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Old 01-04-2003, 07:23 PM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
380SL Idle and Mixture Problems

With the Wayward Woman's new chain, tensioner, and replacement head functioning, I am now trying to solve two problems that I believe are related.

- fluctuating idle when warm
- rich idle mixture

When the car failed its emissions test, it had idle readings of 601 for HC and 6.18% for CO. Pass readings are 220 and 1.20. It passed the 2500 RPM test with readings of 150 and .65.

I believe that these problems are related, so I have tested the vacuum, idle control, and lamda today with the following results.

This is on a 1981 380SL with a M116.962 engine.

Fully warmed up, the idle in Park is 480-580 RPM with vacuum of 12-15 inches. The idle control is getting a fluctuating 3.7 - 4.1 volts.

With the thermo switch bridged, RPM rises to 750 and stabilizes. Vacuum stabilizes at 16 inches.

The lamda check was strange, and I am not sure I did it correctly. First, the valve is getting power. Second, with hot engine, and a dwell meter connected to pin 3 of the diagnostic socket, I am getting a constant reading of 6 degrees, which is like 7 - 8% - which makes no sense considering the rich readings at idle.

Anyone seen this before, or have ideas on where to go next?

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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