Hi Joshua,
The fuel shut-off is a vacuum diaphragm located at the rear of the injection pump. Any leaks in the vacuum system can keep the car from shutting off. Have you tried operating the door locks after turning the car off? Does the locking system work? Anyway...follow thw brown vacuum line at the rear of the pump. It should be attached to the diaphragm. If the diaphragm is bad, the line will have oil in it (pull it off and look.) Also look for any loose lines at the large plastic line to the brake booster.
Regards,
Randy D.

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W.R.Durrance
ASE Master Tech
Mercedes Tech 21 Years
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W.R.Durrance Engineering