dannymiro

My Mercedes 420 Sel has an idling problem when I first start it, it idles really low almost to the point of turning off after reeving it up it clears out then idles high and when I put it in gear it seems to pick up more then slam tranny drop into gear.. Dose anyone have any ideas? .. Please help. Thank You.

Ausmbtech

Check the fuse in the overvoltage relay (OVP relay). It may be blown. Also get a digital multimeter and check the dwell between terminal 3 in the diagnostic plug and earth. It should oscilate. If it is a fixed value then the engine control unit may not be working (faulty OVP relay) which is causing an uncontrolled idle among other things. Other symptoms include slow to start start cold and higher fuel consumption, possible ABS light on dash.

dannymiro

Thanks. I will check it out and let you know my results

laurastone

I meet the same problem, too. Thanks Ausmbtech, thanks so much for your answer.
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herman miller aeron chair

dannymiro

The over-voltage relay fuse is good there is no ABS light on the car dose have a slight misfire like put put put. I have a digital multimeter and checked the service plug on the left fender with the round plug and set the meter at V 20 and there is no movement. Is this the right service plug? and setting for the meter ? The manual I have showed a diagnostic plug on the right fender towards the back but my car did not have one there and this is the only one I found.

Ausmbtech

You need to set the multimeter on dwell not volts. It's either be the % or degrees symbol.

dannymiro

The dwell dose not oscillate except when the RPM drops or rev up but it is defiantly fixed.. Thank You for the help.. I will buy a new over protection relay..

waybomb

To test OVP = pull it out of its socket with engine running - idle should jump way up.

To test ISC = pull it out of its socket - idle speed should jump way up

To test ICV = pull the connector off, idle speed should jump way up.

If you do all three of those, and you get the desired results as stated, then they are <probably> not bad.

Next check all your air tubing for leaks. You can do this with a smoke generator or with Gumout Carb Cleaner. With engine running and warm, spray gumout or blow smoke around all the rubber air hoses. With the gumout, the engine will stumble when you find a leak; with smoke, you'll see the smoke sucked in at the leak.

You have two hoses running parallel to the valve cover, one going across under the fuel distributor, and then the short hoses at the ICV.

There are also "donuts" below the intake manifold. If those are leaking, you have a bit bigger of a job at hand, but it is not difficult, just time consuming.

If none of these find any issues, then you have to check your coolant temperature compensator switch (B11 switch):
B11/2 CTS
Temp C -- K Ohms
20C -2.5K
30c - 1.7k
40c - 1.18k
50c - .84k
60c - .60k
70c - .435k
80c - .325k

But I bet you find the problem when you get to the air distribution hoses.

lkchris

When your engine starts cold, temp sensor advises computer to

- activate the cold start injector to provide more fuel

- activate the idle control valve to provide more air to create faster idle

You can probably imagine the problem if one or the other doesn't happen. Since you need the richer mixture when cold, you can imagine that a vacuum leak would interfere.

aty

What is the normal idling speed, anyway? I remember the SLK 320 I used to have idles around 750 rpm with or without the AC. MY CLK 500's idling speed over the past 3 weeks has gone up to around 850 rpm with the AC on which I noticed is kinda high cause just releasing the brakes when in gear the car starts moving without having to step on the gas. Idle goes down to 750 when I turn the AC off. Yesterday, I noticed with the AC on it was at 750 then it goes up to around 850 again after a few minutes. Same thing happened again today. Could my car be having the same problem(s)?

Thanks in advance,
Al