Help with Codes 18, 28 & 31 - '91 300CE

[rgnprof]

My '91 300CE (M104) has 86K and I am having ongoing trouble with starting and mpg issues...

Duty cycle start at 70% with KOEO and 50% at idle - once car warms up, the cycle drops to 1-5% and stays there until I rev motor...climbs back to 50% and then upon resuming idle drops back and hovers from 1-5%.

Why is this happening?

I pulled 3 codes - Code 18 = "Current to idle control valve is illogical", Codes 28 and 31 = "Intermittent contact in engine coolant temperature sensor circuit..."

I just replaced the 4 prong engine coolant temp sensor a couple of months ago (and, Yes, I reset the codes before pulling these codes this morning...)

Any ideas? I am and have been at a loss to figure this out and I can't find anyone local that has been of much help!

Thanks, ryan