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If your R107 surges at idle, stalls when you come to a stop, or idles at 1,200 rpm when warm, the idle control valve is one of the first parts to check on later cars. It is simple to test and often only needs cleaning.
Which models use an idle control valve
Later R107s, including the US 380SL and 560SL, use an electrically driven idle speed control valve with its own control unit. The 450SL relies on a mechanical auxiliary air valve that opens when cold and closes as it warms. The exact hardware changed over the years, so check the parts catalog for your car before ordering.
What it does
The valve bypasses a small, controlled amount of air around the closed throttle plate. The control unit reads engine speed and temperature and opens the valve more when the engine is cold, when the A/C compressor kicks in, or when the transmission is in gear. That keeps idle steady under changing loads.
Symptoms
- Idle surging up and down (hunting)
- Stalling at stops or when shifting into Drive
- High warm idle that will not come down
- Idle that drops too low with the A/C on
- A buzzing valve that does not change idle when unplugged
How to test it
- Warm the engine fully.
- With the engine idling, unplug the valve. Idle should change noticeably. If nothing changes, the valve is stuck or there is a large vacuum leak masking it.
- Engine off, measure resistance across the valve’s terminals and compare to the manual’s value.
- Check the connector for corrosion and confirm the valve gets a signal (an LED test light or a meter that reads duty cycle helps).
- Pinch or block the valve’s air hose briefly with the engine idling. Idle should fall. If it does not, air is getting in somewhere else; see vacuum leaks.
How to clean it
Remove the valve and its hoses. Spray throttle-body cleaner into it and gently work the internal rotor or plunger by applying a brief 12 V pulse only if the manual says it is safe, or by moving it carefully by hand. Let it dry fully before refitting. Replace cracked hoses and the rubber mount while it is out.
Replace cost
A new or quality used valve usually costs more than people expect, and the control unit is a separate part. Clean and test before buying, and confirm the control unit and wiring are good. If idle is still high after the valve is proven good, look at high idle causes and throttle linkage adjustment.
Quick answers
- Can a bad ICV cause stalling in Drive? Yes. When the transmission is engaged, the valve should open a little more; if it cannot, idle drops and the engine may stall.
- Will a vacuum leak make the ICV look bad? Yes. A leak adds unmetered air and the valve cannot control idle. Fix leaks first.
