Diesel Won't Shut Off When the Key is Turned Off

Could be any number of things causing the problem. Proper diagnosis is a must!

On older Mercedes diesel engines models: 1975 to 1976 115 chassis 300D, all 123, 126, 201 and 124 chassis diesels from 1977 to 1993 the engine shutoff is controlled by vacuum (not electrical). An engine driven pump supplies vacuum to the system and a vacuum operated shutoff valve shuts fuel off inside the injection pump. That means the problem could be related to the vacuum pump, the engine shutoff valve, blocked vacuum hoses/lines, or leaks in the system that prevent adequate vacuum from getting to the valve.

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Problem & Solution

Common among these chassis:


SYMPTOMS: Your diesel engine will not turn off - even after removing the key from the ignition! In some cases it may take a long time and die very slowly.

QUICK FIX: In an emergency there is a lever on the left side of the valve cover that you can push to shut off the engine. How embarrassing in a busy parking lot! If you have just had an oil change done look for small vacuum hoses that came un-plugged around the area of your oil filter.

Emergency Shutoff Manual Control:

Stop Lever on 671 diesel engine: For those of you not familiar with the diesel engine, there is an emergency shutoff lever that will allow you to shut the engine off manually. Open your hood and look for a small lever with a red label marked " STOP." Put your thumb on this lever and push hard towards the engine until the engine stops. If the linkage is worn or maladjusted you may have to push really hard to get the engine to quit. On some engines the label may have been worn off. On older 4 and 5 cylinder diesels look for this lever near the linkages on the top drivers side of the valve cover. On newer engines after 1985, look under the drivers side of the intake manifold just above the injection pump.

Fuel Shutoff Valve on 123 chassis 240D

To be able to fix a diesel engine that will not shut off, you must first understand a little theory of how it works. This problem almost never happens with a Mercedes gasoline engine. That type of engine runs on electrical spark of 12 volt power supplied from your car's battery. When you turn your key off, electrical power is interrupted and the engine quits immediately. Since a diesel engine does not run on electric spark, the engine has to be shut off by some other means. When you turn the key off on your Mercedes diesel, you may have noticed that the engine will not quit immediately. In some cases it may even keep running a second or two after you have turned the key to the off position before it finally quits. That alone should give you a clue that a diesel engine uses a different approach to shutting down. In all Mercedes diesel engines from the first 300D in 1975 up to the mid 1990's, vacuum (suction) is used to turn the engine off - not electricity!

In simple terms - there is a valve mounted on the rear or side of the diesel fuel injection pump. This valve has an arm attached to it that goes down inside the pump. When vacuum (suction) is applied to this valve, the lever moves and shuts the fuel flow off inside the pump. Without fuel the engine cannot continue to run. If the valve acts quickly the engine will shut off quickly. If the valve is sluggish, the engine may take a few seconds before it quits. If this valve is not getting vacuum or is broken then the engine will not shut off at all. The source of vacuum for this valve is produced by a mechanical vacuum pump located on the front of the engine. This pump either has a rubber diaphram or an aluminum piston that pumps vacuum (suction) into the system. The vacuum then goes back to your ignition key assembly under the dash and back out to the shut-off valve on the back of your injection pump. When you turn your key OFF vacuum is routed through the back of the switch and on to the shutoff valve. If the valve is working properly and the vacuum is not being bled off by some leak somewhere in the system - then your engine should shut off quickly.

The number one reason for your diesel engine suddenly failing to shut off is loss of vacuum to the fuel injection pump shut-off valve. And the number one reason for sudden loss of vacuum is a leak somewhere in the lines. This leak can be caused by one of the rubber fittings getting loose or accidently knocked off, or it can be caused by a leak somewhere else in your vacuum system (the most common on the 123 chassis is leaks in your door lock system). If your door locks stopped working at the same time the engine shut-off failed you will know the two are related. To fix your problem you must think vacuum!

Another video to watch is Mercedes Diesel Engine Won't Shut Off When Turning the Key Off: Probable Cause

Additional Resources Available: If our quick fix tip does not help you solve the problem, you will have more complicated troubleshooting and work to do.  The first step requires that you properly test your car's vacuum system. Open your hood and check for loose vacuum lines in and around the oil filter housing. Also check for loose lines on the top of the valve cover. Check the rubber connector fittings to make certain all hard lines have tight connections. If the hard lines are loose in the rubber connections then those connectors should be replaces. We have a vacuum service kit available with an assortment of those connectors. If you have a 123 chassis diesel locate the vacuum lines for your door locks and plug them off to see what happens (they are the two large yellow plastic lines going into the firewall just inboard of your brake booster). If you discover all your vacuum lines are ok then you may have more work to do. It may end up not being a quick fix after all!

Many mistakingly believe that if their lines are connected properly and the diesel engine won't shut off - then they only need to purchase and install a new shutoff valve. By now you should realize that the problem could be caused by a number of different failures throughout the system. It could be something as simple as a disconnected vacuum hose or something as complicated as rebuilding the vacuum pump. I have always said, "Don't just throw parts at a problem." Be forewarned - you can spend a lot of money and never fix it! Diagnose the cause first to determine what, if any, parts need replacing. Kent has written a manual "Diesel Vacuum Source Troubleshooting and Repair" that we definitely recommend purchasing. This manual will walk you step-by-step through the process of diagnosing your diesel's vacuum system. The manual applies to most 240D 300D 300CD 300TD 300SD 190D 300SDL 350SD 350SDL and S350 with vacuum controlled engine shutoff. If you don't have a vacuum hand pump tester we offer a handy kit. We have all the parts you will need to fix the problem, but we strongly recommend you troubleshoot it first!