380 Idle Control Valve Test

[ctaylor738]

My 380SL has been plagued by a lumpy or "hunting" idle since I bought it. The idle will fluctuate between about 500 and 650 RPM, and is worse in Park or Neutral than one of the drive gears. The problem occurs as soon as coolant temp hits 40 at which point the idle speed is supposed to drop from 750 to 500. It does not seem to be a miss, since there are no other running problems.

The fuel distributor, injectors and seals, all air hoses, and all intake gaskets have been replaced. The coolant sensor is working correctly, and all the wiring to the relay is OK.

So it seemed to come down to the idle valve or the idle relay. But how to tell? The valve decreases air flow and hence idle speed as the relay increases voltage to it. The relay was putting out a voltage that fluctuated between 3.8 and 4.1 volts, which is within spec. Was the relay causing the "hunting" or reacting to a problem (like a bad valve) and trying to correct the idle speed? These are big ticket, non-returnable parts, so it was important to find out.

What I ended up doing was buying a low-end model train transformer capable of putting out between 0 and 19 volts. I put my DMM on it and "calibrated" the 0-100 scale to voltage.

So I warmed the engine up. I disconnected the leads to the idle control valve and hooked the transformer up, setting it at 5V. Got a stable but high idle. Cranked up the voltage, and it took approximately 9V to the valve to get the idle down to the 500 RPM range.

So my conclusion is that the valve is bad, requiring more voltage than the relay can put out to hold the idle at spec. Why this would cause the relay voltage to fluctuate is unclear. Maybe the relay would try to lower the RPM, give up, try again, give up, and so on.

As a side note, at 9V, the valve started to buzz and I let it do that for mayber 10 seconds. Since then the idle has been noticeably better. So maybe a little higher voltage is good for these older valves.

Comments welcome.