So a while back I got a p1506 code for “Idle Air Control Overspeed Error” so I decided to replace the IAC valve with a Delphi one. Even after replacing I still get the code and now I have problems with the rpm’s raising way up to almost 3000 when I push in the clutch, and it stays that way until I come to a stop. It only does about 50% of the time and especially does it when the engine is cold. I know already the rpm’s raising up when pushing in the clutch has something to do with emissions but it definitely shouldn’t be that high. Anyways I recently conducted a smoke test to test for vacuum leaks and found a big one coming from the cap thing on the back of the IAC (Circled in the photo) and I couldn’t find any other vacuum leaks. So now I’m on the verge of replacing the IAC valve again but I can’t find any motorcraft replacements specifically for my truck and I don’t want to buy an aftermarket one again. But in looking online I found a few motorcraft replacements made for other vehicles that look practically identical with the only difference being the location of the electrical connector. So

Photo of one of the similar looking motorcraft IAC. Only difference is the placement of the connector
I’m wondering if it would be possible to use one of those slightly different ones instead of buying another aftermarket one. I know it’s probably fine, I just want a second opinion before spending the money on a new one.

 

Only use Motorcraft or Hitachi brand IAC Valve on Rangers, only ones that are true solenoid valves, others are "universal" and have stepper motor circuits which will cause odd issues when used with Ford computers

That cap is the Vent and will just pop off, it can whistle in some 3rd party IAC valves and in a smoke test it would have smoke come out but not as much as you described

The Valve part of the IAC Valve has to move back and forth to set idle
The front side of the valve is exposed to air from throttle body, but the back side of the valve is "sealed" so could build up air pressure behind it, making the valve hard to move, so back side of valve is Vented so it can move freely, under that cap should also be a small filter

On 1995 and up manual trans vehicles(not just Rangers or Fords) there was an emissions software added that should hold idle at current RPM when clutch pedal is pushed down for shifting, and hold it there for 5 to 7 seconds, if pedal is down longer then RPMs should drop to about 1,100rpms and hold there
RPMs will not drop to idle level until speedometer is under 5mph
An idling engine has very high emissions, which is why newer vehicles have "auto stop/start", engine shuts off instead of idling when stopped, it was the only way to meet new idle emissions standards

But in your 1999 the software holds idle high until under 5mph

But 3,000rpm is way too high, which could be a 3rd party IAC valve issue

 

Bumping an old thread.

Would an aftermarket IAC also cause issues like "floating" RPMs and stumbles at idle? I replaced what I believe is my OE IAC with one from Autozone and have started having these issues. Possibly related?

 

Yes, 3rd party IAC valves don't always play well with Ford computers
Used motorcraft from wrecking yard is better than new 3rd party unit