Is this a 116, 126 or 140 SD?

If it always shuts off with the stop lever, it probably isn't running on blow by fumes. Carry a Mityvac and vacuum gauge. When it doesn't shut off with the key, see how much vacuum registers at the vacuum hose to the actuator, and how much it takes to shut off with the Mityvac. If you discover there's less supply than the actuator needs, take a vacuum reading the source - the check valve along the brake booster line.

Sixto
98 E320s sedan and wagon