300CD help vacuum problem with ignition!
 

Okay my 1980 300CD has an interesting problem... i have read about it, but trying to get to the bottom.

The other day I came to work and shut off the ignition... seemed pretty normal except as you might suspect by this post... the engine didn't shut off.

I did some reading and have got to the point where the door locks will shut off the car... even goofier I know..

The vac lines (according to my mechanic) are good... he said the vac control unit that attaches to the ignition might have a bad diaphram... when he pulled the hose today and hooked up the vac. it shut the engine off with the key in the off position.

I am wondering if this sounds right AND if anyone knows what kind of vacuum the engine vac. is suppose to have. Heck any kind of general guideline would be great... my benz dealer doesn't have anything for a Benz that old without looking it up... and you know that would kill them.

Thx for any help!

Rob

RE: 300CD help vacuum problem with ignition!
 

It should have approx 18 to 23 hg vacuum. You have a vacuum leak somewhere. The clue about using door locks to shut down tells me that system is the leak. You might look around the oil filter housing for a disconnected hose or line, which sometimes happens during an oil change. Otherwise each hose that enters the car has to be tested to verify it holds vacuum. Turning the key off supplies vacuum to a diaphram shutting off fuel to the injector pump. Good luck.

RE: 300CD help vacuum problem with ignition!
 

You know we have checked all the lines and they are all holding pressure... could it be the check valves under the back seat... I have heard stories about it.

Start is cold and let it idle... its fine
Runs parked its fine
drive it anywhere, it won't shut off.

Thoughts?

RE: 300CD help vacuum problem with ignition!
 

Do you mean seat backs or back seat? If you mean the front seat "backs" then yes that could be it, but the source is under the hood. Where the hoses enter the firewall by the brake booster take a pair of pliers and squeeze each rubber connector till eng. quits. Plug that hose off with something like a golf tee. then trace it to its end looking for a cracked, disconnected rubber connector.