My diesel is dieseling (doesn't shut off promptly) - PeachParts Mercedes-Benz Forum

jbach36 · #1 11-19-2007, 07:02 PM

My car suddenly started doing this yesterday, sort of keeps running after you turn it off.

What is the cause of dieseling?

How do you fix it?

Thanks,

Jeff 1991 300d, 114k

TimFreeh · #2 11-19-2007, 08:25 PM

Shut off valve replacement W124 602/3 300D 2.5

gdemoss01 · #3 11-19-2007, 09:34 PM

that you ust have a vacuum leak somewhere. diesels only run if they have air--if they don't have air they stop running. your key just cuts off the air so it can't burn anymore.

find the leak.

jkoebel · #4 11-19-2007, 09:58 PM

Quote:

Originally Posted by gdemoss01

that you ust have a vacuum leak somewhere. diesels only run if they have air--if they don't have air they stop running. your key just cuts off the air so it can't burn anymore.

find the leak.

Technically, isn't the key killing the fuel so that there isn't anything to burn with the air?

Seeing how the vacuum cut-off, at least on my '83 240D, is on the injection pump and not the air intake.

charmalu · #5 11-20-2007, 12:55 AM

Take a look at the rear of your IP & the oil filter. there is the vacuum
the shut off, looks like a little can with a tube sticking straight up. there
is a brown vac. line with a short piece of rubber hose. check to see if it
didn`t get pulled off maybe when you were changing the oil filter.

It takes vacuum to shut the little dodad in the IP which turns off the fuel.

As far as these engines dieseling in relationship to a gasser, it`s two
different things. with the diesel, you just didn`t shut it off.

Charlie

ljsense · #6 11-20-2007, 10:57 AM

I just went through this. You'll probably find better instructions if you search the forum, but here's my take.

1. Ask yourself if the problem started after an oil change or some other work under the hood. If so, you probably just have a disconnected vacuum line. look for a plastic line that is not connected to a rubber tube or fitting. Quick, easy fix.

2. If that's not the case, pull the plastic line off your vacuum shutoff valve. (As another said, it's brown and goes from the firewall down behind the injection pump to a little brass colored can.) If there's black oil in there, it's pretty likely your shutoff valve has failed. The part is around $50, and pretty easy replacement on a non-turbo. Order it and two paper gaskets.

3. If you've got oil in the line, it might be causing problems elsewhere. So you might wind up having to replace the ignition vacuum switch. That's kind of a chore, but doable. (I posted instructions about a week ago.)

4. You might just have a vacuum leak somewhere else. Get both a vacuum pump and a vacuum gauge and get into an analytic, detective sort of mood.

Overall, your vacuum pump generates vacuum. It goes first to the brake booster line, that thick black tube that runs from up front near the radiator to the drum shaped thing near the firewall. You can test any fitting directly off it to see if it's pulling vacuum -- should register 20-some inches. Then test various points on the shut-off circuit. See what's happening. Maybe your door lock system is leaking.

Cervan · #7 11-20-2007, 11:05 AM

at first these kind of posts were cute when i got here, but now i just refer them to search lol.

RoysVeggy · #8 11-30-2007, 02:29 PM

Quote:

Originally Posted by ljsense

I just went through this. You'll probably find better instructions if you search the forum, but here's my take.

1. Ask yourself if the problem started after an oil change or some other work under the hood. If so, you probably just have a disconnected vacuum line. look for a plastic line that is not connected to a rubber tube or fitting. Quick, easy fix.

2. If that's not the case, pull the plastic line off your vacuum shutoff valve. (As another said, it's brown and goes from the firewall down behind the injection pump to a little brass colored can.) If there's black oil in there, it's pretty likely your shutoff valve has failed. The part is around $50, and pretty easy replacement on a non-turbo. Order it and two paper gaskets.

3. If you've got oil in the line, it might be causing problems elsewhere. So you might wind up having to replace the ignition vacuum switch. That's kind of a chore, but doable. (I posted instructions about a week ago.)

4. You might just have a vacuum leak somewhere else. Get both a vacuum pump and a vacuum gauge and get into an analytic, detective sort of mood.

Overall, your vacuum pump generates vacuum. It goes first to the brake booster line, that thick black tube that runs from up front near the radiator to the drum shaped thing near the firewall. You can test any fitting directly off it to see if it's pulling vacuum -- should register 20-some inches. Then test various points on the shut-off circuit. See what's happening. Maybe your door lock system is leaking.

I have been searching for hours but RIGHT after my oil change, my 300D would not turn off so I know its a vacuum line I knocked. I have looked pretty hard under hood and found a few things. I will go out either later tonight or this weekend to buy a Mityvac. I took three photos to possibly get some help on some lines I found underneith the hood.

Also, since I knocked that vacuum line off, that would explain why my transmission shifts harder?

Please refer to link:
http://www.flickr.com/photos/roysveggy/

Any suggestions? Thanks in advance!

Hatterasguy · #9 11-30-2007, 03:08 PM

Quote:

Originally Posted by Cervan

at first these kind of posts were cute when i got here, but now i just refer them to search lol.

After 5 years I let you guys tell them to search.

gsxr · #10 11-30-2007, 03:12 PM

In simplest terms, failure to shut off is due to one of the following:

1) A bad vacuum diaphragm in the shutoff pod on the injection pump

2) A bad vacuum pump

3) A broken or leaking vacuum fitting in the shutoff circuit (which is isolated from the rest of the vacuum consumers on the W124).

The proper way to test #1 is to connect a MityVac and apply vacuum to the shutoff pod. It should hold vacuum, and as you pump it to 25" Hg, the engine should stop, AND the vacuum should hold at 25" and not leak down. To test #2, you need to disconnect all hoses from the engine vacuum pump, attach a guage, and start the engine. It should build up to 22-25" quickly and stay there.

sixto · #11 11-30-2007, 06:30 PM

If the brakes work properly with the engine running, can you acquit the pump without further testing?

Sixto
87 300D

gsxr · #12 11-30-2007, 06:42 PM

Quote:

Originally Posted by sixto

If the brakes work properly with the engine running, can you acquit the pump without further testing?

Yes - but only if you are certain the brakes are operating normally. If someone recently purchased the car, and isn't familiar with 'normal' brake feel, this could be misleading as it's very subjective. A vacuum gauge would be better, but otherwise, if the brake power assist seems fine, the vac pump is *probably* ok.

FWIW, my money is on the IP shutoff vac pod, assuming the brakes are normal. That failure is more common.

RoysVeggy · #13 12-02-2007, 11:15 AM

gsxr,

I have been trying to find some pictures on the search, but is the shutoff pod in the center of the engine. This has been frustrating because I know I just knocked off a vacuum line while I was taking out my filter element from the engine bay. Any help?