High idle or a rough, hunting idle on a 380SL or 560SL is one of the most common “it feels possessed” complaints. The good news: it is usually air, idle control, power to the idle system, or mixture control — not a cursed engine computer. Work the list below before you turn mixture screws as a first resort.
Related: K-Jetronic, OVP relay, hot start.
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What “high idle” and “rough idle” mean on these cars
High idle: warm idle sits well above the factory target (often into the 1,000–1,500+ rpm range) and will not come down with the idle screw alone.
Rough idle: the engine shakes, hunts up and down, or feels like a misfire at stop lights even if rpm is not wildly high.
Both can share root causes: unmetered air, sticky idle valves, vacuum leaks into the climate system, or KE-Jetronic control problems on 560s.
1. Idle control valve / idle air path
Late R107s use an idle control path (idle control valve / idle air valve depending on year and market). Carbon, oil vapor, and age make the valve stick open or closed.
- Remove and inspect for sludge
- Clean with an appropriate throttle-body / valve cleaner — do not soak electronics carelessly
- Confirm the valve clicks or moves when powered (bench test per your year’s procedure)
- Check the rubber boots and hoses for cracks that create a permanent air leak
2. Vacuum leaks (engine and climate)
Unmetered air leans the mixture and raises idle. Smoke-test or carefully listen with a hose. Include:
- Intake boots and distributor-area vacuum lines
- Brake booster hose
- Climate-control vacuum pods and lines (a leak here can change idle when you move the mode switch)
- Oil filler and valve-cover sealing if the engine breathes through cracked parts
3. OVP relay and related power
If idle control is dead because the Overload Protection relay or its fuse has failed, idle behavior goes weird along with other electronics. See the OVP guide before you condemn the idle valve.
4. KE-Jetronic / EHA notes (560SL)
US 560SLs use KE-Jetronic with an electro-hydraulic actuator (EHA). Idle and mixture trim depend on that control loop. Faults, bad grounds, or a tired EHA show up as idle and driveability problems that a simple “K-only” parts cannon will not fix. Confirm you are diagnosing KE, not ordering K-only parts.
5. Adjusting idle the right way
Factory idle specs exist for a reason. After leaks and idle control are healthy:
- Engine fully warm, all accessories off, transmission in the correct position per manual
- Set idle speed only with the specified adjustment
- Set CO / mixture with an analyzer where required — not by ear alone
If idle only “fixes” with the mixture wound to an extreme, you still have a leak or a control problem.
6. Cleaning and maintenance habits
- Replace cracked vacuum line as a set when it is original and brittle
- Keep the air filter honest — a collapsed filter changes airflow metering
- After any intake work, recheck idle and for vacuum hisses
Quick decision order
- Smoke / listen for vacuum leaks
- OVP and fuses
- Idle control valve clean/test
- Fuel pressures if mixture smells wrong
- KE control diagnosis on 560s
- Metered idle and CO adjustment last
A digital multimeter helps confirm power and grounds at the idle valve and OVP circuit without guessing.
