I have a 94 E 420 145k, all stock, never molested. One day it started idle high @ 1200, and wont go down till you hit the gas, then it surges from 1200 to 600 ish. I cant figure it out. Cant find vac leaks, wire problems, and I did replace MAS, with no change. I dont have a check engine lite on.

My first thought would be to ask whether the upper engine wiring harness has been replaced. If it's original, chances are it needs replacing.

There is also a switch behind the gas pedal that controls idle speed. I don't know if a bad idle speed contact switch would cause a continuous high idle, but it has been known to cause idle issues. Here's a link on how to replace it.

HOW-TO: Replace Idle speed contact switch

When you say there are no codes, are you checking just the LED self-test (Diagnostic Module) or are you checking all the modules with a tester?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles