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W124 is the Mercedes-Benz internal chassis-designation for the 1984 to 1995/96 version of the Mercedes-Benz E-Class, as well as the first generation to be officially referred to as E-Class. The W124 models replaced the W123 models after 1984 and were succeeded by the W210 E-Class after 1995. In North America, the W124 was sold for the 1985 through 1996 model years.

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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.

Additional information:

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!

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Never trust any garage, after many years and many times of fixing peoples cars after garages have quoted exorbitant quotes for unnecessary work please take it from me as a qualified engineer in several disciplines that i hear that people trust their garages all the time. They want you to trust them, they need your custom. Ok… that being said try this first (it is the most common problem i have seen) and many people have returned from garages absolutely panicking about the quote given only to ask me. Pull off the plug from your airflow sensor (airflow sensors sometimes do not show up on diagnostic for a variety of reasons i shall not bore you with) this action will tell the ECU the sensor has completely failed and will put the ECU into a backup mode and the engine may run normally. Some cheapskates leave it disconnected which causes no harm but will have an affect on fuel consumption. Failing this i am unsure your car has a stepper motor for throttle control, but if it has it is worth swapping it… but before you do so all older engine management systems have a factory reset function, this is because the ECU learns someones driving style over a period of roughly 100 miles. If you are lucky you may find this reset procedure, being an older car, and this may help. It usually consists of something along the lines of turning your ignition on and off say three times, then in the on position pull off the fuel delivery wires, stepping on your accelerator pedal perhaps 6 times, then reconnecting the wire. Procedures are different for different cars but this will factory reset the ECU, clearing out any bugs. If your car runs roughly take it for a spin and it will settle down. However something tells me it may be this… Computerised cars are designed to cut petrol off from the engine at above the range your having your problem with when your foot is off the accelerator and the engine is running above roughly 1800 to 2200 rpm, it then cuts back in when it reaches the above range of revs to prevent stalling. This feature is to save petrol. Older cars usually have a switch, either on the accelerator like a brake light or at the end of the accelerator line (sometimes its just a microswitch) This can cause a cascade where your ECU tries to compensate. So it could be as simple as a sticking microswitch, Hence the intermittent nature of your problem. I can only offer these ideas without looking at your car, and remember, diagnostics don’t tell you everything, perhaps you have heard of planned obsolescence. Anyway good luck.

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Hi there!

Have you tried replacing or at least checking the crankshaft and cam sensors?

I have the same car but mine is a 2.0L 4cyl Diesel. But have gone through lots of them and different engines and never had this kind of an issue.

And I don't remember how was the whole engine assembled on a gas engine but if the intake manifold is plastic try checking it for any vacuum leaks. Or the worst-case scenario pressure tests it.

Let me know if the problem is fixed! And what was the issue if fixed!

I hope any of this helps and will try to suggest other suggestions if this did not help or the problem is solved!

Have a great day! :)

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The car has been to a dealership that I trust several times. I am an Aircraft Mechanic myself and I've watched them check everything out. On one occasion, they had the test equipment hooked up while the engine was hunting as described. The test equipment showed that everything was working normally. We changed the throttle sensor and swapped ECUs from another car. Later that day the engine did the same thing. I've replaced the hoses that have to do with idle/choke function etc as well as the temp. sensors, even though faults with these would have showed up during the test. I've also owned a 2012 C-250 and currently drive a 2010 C-350. My wife drives a 2003 C-230 CL. However, my favourite is the 1993 190E

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@a340mech As I understood the hunt happens rarely right?

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Correct, it does it two or three times a month without any pattern i.e. it will do it when cold, then when it's hot, after a long run on the highway, after just pulling out of the driveway while still warming up, summer, winter, high atmospheric pressure, low atmospheric pressure, full tank of gas, nearly empty tank, uphill, downhill, parked etc. It can last anywhere from a half a minute to 15-20 minutes and then it stops doing it on its own... till the next time. Oh, the fuel pressure has also been checked while it was hunting, but it was rock steady and normal. Fuel filters have been replaced and thermostat has been replaced. Timing was also normal while it was hunting.

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Oh yea, the O2 sensor was the first thing that was replaced... but none of these showed a fault code even as the engine was hunting

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@a340mech That is really weird! Because the question is where does it get the extra fuel and air from to run the engine in more than 1000 RPM?!

And just a question! What kind of fuel setup does the car have? And what fuel are you running?

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Have you checked the Idle Control Valve?

Also check vacuum hoses for leaks.

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Yes, the dealership did - it was one of the first things they checked - and they checked it several times. It would also have left a fault code on the ECM if it was faulty. At one point we were “lucky” enough that the engine was hooked up to the dealer’s analyser while the fault was occurring. It showed nothing except for a higher than normal engine/idle speed. It was as if someone was inside the car blipping the accelerator. We suspected a faulty ground in the accelerator to ECM circuit causing the ECM to sense a varying ground resistance which would make it think that someone was moving the accelerator pedal, but the grounds, as well as all of the other wiring checked out

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Check the wires from the air flow sensor…sometimes there is the isolation broken..

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Unfortunately, the wires (voltages at appropriate connections) and sensors were checked at the M-B dealership, even as the problem was actually happening, but nothing was found. But thanks for the reply!

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It is a standard electronic fuel injection running premium unleaded, as per the manual. Usually, when there’s a problem like this, it is an air leak around the “throttle body,” that is the plate in the intake path that is moved by the throttle i.e. accelerator pedal. This gives the air mass flow sensor the false impression that the throttle is open, that is, someone is pressing on the accelerator, which tells the ECU to increase fuel flow. Except in my case, all of the air paths have been checked and rechecked an hoses have been replaced. Also, since the ECU had been swapped with a known good one, that rules out the ECU sensing a “choke” condition i.e. low engine temperature and thereby increasing RPM. t also doesn’t answer the question of why does it do it intermittently? If there was an air leak, it would be there 24/7/365, not just once in a while.

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Hi there!

Did you check all the air hoses around the plenum? This sounds like an massive airleak of any sorts. What about the Rubber seals on the injectors? I would probably threw them all out and replace them. The old gasket on the Heater on the Plenum should also replaced. There are 2 gaskets on the head: the „normal“ paper gasket and the second metal heater gasket.

You should also think about looking for cracks in the rubber housing (underside) of that KE-Jet. There is also an Good Chance of an leak on the hoses around there.

good luck!

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Thomas Daniel wird auf ewig dankbar sein.
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