1987 W124 260E Idle Hunting/Surging Problem

[87w124]

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.