DIESEL RUN-ON

INTRODUCTION: Have you ever had your diesel engine come to a slow, shuddering stop or not stop at all after removing the key from the ignition switch? Both things have occurred to my 1977 300D but not on a consistent basis. For a slowly running, shuddering engine that keeps going and going, one must firmly press the emergency stop portion of the throttle linkage to mechanically shut off fuel at the injector pump. Rough, slow engine shutdown or running on has at least two potential causes:

1)       a significant vacuum leak somewhere in the car;

2)       an improperly working vacuum pump.

This article discusses testing and replacement of the device which shuts off fuel to the engine. A vacuum leak in this device is not only an annoyance; it can cause expensive repairs in the future. However, the testing method prescribed here requires a vacuum gauge and a hand operated vacuum pump. EXTREME caution and proper safety procedures should be used during testing, as the engine will be running.

BACKGROUND: Older diesels used a push-pull pre-glow start and stop switch to, among other things, shut off the engine by denying it fuel. The driver simply pushed the switch's dash mounted knob. Since about 1977, Mercedes has used a vacuum system to shut off the diesel engine. The shut-off is initiated through use of a small vacuum diaphragm mechanism mounted on the rear of the injection pump. Mercedes-Benz calls it the vacuum box. It is also referred to as the vacuum shut-off unit or the vacuum shut-off valve. This device is connected to the fuel rack inside the injector pump. When the key is rotated to the off position, a valve mounted on the ignition switch opens to allow a vacuum to be applied to the vacuum box. In turn, the lever in the vacuum box pulls the fuel rack to the off position, shutting down the engine. Because a diesel has no throttle plate, it does not create manifold vacuum, as does a gasoline engine. Consequently, a mechanical vacuum pump is built into the front of the engine to provide a source of vacuum for the engine, transmission, door lock, and climate controls.

INSPECTION: With the engine off, make sure the rear of the injector pump and vacuum box are free of oil, dirt, and other contaminants. Unplug the rubber connector and its attached plastic tube from the vacuum box. Inspect the inside of the connector and vacuum box pipe for evidence of engine oil. Use a clean toothpick to swab and probe for oil. Oil inside the vacuum box is a sure sign of a failure, as is a generous amount of oil in the line. Small traces of oil in the line can be residue from a prior vacuum box failure.

TESTING: Attach a spare length of clean rubber tubing to the vacuum box and connect it to the hand vacuum pump. The reader is advised to buy a hand vacuum pump such as Mighty Vac or equivalent available at any auto parts store. After starting the engine, apply vacuum with the hand pump. Does this successfully stop the engine? A major vacuum box leak exists if no amount of vacuum shuts down the engine, or if the vacuum created leaks down quickly. If the vacuum seems to hold, repeat the test several times to see if the engine shuts off. Remove the spare tubing from the module: look for black engine oil that might have been sucked into its lower end. Any trace of engine oil in this previously clean tube indicates a leak necessitating module replacement regardless of whether the engine reliably stopped during testing. Engine oil internally lubricates the injection pump and a vacuum can suck oil or oil vapor through a slightly damaged diaphragm or seal in the vacuum box. Oil in the vacuum system is potentially devastating if it contacts rubber parts.

REPAIR: The repair is quick, easy, and cheap. Remove the four bolts and ring attaching the vacuum box to the injection pump. Carefully loosen the vacuum box and the attached actuating rod that extends into the pump's rear. Note that the vacuum box "L" shaped end slips behind the pump control rod, which is unseen inside the pump. Practice re-installing the old vacuum box without actually removing it from the pump until you have confidence to continue. The vacuum box will not fit flush against its mounting surface if the rod is improperly positioned. Remove the old vacuum box and the steel ring sandwiched between two gaskets. Clean the pump rear mating surface. Replace the old gaskets and slip this assembly onto the new vacuum box. Make sure the cutout on the washer and gasket face down. Install the new vacuum box. Inspect the rubber connector between the vacuum box and plastic tubing. If it is a soft, sticky, cracked, split, wet with oil, or in some other way deteriorated, replace the connector. A small diameter fuel line is a good replacement choice because of its resistance to heat and oil. Re-attach the vacuum line. Now test the installation for proper operation.

On diesels, if the line from the vacuum pump to the air cleaner is fouled with engine oil, the vacuum pump diaphragm may be leaking. If so, engine oil will eventually migrate throughout the vacuum systems in the car, causing the rubber parts to rapidly deteriorate. If you see oil dripping from the "ignition" steering lock switch on your diesel, the vacuum pump diaphragm may be leaking or the engine shut-off actuator on the injection pump may be leaking (and the engine may not shut off). The lubricating oil in a diesel can eat away the rubber parts in the brake booster, door lock actuators, vacuum cruise control actuator, and climate control flap actuators. So it is a good practice to check the system from time to time.

Fortunately, M-B has come up with a neat little device to help you monitor the condition of your (diesel) vacuum pump and system. It is a small in-line transparent air filter that can be installed in the vacuum line where it connects to the T in the line to the brake booster (71). Simply cut the rubber vacuum line about 2 inches from the T and insert the filter. The filter paper in the strainer should "point" toward the vacuum pump. See The STAR, January/February 1992 for more on this handy device. (Gasoline engines without vacuum pumps do not need this filter)

Now, if the diaphragm in the vacuum pump should begin leaking, oil will be clearly visible in the filter, alerting you to the problem and preventing oil from entering the vacuum systems. The filter is part number 000 078 06 56.