every time and the door locks would hold pressure for about fifteen minutes. The leak is in the passenger front door, and I was planning to fix it.

I ran a can and a half of Lubra Moly and changed the air filter. When I finished, the engine would not shut off, because I had inadvertently broken the two nozzles off the metal tube that runs from the Vacuum Pump to the power brakes. I replaced this, and reconnected the tubes and still it would not shut off. I have determined that I have a vacuum in the line I replaced, a steady 20 lbs. I can also shut off the engine by sucking with the mity Vac on the shutoff valve line. What I can't do is to shut off the engine in any way other than sucking on that line wioth the MityVac or pushing the off lever under the hood. The door locks do not work, either.

I tried pushing a golf tee in the yellowish device that connects two yellow tubes to one that I think controls the door locks, to remove these from the problem, as someone suggested in another post. I have also tried connecting a line from the vacuum pump line directly to the shutoff valve, None of this works. The diagram given for a 1986 California model does not resemble my car. Above the shutoff valve, which is metal and invisible from topside, is a white plastic thing with two tubes coming out of it, one goes to a four-outlet connector that connects to the vacuum pump line.

The other line off the vacuum pump goes to a triple connector. Both of the lines off the vacuum pump have a steady 20 lbs. vacuum with the motor running, and these were the only ones I found disconnected.

The shutoff switch connects to a brown and blue line that goes into the firewall. There is a lot of play in this line, but it is connected to something inside.

I have had similar problems before, and when I did, I replaced all the lines that appeared deteriorated or cracked, one by one, once I got it running.

I have spent about three hours on this so far. I normally get everything up and running within a half hour, but this time I am stumped It seems pretty certain that the problem is a tube that is disconnected that I cannot see, or a tube that is connected to the wrong place. I appreciate any help anyone might offer.

I have not removed or swapped more than one pair of lines at a time.

toomany MBZ · #2 01-13-2009, 08:31 PM

Make sure you seal off the vac line before the yellow check valve.
The '86 is a different engine, so nothing from that will help.
Pics may help.

Richard Eldridge · #3 01-14-2009, 03:31 PM

In my car, there are two yellow tubes connected to the rear (ie the side closest to the firewall) of the yellow check valve.

When the engine is running, the vacuum is sufficient to pull the locks up and down.

rrgrassi · #4 01-14-2009, 03:35 PM

Have you tried hooking up a miti vac directly to the shut off valve to test it?

ImBroke · #5 01-14-2009, 05:51 PM

Sounds to me like you pulled one of the brown lines off of the "ignition" switch. Since you can apply vacuum to the brown line going to the back of the IP and get the car to shut off, the shutoff valve is working. Pic of how you're tryiing to isolate things would be helpful.

Richard Eldridge · #6 01-16-2009, 05:22 PM

When the motor is running, there is 20 lbs of vacuum coming off the line that connects to the line going into the ignition switch (brown with no stripe).

You can stop the engine with around 7 lbs of vacuum, using a MityVac. Too much for me to suck, but the MityVac can shut it off.

Both of the brown lines are plugged into the ignition switch. The one with the blue stripe is closest to the ignition switch and is marked by a blob of blue paint.

The line going in holds a vacuum of 20 lbs or more when the engine is running and the ignition switch is on. When you turn the switch, it drops to zero.

The line coming out (brown with a blue stripe) has about 5 lbs of vacuum. Insufficient to turn off the engine, as over 7 lbs seems to be needed.

If you suck on the input line, even with the MityVac, you cannot pull enough vacuum to turn off the engine.

I suppose that this suggests that either the line to the shutoff valve is leaking, or there is a problem with the ignition switch.

I have no spare ignition switch or sufficient hard plastic line to replace the possibly defective line. I suppose that the switch can be cleaned or repaired in some way, but I don't know where to start, other than squirting something like WD-40, tuner cleaner, or brakepad cleaner down one tube or another.

This all seems strange, because everything was okay before I started, and all I did was do a Diesel Purge, replace the PS filter and the air filter, and then replace the vacuum tube line, which somehow I broke using the Deisel purge bottle. Nothing I could have done while doing a Diesel purge should have damaged a hard plastic line or a switch. It might have pulled a line loose, but both lines seem to be firmly inserted in the ignition switch and elsewhere. All old rubber tubing was replaced about a year ago, except the fourway and threeway tubes.

I guess a trip to the junkyard might be needed here, for more hard plastic line. Sure to require two trips to the dealer, one to pay and another to pick it up. In the Junkyard, line can be nabbed for very little and tested on the spot.