Have 94 E420 with the dreaded rough idle. The following parts have been changed air mass meter, fuel pressure regulator and the throttle body module and it still idles rough. I did notice that the check engine light does not work at all. The car has been to an independent shop, mechanic was a mercedes shop tech for 25 plus years. When he hooked his diagnostic tool up the car, received the following message 'Check the connection could not communicate with the computer.' Last option is to take it to the $dealer for diagnostic check. Please note the car was brought used don't know when it was tuned up last.

Until there's communication with the scanner the dealer will not do anything either.....well....that's been my experience anyway.

The independent should look at the system to see if there is a comm' break to the port....this needs to be a set time agreed with you so you have a maximum amount of billing up front.
This is what I do for my customers when things are not clear cut.

This is unlikely to happen at the OKC MB facility .

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Lasly- Which independent shop did you take it too? thanks for your time.

I think the name of the shop is Euro Auto Service, they are located at 2600 Huntington Ave in Alexandria, VA. Let me know if you need a phone number for them.

Laslywhite: I am taking mine to Silverstar in Vienna for the same problem I'll let you know how it turns out.

Thanks, I really what to know what the problem is instead of just swapping parts.

You did not say that you changed the air filter, plugs, wires, cap or rotor! Have you done any of these - at LEAST the plugs and filter?

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2024 CR-V Hybrid

Previous:
1972 280SE 4.5
2018 Durango R/T, 2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Might be a shot in the dark, but try changing the wiring harness. I believe Mercedes continued to use the biodegradeable wiring harness up through 94'.

I've had the dreaded idling problem every year since I've owned my car. They were fixed only to have another part fail the following year.

1. Spark plugs, distributor, rotor
2. OVP
3. Alternator (things changed that didn't help...found loose hose behind dash)
4. coil (things changed that didn't help...FPR hose)
5. Valve seals (things changed that didn't help...spark plugs, MAS, O2 sensor, fuel pump & filter)
6. This will be my sixth year of ownership...hopfully I won't have any idling issues.

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JR
1986 Gold 300E 4Dr automatic

I had the same thing happen to my Benz. Please read my post...

1994 MB E420 issues idle/surging/stalling LONG

Replaced the two vacuum hoses that I found broken under the power steering pump. Over time the hose became very brittle and just moving them slightly caused them to snap. Thanks to everyone that posted to my questions