Mercedes-Benz W123 diesel does not stop with the ignition key
Why does a W123 diesel not stop when the ignition key is turned off?
The diesel's shut-off lever is moved by vacuum. If the shut-off diaphragm leaks or a vacuum hose is detached, the lever does not move and the engine keeps running. The shut-off diaphragm is on top of the injection pump, and replacing it is usually a quick job.
How can I test the shut-off diaphragm for a leak?
Suck on a piece of hose from the diaphragm's connector. If there is a hole in the diaphragm, the vacuum leaks and you feel it immediately. An intact diaphragm holds the vacuum. A leaking diaphragm can also be told apart from other faults by blocking the vacuum pipe as a test.
Which hoses should I check if the diaphragm is intact?
Check the thin vacuum hoses going to the ignition switch and to the shut-off, and their connectors. A loosened connector or a detached hose leaks and prevents the engine from stopping. The leak is located by disconnecting and blocking the hoses one at a time. The valve at the base of the ignition switch can also be faulty.
Are the vacuum pumps different in different model years?
Yes. The pump of the 1983 model year is a piston pump with one hose connector, the older one is a diaphragm pump with two connectors. When in good condition, both work equally well.