Hello -

I am new here and new to MB. My recent acquisition, 1987 260E has an idle hunting/surging problem and I am hoping to pick your brain. I did search previous threads but could not find any that fit to my case.

Symptom: Engine starts fine, goes to first idle. When warmed up, rpm goes down to about 600-700. Immediately after, idle surges to 1,600-1,800, comes down to 600-700, and repeat this every 1-2 seconds. During this, I see higher water vapor condensation from exhaust and there is no warning light coming on. Changing gears, turning A/C on/off, etc. does not affect this at all. If I disconnect idle control valve (brand new), then car simply goes to stable 600-700 rpm and drives perfectly well.

Background: I bought this as non-runner from older seller. Car was not used for last 1-2 years. I found dead fuel pump relay (replaced), and brown OVP relay fuse (only fuse replaced). Once started, this idle hunting problem came in. I suspected a faulty idle control valve so replaced with a new Bosch unit but that did not help at all. I wondered if there is a vacuum leak (intake manifold, etc.) and the system was trying to compensate for it, but spraying carb. cleaner at various points did not change rpm at all. And, it idles and runs perfectly when idle control valve is disconnected. One last thing. When shaken, OVP relay rattles little.

So, anyone has suggestions? I heard any rattling of OVP is not good. If OVP relay is faulty, does that cause idle hunting? As I noted, there is no warning light comes on, i.e., ABS, O2, etc.

Thank you for your input in advance.

Check the throttle linkage. There is a switch that signals idle condition. I dont remember which way it works, but when on idle it is in one state (closed or open) and when the gas pedal is depressed it is in the opposite state. Maybe you just have to grease the linkage.

Saumil

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Saumil S. Patel

Actually, I just remembered, there are two switches. One is on the throttle body and the other is near where the accelerator cable comes from. Check both.

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Saumil S. Patel

Thank you - I will check both switches this weekend and post findings. I assume a faulty switch may give conflicting signal to the control unit, i.e., both throttle closed and open simultaneously.

Could also be the idle control valve sticking, you can disconnect the valve, unplug the air hoses and unbolt it, then spray it liberally with solvent. If you have a 9v battery, you can connect the poles to the leads on the valve to energize it and help expose more of the valve surface. Do this outside and be careful about sparks - don't want to set yourself on fire!!

Can also try cleaning out the mass air meter - if you remove the air filter housing and depress the meter plate, spray a bunch of air intake cleaner up towards the meter plate hinge. You'll need a spray can with a plastic extension to get up there. Have some paper towels to soak up the draining solvent from the bottom of the housing.

Anthony

that really sounds like an idle control valve issue...really does...

Hi everyone -

Well, it seems to be an idle position sensor issue as Saumil suggested. I cleaned idle control valve and even bought a new one. Having some experience with K Jetro, only suspect for fluctuating idle has been idle control valve (or as Porsche calls it, auxiliary air valve). I did not realize KE Jetro has idle position sensor switch as well as throttle position switch.

When I cleaned idle position sensor switch's connectors (to wiring harness), problem went away immediately. Switches are still original to this car so both switches probably should be replaced. But for now, it runs perfectly.

Thank you everyone for your input.

I do have one more questions though. When I replaced idle control valve, I was also trying to replace hoses. One has a connection for start-up valve and start up valve is secured by 2 hex bots. I managed to strip one of them (after soaking in PB Blaster for 2-day). Anyhow, do you have a recommendation for extractor/method? As there are electricals around, heat is not an option.

It seems thatI spoke too quickly. Car was idling just fine for a whole day but idle hunting came back next day. I am still suspecting 2 switches - throttle position sensor (variable) and idle position sensor (on/off). Does anyone know testing procedure for these switches? Also, I cannot find throttle position sensor by itself - do I need to purchase the entire throttle housing, if the throttle position sensor is faulty?

Thank you!

I must apologize - I should have searched before posting my question. I did find a few posting on testing TP switch. Thank you all.

Well, I checked both throttle position sensor and idle fuel cutoff switches. Both are working good and consistently. Then, I remembered reading about a stiff linkage, so liberally sprayed lube on it. Now, idle huting is gone. Probably throttle linkage was not allowing for throttle to return all the way and TPS was not sending a "fully closed" signal.

New pro0blem is that it now idles consistently high at around 1,200 to 1,500 rpm, after a warm-up period. It fast-idles at around 1,800 and goes down to 1,000 or so. Once fully warmed up, it comes back to 1,200 to 1,500 range and stays there. With little fluctuation but not something I call hunting (rapid successions of ups and downs).

As I mentioned, engine runs perfectly at smooth 800rpm when idle air control valve is unplugged so I don't think it is a vacuum leak.

Any thought?

At idle, the voltage from the mass air sensor, i.e the potentiometer should be around 0.7V. There are 3 terminals on it, one is ground, one is 12V and the third one should be at 0.7V (w.r.t. ground). Check this voltage, and if it not 0.7V, adjust it, there is a little screw on the sensor.

Also when on idle, make sure that the air sensor's plate is at the top.

Saumil

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Saumil S. Patel

Hi Saumil -
Thank you - I will check into that.
My idle hunting problem came back again, out of blue. 800 to 1,800rpm, up and down rapidly and repeatedly. So, I took my propane out, with rubber hose attached, to fish for a vacuum leak. Propane did not do anything but I must have touched something while I was doing this - car started to idle perfectly, all the sudden (without any help from propane). I am beginning to think there might be a frail wire, as this happend when my hand touched wire harness around the throttle body.
I hope I can re-create this scenario.
In older thread I read, there was a mention of vacuum line running from intake to auto box to cushion a shock from gear engaging. Is there one on 87 W124? I really should check that also, as my trans engages little harder than I like, even at 800rpm (very hard at 1,500rpm). While I cannot be certain, I feel rpm surges when I shift from park to any other gear. Could this be happening?

I'd check resistance on the crank position sensor and see if its out of range or weak at all.

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Maybe the wiring to the idle switches is flaky or the connector is not making a good contact with the posts.

When you turn on the ignition but before cranking, does the oxygen sensor light come on ?

Here is the reason I am asking:
Oxygen sensor is continuously monitoring the oxygen content in the exhaust and sends the signal to the computer and the computer adjusts the air/fuel ratio accordingly. This is a feedback system and if the signal from the oxygen sensor is delayed in any way (just a sluggish response) then the feedback system will oscillate. So either the computer is getting the wrong signal or the actuator, i.e. the governor (the small black part that sits on the fuel distributor and has wires running into it), is not obeying the computer's command properly. Check the wiring for oxygen sensor as well as the governor.

If you stare at the air sensor's plate, this plate is not moving when the RPMs oscillate, right ?

Unmetered air getting into the intake may also cause errors in air/fuel ratio calculations. You can spray some carburetor cleaner around the air sensor's base, and other places around the intake manifold, and see if RPMs are affected. But periodicity points more to the oxygen sensor's feedback loop.

Saumil

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Saumil S. Patel

I am away from home this week but like to respond to one inquiry from Saumil - Actually, air metering place DOES oscillate in sync with RPM. I think computer is sending signals to Idle Air Control Valve to open and close. If I simply unplug Idle Air Control Valve, then the car idles perfectly at around 7-800rpm. Not even a fluctuation to worth mentioning. Of course, with Idle Air Control Valve disconnected, it will never goes into warm-up high idle either and cold start takes little more cranking.

Occurrence of his rapid idle oscillation is not consistent and independent of temp. At one time, it idles very well for some time and start doing up and down when motor had been warmed up for some time.

Because it idles perfectly when Idle Air Control Valve is disconnected, I think it is an electrical gremlin of some sort...

Re. PS2CHO's comments - I never suspected the crank position sensor. Let me look into it.