R107 Workshop
Mercedes-Benz R107 · 280–560 SL · 1971–1989

Rough idle when cold

Engine runs lumpy or hunts when cold but smooths out as it warms up.

Applies to all R107 fuel-injection systems. The flow first asks which system you have. On KE-Jet the cold mixture is enriched by the cold-start valve plus warm-up regulator / EHA. On D-Jet it is the cold-start valve + thermo-time switch + MPS. On K-Jet it is the cold-start valve plus warm-up regulator and air-flow plate rest position.

What the diagnosis checks

  1. Which fuel-injection system does your R107 have?
  2. Confirm: 1986–1989 560 SL / 420 SL with KE-Jetronic (CIS-E, has EHA)?
  3. Does it smooth out completely once fully warm (≥ 80 °C)?
  4. When was the cap, rotor, and plug wires last replaced?
  5. Is it a 1976–1985 K-Jetronic car (450 SL / 380 SL)?

Possible causes it points to

  • Worn distributor cap, rotor, or wires
  • Warm-up regulator / EHA enrichment
  • Vacuum leak (intake boots, brake booster)
  • K-Jetronic (1976–1985 450/380 SL) — purely mechanical CIS
  • Early D-Jetronic (1972–1975 350/450 SL) — different system

Parts for this job

KE-Jetronic fuel system spares

Background reading

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