Idle problems while going 30-45mph

[mrsolo2000]

I have a Bronco II with a 4.0l OHV A/T being run with an ECU, intake, and ignition system out of a 1991 Explorer. I am having trouble with idle control. Under 30mph, idle works just fine. RPMs drop to about 750rpm and are steady. Above 45mph, idle drops to around 1200rpm (or higher if the speed is increased) and is steady. In speeds ranging from 30-45mph, the ECU struggles to find idle and it hunts between 750 and 1000rpm until the speed drops below 30mph or throttle is given. Even weirder......I have an OTC UB-40 breakout box. When that is hooked up while driving, no idle issues at all. IAC valve is working. Any ideas on what I should be looking at to fix this issue?

 

[SenorNoob]

The only way I can see it changing when the breakout box is plugged in is if there is a bad connection there.

 

[mrsolo2000]

The only way I can see it changing when the breakout box is plugged in is if there is a bad connection there.

I came back from the test drive with the breakout box and did a continuity test and the line was good from the ECU harness to the IAC harness. I am going to be pondering this all night tonight.

 

[SenorNoob]

I'd look for slightly damaged pins. Might ohm out fine but have bad contact when connected. Probably on the speed sensor.

 

[mrsolo2000]

I'd look for slightly damaged pins. Might ohm out fine but have bad contact when connected. Probably on the speed sensor.

The VSS?

 

[SenorNoob]

Yes. It won't affect the speedometer, but the computer uses it for idle strategy.

 

[mrsolo2000]

Any common issues with the VSS that you are aware of?

 

[SenorNoob]

Not particularly. With swaps the gear on the end get trashed sometimes. But that would show on the speedometer. It's just the only way the PCM knows what speed you are going.