Mercedes W124 - Unstable Engine Idling

Gas or diesel? Which country version? Engine with hydraulic valve actuators and Cam shaft on top? Or the adjustable one with rocker arms?

it is on gasoline. Malaysia but origin could be from UK (re-conditioned). can shaft, in line six straight.

Mileage? No adjustable valves? Is it a version with AC? And if yes when does the problem occur?

AC on/off or in whatever case?

You might also check if you have installed the right spark plugs.

It has fuel injection? or carburetor?

How warm is the engine when it appears?

Also with open hood and without air filter?

Is the filter loop still original?

There might be temperature compensated (think bi-metal) regulated flap in the air intake. Maybe it doesn't react?

it happen regardless of the air cond.

it is fuel injection system

not open hood and with air filter(yes original)

not bi-metal. a thermistor sensor at the air intake funnel before the filter.

does anyone willing to share the wiring diagram if you happen to have it? i suspect the it could be the wiring shorting somewhere but the electrician could not detect any.(they just poke around without reference to any diagram)

If you suspect it to be that. Check with stethoscope against each injector separately. If you have none, a long screwdriver with the handle against you ear (press) and the point against the injector will tell that also. You can try to detect if 6 cylinders are working. One might be out sometimes. You can locate it too with removing the spark leads one at the time. (make sure you are good isolated -rubber gloves- dry cloth e.g.) Your RPM meter can show low because the RPM of the whole system slows down, but also because of 1 cylinder. Does the engine rocks a lot? You might find a diagram on forums. I only have diesel versions here.

If you remove the electrical leads, measure the resistance of the coils with an ohm meter, if you have one a impedance meter will tell more.

Really sounds like a timing problem have you checked that? Maybe the techs at the workshop didnt check that.

you may be right, worth checking, doesnt take long to check and eliminate from enquiries.

GA for a really great post.

Also I would see if it has a blocked catalysator as well.....run it with the cat removed and a piece of pipe bridging the gap.....will be louder, but will test that cat out easily.

Also, if the engine is worn, the burnt oil smoke will now be seen as a blue emmision (and that then will also be the reason that the cat is blocked!!)......someone should drive behind to observe the exhaust better.....

You will find this very interesting:

http://www.peachparts.com/Wikka/DoItYourSelf

this link is specific to engine control DIY dianostics with step by step tutorials:

http://www.peachparts.com/Wikka/EngineControls

My second thought was of modulator valve adjustment:

http://www.peachparts.com/Wikka/Trans722VacMod

thank you all so much for the tips.

looks like i need to do a lot of testing. will post more findings when i done some tune up and test.

this is great guys....

I suspect your problem is either fuel or ignition timing. It's a guess, but based on your temperature related symptoms, I'm thinking that the problem is in what the Germans call the enrichment valve. In the old days, it was called a choke. This valve supplies extra fuel the engine needs to run properly when cold. As the engine temperature increases, the valve is supposed to lean out the mixture as a function of temperature. It sounds like you are running on full rich all the time. These valves can normally be rebuilt. Please let us know the results of your investigation.

Hi there , I had a mercedes 260e with exactly the same fault. theres a device on the cylinder head with two leads coming from it,mine was at the back . This device monitors the temp of the coolant and changes the signal sent to the engine management system as the coolant heats up, mine was faulty and the engine idle got worse as it heated up.Im sure thats your problem ....Good luck Al.

don't mind if you can post a picture of the divice that i may get a clearer picture of the device? many thanks.

How about vapor lock as part of the problem. If a part of the fuel line passes close to heated part of the engine the fuel in the line will partially vaporize causing uneven fuel distribution.

The Doctor's (mechanics) should see the "fault" on the Diagnostic screen. If they have monitored the diagnostics through the warming up period when the fault occurs!

The vehicle's CPU is either receiving unstable information from the temperature sensors and exhaust sensors or ....the brain (CPU) is failing on warm up?

It may be time for a 100% overhaul to specifications.....

If only this model had such echnology available eh?

hi all,

did some testing yesterday. took out the air filter assembly, look for the temperature sensor as mentioned earlier. i saw only a single wire switch and a 4 pin sensor on the engine. tried to unplug both sensor one at a time but no improvement.

later saw some wire running from the trottle body, so suspect the sensor and pull out the cable, the engine rpm increase to 1300 from jerking 450 ~ 1000 rpm.

when engage air cond and gear, the engine goes out again.

really hard to start. need to floor the eccelerator and crank it for about 10 ~ 15 sec to start the engine.

almost give up now. anyone with similar experience to share?

thanks

vacuum leak

any particular location to look ?

anyone know where is the adjustment screw for the fuel is located? I only saw the air adjustment screw(allen key) at the center of the air filter assembly. thinking of increasing the fuel feed to increase the rpm to see if it could stabilise.