92 E150 rough idling

My van is 1992 E150 4.9L V8. Idling is very rough. IAC valve had been cleaned and checked with 12v. The harness from ECU to the IAC socket is good as resistance was measured. The idling is still very rough after installing back. What else would be possible? Do I really need to replace a new IAC valve as the van has been running 240000km? Any suggestion would be appreciate. Thanks

I finally fixed the rough idling problem. It was the throttle body position sensor. There are four screws to remove the throttle body, few hoses in order to replace it. It's really time consuming jobs, but its worth.