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?

TIA