86 300E won't idle/start

My 300E (1986 automatic) won't run because it basically doesn't hold an idle. As soon as I start the engine, it turns over-everything sounding fine so far- I watch the rpms make one rotation up and then fall to zero- the engine stops. Pushing again on the accelerator to try to "catch" the egine into idling makes no difference. What would cause this?

austinite