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