94 E420 - Rough idle, but drives great on the interstate
 

My dad has a 94 E420. It ran fine until he revved the engine up manual using the throttle. Now it idles rough but runs great on the interstate. Not sure if it was coincidence or did he actually mess something up. It does have ASR. He took it to Mercedes and they said it was not showing any codes.

Any idea's?

That car has 2 distributor caps & rotors. They are always a good possibility as well as the spark plug wires.

Alright, so my dad has changed the MAF, both distributor caps, rotors, coil packs, spark plugs, and wires.

It is still doing the same thing, which is driving rough in low RPM's.

It is not throwing any check engine lights or any codes (been hooked up at Mercedes twice now).

The proper spark plug for the E420 engine is a Bosch F8DC4, a non-resistor spark plug. Other type spark plugs could cause problems. Listen for clicking of the OVP relay ... that can cause a rough idle, at least it did in our 1995 E320.

What is the OVP relay?

OVP is the over-voltage protective relay, but I misspoke because I checked a parts website and a 1994 E420 does not have one. So that cannot be your problem. On a 1995 E320 the OVP is behind the battery in the engine compartment.