W124 dieseled for maybe 15 sec after shutoff
 

I'd previously seen posts of people's cars running after they turned the key off, or some that won't shut off at all. It's a bit nippy outside today, maybe 45 degrees. I started the car up cold, backed out of the garage just 25 feet to put it in the driveway while I clean out my garage. Turned off the key, and it continued to run for about 15 seconds, first time ever it's done that.

I figure it might be a combo of the chilly weather and that the car was only started for a 10 second drive.

Am I right into thinking this is a one-time-thing, or is this a foreshadow of bigger problems later?

Thanks,

Jeff 1991 300d, 102k

Such a short drive probably did not develop enough vacuum to give you a crisp shut down. I wouldn't worry about it until the problem starts reoccurring.

You might have a vac leak that is just starting to show itself. Sorry I can't point you in the right direction as I'm not familiar with your engine or chasis. There's probably some vac diagrams on here somewhere, I'm sure. Having a MittyVac is dead useful for finding vac leaks.

What do you do with vac leaks?
 

When you have a vac leak, what do you do with it? I mean, is it normally in a rubber hose (simply replace a hose), or in the vac unit itself?

Also, I see a bunch of different Mity Vacs on ebay, is this the kind to use?

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=110098739599&fromMakeTrack=true

Thanks,

jeff

Your vacuum leak is probably somewhere in the engine compartment. (With a W124, the door locks are on a separate vacuum pump (under the rear seat) so at least you don't have those to worry about.)

Go through the engine bay, tracing the big vacuum line from the pump on the left front of the engine to the brake booster. Check every line that tees off of the big one and every place they go. The vacuum lines themselves are hard plastic and fairly rugged, although they can be burned or cut. Replace any lines that look damaged.

The rubber connectors that connect several lines can get leaky over the decades and should be replaced if they feel soft. ("Soft" is a relative term and you really need a new one for comparison. They are a few dollars apiece -- you can get them from Phil.)

While you're looking over the vacuum lines, remember that the system is full of restrictors -- little things that look like plastic pieces joining two lengths of tubing. Not only does each one need to be there, but there are several different "sizes" and each one needs to be correct for the system to function correctly. A diagram of all of the vacuum lines is a valuable asset. They are usually included with the factory service manual and may be on line some place.

You should also check the "ignition" switch as it has a vacuum line -- that's how the signal gets to the engine to shut it off. Make sure the line is in good condition and isn't full of oil -- that can happen if your vacuum pump is failing. Trace the line from the switch to the IP, where there is a vacuum actuator that closes the IP, that's how the engine is shut off.

Good reply!!
 

Man, thanks for the clear reply. I only hope now I can figure this stuff out. I imagine like most repairs, once I see it, figure it out, then looking back on it, it will seem easy.

I had a brake problem a while back, hard pedal. Some thought it might be the check valve, or the vac pump going bad. Others have said the vac pump either fails or it doesn't. So if it's working, it's working. Is that true? I hear they're expensive.

jeff

These are the vacuum pumps for your car:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ42140X90VP&year=1991&make=MB&model=300-DT-003&category=All&part=Diesel+Vacuum+Pump&appEngines=_any

It does seem strange that a loss of vacuum to the brakes would "fix" itself. This may be related. It will be easier to find if it gets worse/consistent.

No one mentioned the shutoff valve on top of the IP. Mine was bad.
Thanks

Also, to tell if yours is bad, check it by connecting a vacuum tester (mightyvac or Harbor Freights $20 special that will probably break but you can always return it for a new one). Anyways, connect it to the shutoff valve. Pump it till you get a vacuum. If it holds a vacuum, it's good. If not,
A. Your shutoff valve is bad. They're about $25
B. You don't have a good connection to your vac tester. Check that connection and do again.

Thankfully, the shutoff valve is right on top of the injection pump on the 603 engine. This is a harder job on the 617. So if it's the shutoff valve it's not a big deal to swap out for a new one.

Mine did this also. After a few days, it would not shut off. my vacuum line coming off the main line was plugged. On a suggestion by a competent forum member I used a small drill bit by hand (no drill) and cleared the blockage. Works just fine now!