Diesel shut offf valve replaced, engine stops but doesn't start again...

[Jacques405]

Hi fellow Benz enthusiasts.


My '87 W124 300TD wasn't shuting off, so i was opening the hood and using the stop lever. When i was lazy, i would stall it... I did this for a few months.


This morning I had a bit of time so i changed the vacuum actuator. It went ok and the engine now stops.


The problem is the stop lever stays in the stop position, the car doesn't start again...


If i remove the vacuum line the lever goes back up and the car starts.


With the vacuum line disconected, the stop lever works ok, it comes back as it should.


It looks like the key assembly is sending too much vacuum or vacuum for too long so the lever doesn't come back after stoping the engine...


Even after an hour the actuator and line are still holding vacuum, and the stop lever is down.


Where should i go from here?


Any feedback greatly apreciated.

[tyl604]

Did you unhook any of the three-way and four-way rubber connectors? Maybe you got one crossed up when put back together.

Not sure what you mean by vacuum actuator. Are you talking about the stop valve that goes internal at the back of the injection pump? It has an exterior nipple connected by plastic hose to the ignition switch; when you turn off the ignition, the vacuum pulls the stop valve to stop the engine. Or maybe you are talking about the VAV, vacuum actuator valve which has a clear plastic dome???

[Alec300SD]

Most likely, oil from the interior of the IP was able to be suctioned past the failed diaphragm on the shutoff valve on the IP,
and down the vacuum hard line into the cabin where it pooled in the vacuum switch on the steering column lock.

The oily residue in the vacuum switch is creating a seal that the internal spring of the vacuum switch cannot overcome when the key is removed from the ignition.
Hence the inabilty to start the engine, until you release the vacuum.

You need to remove and clean out the oil from the vacuum switch on the steering lock.
The vacuum hard lines shoud be cleaned out as well.
The rubber vacuum connecters to the vacuum switch are probably oil soaked, and should be replaced.

[Diseasel300]

Or the vacuum lines on the back of the key switch assembly could be reversed.

[Jacques405]

Thank you very much indeed for the replies gentlemen.

tyl604, by vacuum actuator, i mean the shutoff valve, the brass cylinder that goes half way in the injection pump. I changed it thanks to a midget 10mm wrench and didn't remove anything else, didn't remove the ALDA or touch any of the rubber connectors apart from the one on the shutoff valve.

[Jacques405]

Alec300SD, thanks for the clear explanation, i worry your hypothesis is the right one... It does makes sense as i have been using the car with the dead shut off valve for a few months, sometimes removing the key and leaving the engine running...

I am not excited about touching the steering column block assembly, i am tempted to try an "African style" fix and spray brake cleaner into the vacuum line at the shut off valve... As there is vacuum, the brake cleaner would be sucked into the lock assembly and dissolve the oil...


Diseasel300, i didn't touch the vacum lines and everything was working ok before.

[vwnate1]

Instead of brake cleaner, maybe try some of the purple de greaser stuff Rich mentions.....it won't damage anything and is incredible as dissolving old grease and oil .

[Alec300SD]

Don't use brake cleaner on the lines, especially not with the engine running.
Brake cleaner will degrade plastics.

If you want to try cleaning it out in situ:

Disconnect the vacuum lines for the shutdown vacuum circuit (from the shut off valve and the main vacuum tee).
Get a large irrigation syringe (60 ml)and force flush a weak Dawn dish soap solution (or ZEP citrus degreaser solution) through the piping multiple times until the discharge comes out clear.
Flush in both directions.

Rinse out the soapy residue with water.
Dry the piping with an air compressor or wet/dry shop vac.

Disclaimer, I have not tried the above procedure, so it may not work.
I'd first verify that there is oil at the vacuum switch by removing the vacuum connectors,
and inspecting the interior of the vacuum tubing for the presence of black engine oil.

[BillGrissom]

The 2 vacuum tubes to the vacuum switch at the key switch aren't symmetric, if like my 1985 300D. Insure you didn't swap them. That switch is a single pole double throw (SPDT). The hose to the stop actuator must be to the common of the switch. The switch connects its common port to either vacuum or atmosphere.

[Jacques405]

Thank you very much for the replies, it is really helping.
I am going to try the in situ degreasing as soon as i can...

[tyl604]

You might want to look for oil first before you do all that work.

[Jacques405]

Yo Bill, interesting info...
But i didn't mess with any tubing, i only changed the shut off valve at the injection pump.
I didn't go anywhere close to the key switch...