[W126 Coupe] Strange Idle problem

[David Doubtfire]

This is the first time I have used this mailing list so I hope someone 
can help me.

I recently had a high idle, hard start problem with my car (Euro 500SEC 
1983) which I had recently purchased. I was offered a second hand ICV 
from a 560SEC (US), which I was told would probably work. I put it in, 
and hey presto, the car started first time from cold, and after a quick 
test drive the idle settled down to a steady 500rpm, and would start 
easily when hot. A few day later I took a short trip and the car 
suddenly stalled when I stopped at a set of lights, and later stalled at 
speed when I took my foot off the throttle.

Back at the garage I checked everything and nothing seemed amiss. when 
hot, the engine would start easily and idle at 500rpm. The revs would 
then start to rise and fall, with the revs going higher and lower until 
it would eventually stall.

I took out the Fuel Pump Relay (FPR) and bridged the pins to keep the 
pump running and try to eliminate it as a cause, but the symptoms remain.

Firstly, does anyone know if the 560 ICV will work OK in a 500 (euro), 
and has anyone had a problem like this. Once running the engine runs 
sweetly.

Many thanks

David