I have a 99 F-150, 4 wheel drive, 4.6L engine, manual transmission. It idles at 1800 to 2000 RPM but only when the truck is rolling and is out of gear or between gears. For example, if I get off the highway, take it out of gear and roll down the exit ramp the idle is around 1800 RPM. Once I slow down to below 2 to 3 mph, or stop, the idle immediately drops down to 775/800 RPM.

This is very consistent and happens every time I take it out of gear, such as pulling up to a stop light. Once I am just about stopped the idle then drops down.

The check engine light is not showing. I changed the IAC but it was no help. (I later realized this was a waste of money.) If I drive with the IAC unplugged the idle immediately drops down to 500/600 RPM when out of gear. Obviously the main computer is telling the IAC to keep the idle high.

I checked the TPS voltage output and it was fine. I checked for vacuum leaks but found nothing. I am not sure about the MAF, I did check its resistance and it was not broken.

I did have the ABS light on. This went away after I changed the speed sensor on the rear differential. However, this had no effect on the fast idle.

Is this normal? I have only had the truck for a few months and I honestly can't remember noticing this when I got it but maybe this is normal. It seems like a waste of gas. It can't be good for the clutch either as the engine is racing between shifts. Any suggestions?

I did alot of searching on the forums but didn't find anyone with a similar problem. Thanks in advance for any help or suggestions.

 

There is nothing broke, that's what the computer is programmed to do (part of the dashpot strategy".

Steve