I'm posting this for general info for others who may be experiencing similar symptoms.

Ever since replacing the engine on my 1988 351 I've had this anoying condition. Truck would start great the first time and go to wherever fine. On the return trip the truck would start fine but within a few miles it would be racing at 1500 to 2000 rpms. If I pulled the IAC connector the rpms would fall back. I tried everthing I could think of but this condition happened without fail on the second startup of the day for a year.

I was pretty sure it was a sensor so when I replaced my leaking heater core I put back my original Ford ECT which I had kept having replaced it with an Advance aftermarket ECT during the rebuild. That fixed the problem. I wanted to test the old ECT so I cut off the brass tip from the connector. What was left was two tiny insulated wires epoxied at the tip. I put the connector in my vise, connected a ohm meter and put the space heater to it. The temperature in my barn was below freezing so the initial ohm reading was very high. As the sensor heated up the reading went down as expected but after a minute or so the meter needle did a couple of bumps. I started to move the wires with a wood dowel and what do you know - I lost continuity. The wires had a break inside the insulation that failed to pass current when hot.

This is the third sensor from Advance that's failed prematurely. The ACT sensor leaked air internally within 200 miles. The stator in the rebuilt distributor went bad within 4000 miles. The ECT probably was defective right off the bat. Also the truck missed terribly on two different new sets of NAPA/Beldon wires. As soon as I put in Ford wires that condition stopped.

To their credit, Advance and NAPA both made good on replacement parts without question. But what I've found is that Ford electrical wires and sensors work better for my truck. Even the 20 year old ECT outperformed the new one.

Go figure!

 

often posted here to use ford parts.