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...

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.

Huh... The plate is moving - I am not exactly sure how the idle control valve modulates the air in the intake (some one else can help) BUT I would think that it would not involve changing the plate's position, more likely to bypass the whole throttle assembly.

I thought the plate would only move via the throttle body or equivalently the throttle cable OR perhaps cruise control (could there be a problem here?).

Saumil

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Saumil S. Patel

Well, my car did not misbehave even for a second all this weekend. It ran perfectly, so I could not test anything. O2 warning light came up fine, went to fast idle, came down to 800-ish rpm range and pretty much stayed there for 2 days (except for occasional trips to 1,000 rpm right after I disengaged gear into P or N).

I wonder if my problem had anything with a bad gas as car still had some of 2-year old gas in it (mixed with fresh gas). I also plan to replace fuel filter next change I get.

I plan to tackle this problem again as soon as it comes back. Since I am in Chicago area, it is getting close to a winter storage time. If I cannot report back to you guys for a while, I apologize in advance. I will close this thread one way or another - though it may take little longer than I planned. Meanwhile, thank you all.

hey i could not help but try to help coz for the past 6 months i have been having the same problem with my MB w124 260e 1987, it can be a real bore coz it goes n comes back and i mine the same way like yours mine has also been asleep for almost 2 years. i think you should change your 02 sensor to a new one and your problem will go away. if you have checked the idel sensor and it seems to be ok then am sure it is the 02 sensor sending a bad reading when i changed mine the problem went away.

If the throttle plate is sitting too high, it simply could be struggling to get enough air so the ICV keeps opening+closing to stabilize the idle. When it is unplugged the system is in open loop so the throttle is pushed open further. Or at least as far as I understand. So I would make sure that the throttle plate is both centered, and the HEIGHT is correct. The height is adjusted via the two small "pins" to the right of the fuel distributor. They are 2 circle button-looking items. Unless you are sure though leave them alone as the only way to push the plate back "up" is to remove the assembly and push the pin from the inside.

Here is the FSM PDF on plate adjustment:


If you don't have a feeler gauge, I just used a piece of paper as it is roughly .5mm

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Saumil, ps2cho, thank you for your input.

I may be missing a point here but I think Idle Air Control Valve takes air in after the sensor plate but before the throttle, then to intake plenum (after throttle plate), so its opening should have the same effect as throttle opening, and to allow for more fuel for higher rpm via metering plate opening.

What puzzles me is that engine idles absolutely perfectly when Idle Air Control Valve is unplugged. And when plugged in, RPM oscillates rapidly. So, metering plate mis-adjustment is somewhat unlikely. I actually pushed in metering plate a bit at one point and doing so did not stop RPM oscillation as Idle Air Control Valve kept opening/closing. I will double check this this weekend though. Maybe I need to push metering plate harder.

I owned a number of cars with K and L Jetros in the past (not KE though) and this issue was always due to sticking Idle Air Control Valve. Just cleaning it with a heavy dose of carb. cleaner fixed it - of course those valves were simple opening plate with bimetal strip and heating coil attached to it - not a servo motor like this KE system's). This time, I have a brand new valve that actually is doing exactly what CPU is commanding. So, I keep thinking that either CPU is getting a wrong info (frail wire, corroded connection, fried unit, etc.) or proper signal is not reaching the valve (frail wire, short, etc.).

What do you think?

By the way, when motor does not do this oscillation, it runs oh so sweet. Smooth, perfect idle to nice even delivery of power to higher rpm. It is quite quiet, not a miss of any sort. Just perfect...

Then, out of blue, it may start oscillating.