My 88 300E has had this ever present rough idle.It is to the point that it is very manageable some days are smoother than others, anyway I've change everything from the OVP, fuel distributor, ECU, wires, distributor, a few injectors to name a few and have had the fuel/air mixture checked and set properly. Could a new ECU basically knock out the gremlins that seem to be ever present with my car?
Since the ECU is probably big $, I'd suggest looking into the following, if you haven't already:
Fuel pumps
Fuel pump relay
Fuel filter
Fuel accumulator
Fuel pressure regulator
Idle control valve
Sorry if it seems basic, just want to get the cheapest possiblities first.
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The fuel filter and idle control valve was replaced. Also the fuel system was checked for proper pressure everything was normal. When I got the ECU replaced the tech said that the car wouldn't hold a steady idle and he is leaning towards an electrical issue? He told me to drive the car and see what I think, He said he wanted to try and save me some money. Getting into an electrical issue could be costly and the car it is not running all that bad some days are worse than others.
I'm no Mercedes expert... I'm still 10 days away from even being an owner... but I know a thing or two about ECUs. I used to work for Cummins Engine Company as an electrical engineer, in the design/development of ECUs and associated electronics (sensors, wiring, etc).
Anyway, the point I want to make is that you should remember than when you replace the ECU, you are ALSO replacing the software that is loaded into that ECU. Before replacing your ECU and spending huge bucks, you might want to check with the dealer to see if a software update (a "new calibration", we used to call it) is available. It's always possible that part of your problem lies in the software, not the hardware.
You would be astounded at the number of things that are adjustable in your ECU's software. At least, that's the case with Cummins engines 1998-present, and I'm sure it applies to other makes/models/years of engines as well.
Just food for thought from a newbie here.
One final comment on ECUs. You know the ECU that is in 1998/1999 Dodge Ram trucks equipped with Cummins engines? Those cost Cummins $211 each to buy from the manufacturer (Motorola); yet, like every other ECU I've seen, the end customers (you and I) pay upwards of $1000 for replacements. Astounding margins on some of this stuff.
For the most part I got the rough idle take care of. There is a MB wholesaler here who I have dealings with, he got me a ECU and some engine mounts. Most of the vibration is gone in the car,what vibration that is there is small. I may go after the Transmission mount later in the year. The last benz tech who worked on my car wants to dig into the electric side of the engine, he said the car holds idle but not like it should? I've spent enough and the car is running better than ever so I'll see what happens.
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