Someone please help,my merc won't turn off,i know that it has something to do withvacuum

Could you shut it down with the stop lever?

There are two points of failure in the vacuum system the shut off switch inthe dashboard that is attached to the key and the shut off valve that is attached to the rear ot the injection pump.

To test the system remove the vacuum line from the shut off valve on the ip and attach a hose to the valve. If you can shut the car off by either using your Mitivac or by sucking on the hose then the valve is Ok and the problem is in the vacuum lines or the switch in the dash. If you cannot shut the car off by sucking on the hose, then the problem is in the valve.

To test the switch and lines, attach a vacuum gauge tothe line that attached to the shut off valve and have somebody turn the key to off while the engine is running. the vacuum should spike as the switch sends vacuum down the line to the gauge. If you are not getting vacuum, then start by tracing the hoses as one might have disconnected.

If you need to replace parts
The valve is easier to replace, but is more expensive than the switch in the dash.

There are threads with step-by-steps on how to do those tasks. Do a search for shut off valve and shut off switch to find them.

Hope this helps...

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It's also possible that a vacuum leak elsewhere in the system, such as in the climate control system, could cause problems in shutting off. You can test these systems either with a Mity Vac or by excluding them from the vacuum system by plugging the line under the hood that does to the climate control system.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

On the W126 the doors are on a separate electric vacuum pump located in the trunk.

Is this shut-off problem a recent occurance or something that has gradually became worse?

If recent, try checking the vacuum connections under the hood, especially near the oil filter as it is very common for those to get pulled loose during the filter change.

If it is a leak that has gradually become worse, for ~$10 you can replace all the flex vacuum fittings under the hood and likely cure the problem. This will give you a good base to work from, at least, then use a mighty-vac from there to isolate. This saves a lot of those intermittent headaches.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I had the same problem on my SD. I pulled vac on the valve with a mity vac, it shut down. Pulled the lower panel below the key switch, was dripping with oil. Replaced the valve $54 you need two gaskets also, shuts off fine now.
So just because it works when you suck on it, doesn't mean it's OK.

The pic is of the one I pulled from my '82 300 D. New gaskets tho.

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