Why does my engine hunt at idle?
I have a 1993 190E (2.3 4 cyl.) Regardless of outside conditions and how I’ve driven the car, when I stop, the idle fails to come down below 1800 to 2300 RPM and it hunts up and down between 1800 and 2300 RPM. f I take my foot off the brake, it accelerates and gets to whatever speed 1800 and 2300 RPM will allow. A few minutes later, everything goes back to normal RPM and all is well for a few days. Then, for no apparent reason, it does it again. M-B dealership has been unable to find the fault. I.we have tested all pneumatic hoses that have to do with idle, they replaced some temperature sensors and I’ve replaced the throttle body. I have a very good relationship with my dealership - have bought seven M-B cars there and I trust them. I wanted to buy another 190E from a private seller and that car drove normally except at the very end of the test drive. As I pulled into the seller’s driveway, that car did the same thing. The seller said that it had never done that before. I didn’t buy the car. But I’m wondering if anyone else has experienced this and has anyone diagnosed the problem successfully. Please don’t come back with lame “does the car have enough gas in the tank” types of suggestions. I am an aircraft mechanic and not a complete idiot around mechanical things. But this phenomenon baffles me as it does a dealership full of M-B mechanics. Thanks in advance for any help that you might be able to provide.
The relay pack ( almost directly below the heater/AC fan in the cabin) for the daytime running lights had been removed as one M-B Tech traced a wire that was in the circuit that sensed idle RPM and went through that relay pack, presumably to turn the daytime running light s on. That made no difference to the problem and the relay pack was reinstalled. I subsequently removed it (it’s really simple) and turn the headlights on and off manually
All temperature sensor have been replaced and the air flow sensor was replaced at the same time as the throttle body.
There are no faults showing when the ECM (Engine Control Module) is interrogated. An ECM from a junk yard was substtuted, but the problem did not go away. (The chances of another ECM having the exact same problem are somewhat remote) I’m back to running with the original ECM
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Thanks to everyone who responded to my problem!!! Almost all of the suggestions that were posted I had tried prior to posting my problem here, but thanks anyway. The problem persists to this day, but it doesn't seem to be happening as often, perhaps only four-five times a year, and I can live with that. Given that, I'm almost convinced that the problem is caused by an intermittent electrical condition, perhaps higher/lower resistance in a connection somewhere, that results in the ECU receiving incorrect information, causing it to command higher idle RPM. If I'm going to trace it down, it will involve the use of a megger to check insulation break-down and/or connection integrity in the electrical harness. Finding the appropriate schematics will be "interesting." In any case, it will be a tedious process. I posted this in the hope that someone might have encountered the same problem and coould give me advice that would save me some time. Thanks again to all!
из Thomas Daniel