I have a 1989 F-150 with the fuel injected 5.0L and automatic transmission. It has been sitting for a couple of weeks, and just today I started it up to take out some garbage. I noticed that when I slowed down and the engine got to about 900 rpms, the rpms suddenly jumped back up to 1100-1200. When I placed the vehicle in park, the rpms shot up to 2500+. When I restart the truck, the idle started out at about 1500rpm or so, then went right back up to 2500 rpms.

I took off the IAC and cleaned it up, and it appears to be functioning. I also unplugged the IAC while the truck was running, as I was told if there was an intake leak the truck would continue running. However, when I unhooked the plug from the IAC, the truck died out.

The last time I used the truck it ran fine, so I'm at a loss. Any ideas or suggestions?

 

If you figure it out please let me know. I've had the same problem with my truck for a while. I replaced several things (returned most when they didn't work): O2, IAC, TPS, cap/rotor/plugs, ACT, and the water temp sensor.

At this point I suspect it to be a problem with the thermocater but have not yet figured that one out.

 

Today when I started my truck, the rpms shot up to 2000, then immediately came back down to 700-800. It was idling very rough, and my "Check Engine" light came on. If I would give it any gas, it would bog down a bit, then pick back up. Again I tried unhooking the plug from the IAC and the truck would stall out.

Anyone have any ideas at all? I'm completely stumped here.

 

If the CEL came on, read out the codes.

 

Well, going by my CEL, I'm getting a Code 23: Throttle position sensor. It's not able to idle for long at all anymore, and will stall out if I try to drive anywhere. Does this sound about right to anyone else?

 

TPS doesn't last forever - if the connector is clean and tight, you prolly need to replace the TPS - it's somewhere around $50-$55 at rockauto.com.