How many inches vacuum do i need in my 1994 w140 mercedes? it has a 300 diesel engine . eng number isOM603.97. i have no power brakes and motor will not shut down. by disconnecting everthing but brake booster and shutvalve i get 12 inches of vacuum. is this enough?

Does it have a throttle cable? if not the idle is controlled by an computer controlled electronic throttle control stepper motor

im sure the timing cover at front of the engine
http://www.importecwarehouse.com/Mercedes-Cam-Camshaft-Position-Sensor-Front-of-Eng-p/mb0021539528.htm

maybe you have to change the
Mercedes W140 Central Door Lock Vacuum Pump

Try or (have your mechanic) remove oil pressure switch and clean. They gum up and it's a safety feature that when car is in accident and/or roll over, when oil pressure drops (eng quits) power to fuel pump shuts off. Sticking oil pressure switch is common on Benz Diesels

If your model has a Vacuum Reserve cannister, its probably rusted out on the bottom. This part use to look like a Coffee can and has a Vacuum line to it. (Plastic would have been a better material.)

If that is not the problem, you can hook up a manual Vacuum Gauge tester to the line that hooks up to the pump on the engine. The line should hold Vacuum. If it does not, start backtracking all of the hose connections. Eventually you will find a line that does not hold Vacuum. It could even go to a Power brake booster.

I had bought a used Diesel truck and the guy paid big money for a new engine-driven Vacuum pump. I kept dropping my Cruise Control up hills. I used a Vacuum tester and found my Vacuum Reserve cannister was rusted. The seller paid $300+ for a part not needed.

5W-30 Full Synthetic

Try this site from Diesel Giant. Good pics and layout. They also have a schematic in pdf. http://www.dieselgiant.com/mercedesvacuumtroubleshooting.htm

prees the brakes and test the brake switch wire for power and u do have power there locate the brake module

Those old engines were made to last - compare the weight of the motor to one of similar capacity nowadays; the old ones were made of cast iron which is quite heavy, the newer ones like the C270CDi Mercedes motor weighs about 30-40%less due to aluminium parts. Longevity suffers, but you need less power to drive the engine, giving you more power to drive the car. Furthermore, the common rail injection used nowadays are a lot more flexible and powerful, due to the fact that the exact amount of diesel needed per cylinder stroke is injected, whereas the old system injected a fixed amount per stroke, whether you are doing 20mph or 80mph. So, it overfuels at low revs and underfuels at higher revs. To prevent it from burning too lean the engine has a rev limit, roughly where the mixture is running at "100%" effectivity. The newer motors use computers to make the decisions based on 83452 different inputs, the older ones just measured the diesel needed and the air supplied.

So, the sad news is that the only way to get the car to perform a lot better is to put in a new moter eg. C270CDi, or put in a REAL motor like a 350Chevy :-D but leave the 300D badge on and see the other guys $h@ their pants when this "diesel" smokes them!

I think you can get this diagram on All data, about 12 bucks or so a year

http://www.alldata.com/products/index.jsp
Good Luck