My brother in law has a 1985 300D that keeps running. I said the fuel
pump must be getting power but he said it doesn't have an electric pump.
He said there is a 'button to push' under the hood to stop the engine
and that works but there must be a proper fix.
Yep, it's a vacuum leak, or a bad fuel cutoff bellows - if you buy,
or have access to a small hand pump vacuum tool, (a VERY handy tool
if you are priviledged to own one of these cars) you can sort it out
fairly quickly. Turning the ignition key off on these cars does two
things. It sets the electrical part of the car to the shutdown
configuration (power to the clock, interior lights, cig lighter?, and not
a lot else) BUT, the ignition switch also has two vacuum lines running to
it, which (ideally) control the fuel shutoff at the injection pump.
These car's engines and fuel systems are purely mechanical - electricity
is needed to A) heat up the glow-plugs on startup and B) spin the starter.
After the engine is started, you can remove the battery, shoot holes in
the alternator, etc. These cars also use vacuum for a lot of things,
which is weird, because diesels really have no true "manifold vacuum",
unlike gas engines - they have to rely on a dedicated vacuum pump.
Vacuum is used in the door locks, climate control, cruise control,
brake boosting (duh), shift modulation, and, oh yeah, engine cutoff.
Fortunately they abstained from vacuum windshield wipers - anyone
here old enough to remember those jewels (hint - decelerate to make
the wipers go faster)?
I've always thought these cars were ideal for UFO encounters - "OK,
green alien scum, you shut down my Becker, but I can still drive
over you!" Also handy against the new microwave EMP guns being tested
for law enforcement usage - "OK, LAPD scum, you shut down my Becker,
One thing that will be interesting may be a MB diesel going down the road at
100MPH, cruse control set, and the drivers brain short circuited by the
That could be painful to a lot of people in the way.
When I set my cruise in my 81 300D to 100mph, it just giggles
at me. "You want me to do WHAT?". Plus, fortunately, I short-circuited
my brain back in the '70s, so that's not an issue.
On Tue, 10 May 2005 11:56:50 +0000, Paul McKechnie wrote:
If you have a W123 it is normal that you can only open and lock the door a
few times when the engine is not running. The W123 does not have a separate
vacuum pump which is electrical driven as in the newer models . There is
only a little reservoir. When you have the engine running, the locks should
Okay, but can you open and close them at least four or five times after the
engine has shut off? If not you have a leak somewhere. Also, you should be
able to unlock all the doors with a twist of the key in the driver's door
the next morning. My '81 240D lets me open and close the locks at least
three or four times the next morning. My wife's 300D only lets me do it
once, or at the most, twice the next day. I suspect a leak in one of the
door lock actuators or one of the check valves is bad. When they stop
working completely I'll probably start looking.......
Yes there is a proper fix. The engine's shut down is accomplished by a
"shut down device" a small vacuum powered bellows inside the aft end of
the injection pump - a vacuum line is (or should be) attached to it.
After twenty years the bellows is worn out and the device needs to be
replaced; thats about an $80 part plus 3/4 hour labor.
The shut down device's bellows simply pulls the fuel rack inside the IP
to its "OFF" position and the engine stops for lack of fuel.
Meanwhile, there's a STOP lever on the throttle linkage that will stop
the engine when its pressed.
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.