So my 85 CFI was converted to SEFI using an 86 GT harness years ago. Slowly the dreaded hanging and surging idle has gotten worse with all the upgrades even after Mass Air conversion.
I tried everything....vacuum lines all replaced, gaskets all replaced, charcoal cannister. Reset idle, checked codes....on and on and on. And beer...lots of beers to offset hair pulling aggravation.
Then today my Ford tech longtime friend and I were in the garage and I told him I just didnt get it and I just didnt get it. He said, hey ya know what...that diode in the IAC wiring harness could be the problem. He said there was a TSB years ago for that, the harness diode fights the IAC's because all the updated/new IAC's have the diode built in. The TSB says to swap/reverse the wiring in the connector to the IAC. We never did that in all the years it was present....guess what, firing up smooth as a baby's bottom!!!
Most if not all the IAC's now have the IAC built in....if you have the diode either remove it or switch the wiring into the connector to deactivate the harness diode and see what happens. Oh man I think Im just gonna go out to the garage and crack open a few and watch it idle for an hour!!! Hope it helps at least one other on here.....
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