I'd suspect that the throttle linkage is sticking partially open. It doesn't take much for an unloaded engine to race to redline. Solely a fuel leak would cause flooding, solely a vacuum leak would cause an excessively lean condition, and it would have to be pretty massive. For it to make it all the way to the redline, air has to be passing through the throttle plate for the CIS plate to meter fuel.

That's my thinking anyway. It's not unheard of to have the overly-complicated linkages stick or bind up.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]