1995 E300 Idle fluctuating
 

Sorry guys,E300D!

What regulates/controls the engine idle speed?
Engine used to idle at 600 rpm if the transmission is in park or in gear with brake depressed. Now if it is in park it idles at 700rpm and drops down to 400rpm when in gear with brake depressed!

Thanks for any help.
Rick

theres a part mounted below throttle body which needs cleaning from time to time.Mine bumps 300 rpm when cold only.Even in Drive.I also hear that the wires inside the throttle body can have its insulation crack,and these wires touch and can cause idle bounce.They sooner or later need new wires. I can't see my idle controller under throttle body so for now I let it alone.Probably a shot of throttle body spray at the right place would clean the gunk.

Dumb question,where is the throttle body?

Rickster 1:

You have posted your question in the Diesel Discussion; is the car an E300D?

Sorry about that, yes it is a E300D

Throttle bodies are associated with gasoline engines; diesels (most) do not have throttle bodies.

Diesels do not have a "throttle body." They do have electronic idle regulation and it sounds like yours is not working. Unfortunately, the IP in your car, like mine, is buried under the intake manifold. The problem could be caused by a loose or dirty pin at one of the connections on the IP or by a failure of one of the components in the ELR system. Dealers and shops used to have a plug-in diagnosis box for such things but after 18 years I don't know. About all we as owners can do is to check all of the wiring and connectors.

Jeremy

Sounds like your idle control circuit is not working. What is the condition of your wiring harness?

Where is it located, I will check it out.

It's bundled in with the main wiring harness.

As Jeremy mentioned above, it could be something to do with the ELR or a connection down by your IP. It is not easy to access the IP on the OM606.xxx because of the inlet manifold.

It might be best for you to check the condition of our wiring harness as a first step or get it checked out by someone. The next step gets a whole lot more involved.

The idle control wiring runs from your IP to the main connector which is located by your battery (or by the brake booster for us here in OZ). It then travels to the ELR. It is a brown/blue wire. The ELR reads other things such as coolant temperature in head and rpm sensor.

Depending on your level of tinkering skill, you would have to actually remove both ends of this wire (the end going into the connector from the IP and the end coming from the ELR) and basically bypass the main connector so as you can measure the current passing through. It's a closed loop-you should have approx 600 milliamperes running through at idle with no additional loads running. As you load the engine with A/C, turn steering to full lock etc. the current will decrease.If you accelerate up a bit you will see the current decrease to approx 200 milliamperes also.

There is no documentation for this that I know of. Well I'm sure there is but not available to us.

the idle speed is controlled by an ELR unit, its a relay type box behind the battery curtain.

Its fed by the overvoltage protection relay and its job is to make sure the engine remains at 640 rpm at idle whether in gear, A/C running, power steering loaded etc..

Without its assistance the engine should be at about 550 rpm - measure with a tach tool or a MM that can measure Hz. - the rpm sensor reads 144 teeth every rotation of the flywheel.

Make sure the wiring is good and intact - as shorts in the 4 wire coolant sensor circuit can cause the idle speed, EGR and also the manifold flaps to act completely bonkers.

You mention the car idles at 700 rpm, this makes it sound like the alternator is not working - there is no load on the engine at all.