My '04 Ranger 4.0 was idling high for a couple of days and was slamming into drive from park. When shifting out of first gear, the RPMs would get abnormally high before it would go to 2nd. After that, the truck was fine. I figured the IAC might be going bad, but I wanted to make sure there wasn't a vacuum leak, first. I unplugged the IAC, and the truck immediately stalled out. Ok cool, no vacuum leak, likely. I plugged the IAC back in, and the truck just turned and turned and wouldn't start. I swapped the old IAC for a brand new one, plugged it in, and still the truck won't start.

I'm having trouble finding someone with my exact situation, so I'm hoping someone on here can help me out! Why would the truck start just fine before unplugging the IAC, and not start at all after unplugging it and plugging it back in? The engine sounds completely fine, and it really wants to turn over. Sometimes I almost think it will, but then it drops out. I removed the inertia switch a long time ago, so that isn't the issue.

If someone can give me somewhere to start troubleshooting this, I would greatly appreciate it!

 

I found the culprit. Super easy, and I should have checked it sooner. Replaced the TPS and all is well. Not sure why unplugging the IAC made the TPS finally go bad, but it must have been on its last leg.

 

Your symptoms correlate with a failing TPS, not an IAC. The IAC won't affect shifting but the TPS will.

 

How many miles on your 2004?
I ask because I have a 2005 4.0 and would like to know my TPS's approximate lifespan.
Also, did your harsh shifting go away?