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Old 02-09-2003, 10:40 AM
ctaylor738 ctaylor738 is offline
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,315
380SL Still Won't Idle

In the spirit of "just the facts:"

Car ("Wayward Woman") is a 1981 380SL. Engine is from a 1984 euro 380SEC, but with US injection and emissions.

Problem: rough idle when warm. Not a miss, but a "cycling" between 400 - 600 RPM. Engine will start to die, then surge. Worse in Park than in gear.

New parts: plugs, wires, cap, rotor, fuel distributor, O2 sensor

Other info: Runs excellent otherwise. Mixture adjusted, lamda control cycling correctly. OVP relay working. Vacuum is 16" with idle stable at 750. "Gently pressing" on air intake plate does not smooth idle. All wires test OK, no corrosion found. No detectable vacuum leaks.

Tests done: Idle air valve is getting rapidly changing 3.76 - 4.8V when problem is occurring. Manual says it should be 5V +/- 1V. Bridging the contacts on the coolant switch (simulating temp < 42 degrees) brings the idle up and stabilizes it at 750 RPM. Temp switch shows 0 V output with engine warm. So, alles gute?

Dilemma: Is the idle controller defective and sending bogus signals to the air valve? Or is something else, like a vacuum leak, or a bad injector causing a feedback loop where the RPM drop and the idle controller "catches" it and overcompensates.

One way it's a $200 bet on a non-returnable electrical part. The other, it's a $200 bet on injectors and seals (4 of which are new).

Any additional tests or places to look?

Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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