The 85 300 SD keeps running after the key is turned off How difficult is it to replace the fuel system cut off solenoid?

Answer

Your vacum system could be of failure or a single vacum line or vacum lines could be leaking thus leaving no vacum to shut the engine down.

Answer

The car will not shut off because it runs on a vacuum system. There is a manual "shut Off" red button on top of engine for manual shut off. Problem usually is: 1) Door lock seals have cracked and letting air into the lines or 2) vacuum pump is no good. For 1) need to replace door lock (is a pain to do). Simple fix is open hood. Above steering column is a bunch of vacuum hoses. You'll see the main one coming from the vacuum pump and then branches to many other lines. These hoses are designed so you can pull them apart. First check, follow main line from vacuum pump and pull apart hose and put finger on to see if you are getting good vacuum. Second, Yellow? line going into firewall (under dashboard) goes to the doors. Pull this line off and put finger on to feel vacuum pump sucking. Engine might now cut out if the problem is the door seals. Temporarily I have put a piece of plastic over the hose with rubberband. Obivously you loose automatic door openers but great temporary fix. If it is vacuum pump, don't bother with vacuum pump seal, just get the whole new part. It is relative easy BUT need good alen socket. Also, crank engine so when you install the pump spring isn't letting the pump go down. (just crank the engine w/o starting before you put pump on so pump sits flush, you'll see what i mean. Go to www.alloemautoparts.com for best prices.

New Answer

The fuel cut off solenoid is a simple bolt on, however long slender hands and fingers are a great asset. If installed incorrectly the engine can 'run away' meaning rev uncontrollably until destruction. The stop lever will not work in this situation. I suggest you have another person start the car while you prepare to block the turbo inlet with a damp rag over a small piece of 2x4. Should the engine attempt to 'run away' you can block airflow and suffocate it to stop.

The most likely reason is a door lock vacuum is leaking, lock each door one at a time and see if one door eventually builds up enough vacuum to shut off the engine, then you know which door vacuum unit to chasnge inside the door panel. The rubber boot becomes brittle and leaks. Or there is a vacuum hose pulled off or cracked inside the engine compartment. Last resort is the vacuum shutoff behind the injector pump.