My car has a similar idle problem and the fault codes and these are what I had initially:

"7 TNA (Engine RPM) Signal
14 Road Speed Signal At CIS-E Control Unit
18 Current To Idle Speed Air Valve
27 Data Exchange Fault Between CIS-E Control Unit and EZL ignition Control Unit"

Cleared the codes and the only one that still comes up is 14. From what I have read, most common issue is the failure of the hall effect sensor that is on the back of the speedometer. Other potential issue is the CIS computer itself.

I have a likely good used one that is waiting to be installed just haven't been able to do that in the past 4 months. My issue really isn't that bad atm.

Part number is 0075422917, should be about $55 at the dealer.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.