high idle still - stumped

[jchip]

My 91 420 SEL idles at 1800-2000 rpm in park/neutral when warm and 850 rpm in drive. When cold, it still idles high (above 1000 rpm). I've cleaned and checked the operation of the idle controil valve which seems ok. I swapped out the idle control module with a spare from my local tech but still no change. The module is getting power. When hot or cold, the duty cycle on the idle control valve is at 70% . It should be near 35% when warm. This makes me think something is telling the circuit that the engine is cold and is holding the valve open but the temp sensor checks out ok as well. I'm stumped. Any thoughts anyone?

J.Chip