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.

Have your tech's checked the distributor caps and rotors? I've not experienced it myself, but I've read here more than once that the 119 engine is quite sensitive to those parts getting old.

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Fourings

84 Audi Coupe GT with Turbo engine, EFI (buh-bye CIS), Jamex seats, Koni's/H&R's, e-code lights. 210k and counting.
1992 Anthracite 400E, 161k, "Grey Ghost".
1986 VW Quantum Syncro, 192k, gone to the crusher in the sky.
1989 300TE 172k. (Was mine, now also Mom's car)
72 280SE 4.5 (Mom's car)

i just did the head\valve job cuz the compression was a bit off in a few cylinders. had to do the values and shave the head flat for a .016" warp. cyl. got it running and #2 was dead. rechecked compression and all where within 2# of 165 psi. so started pocking around with the carb cleaner for a leaking intake manifold or something. leaned on the tube to the injector and it smoothed out. took oil to the seals around the little critter and it smoothed right out. so i pulled the injectors and found the Oring was as hard as rock and the insulator as well. i took all of them out and oiled them up good and put them back in and it ran great. so try an oil can to the black insulators and see if it doesn't smooth things out. new insulators and Orings are about $35-$40 and a lot cheeper than a tech's time so give it a try. good luck. jim

I found two loose or broken hoses under the power steering pump. One is black and the other one is a clear to smokey colored tube. On the driver's side inner fender there is a part numbered "015 540 60 32" it has 4 connectors and the clear tube connected near the center of it. Maybe 4 inches away is a part mounted on some kind of damper with the letters "MOT" and an arrow on it there are two black tubes coming out of it and a connector on the top, one of the tubes goes under the fuse box in the corner and the other one is in the same loom as the clear tube and runs under the power steering pump. I believe these are part of the emission system but not sure, any ideas? Thanks in advance

In mine the MOT is the charcoal cannister purge valve and that vacuum hardline comes off and causes all kind of idle misbehaving

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