Guys,

The 300SD isn't the only car that did this me. I bought a used 1991 Pontiac Grand Am which appeared to overheat at the used car dealer's lot. I told him that the fan wasn't coming on in time. He wired the fan directly to the ignition, and the fan always ran when the engine was on. Well, the engine ran about an extra second after the ignition was turned off.

I don't know what the cause of overheating was, but I removed the jumper wire, and the engine turned off normally. The car never overheated again.

By the way, I'm using an aftermarket mechanical thermostat to control the fan on the 300SD. Even when the fan was set to come on at a lower temperature, if the fan was on, the engine would take longer to shut down.