I have a 2000 f150 5.4 the intake went bad i replace it. Everything went well but now the truck does not want to slow down i can be going down the road at high way speed and put the truck in N and the rpm jump 500 then come down then when i put the truck back in drive then it slows down. What could make it that? Please help the mpg has went down a lot. And the idle is about 1100 after i start driving.

 

Possibilities:

Sticky throttle cable or weak return spring
Dirty/sticky throttle body
Large vacuum leak
Sticky IAC

Have you checked the OBD2 for codes?

 

I cleaned the throttle body. But the throttle cable does seem to have some slack how can i make it tighter? And i have spayed starting fuild around the engine and don't find anything but I'm getting ready to do again. No cheack engine light.

 

Are you sure that it isn't acting normally? I believe the throttle is set up so that it will not close quickly for emissions reasons.

 

The throttle is pure mechanical, there's no dashpot to hold it open. The IAC will open to gradually return it to idle, and it should be a lot less than 1100 rpm. SOMETHING is keeping the throttle itself open or the IAC is not working correctly.

 

glc, Thanks for the explanation. When I lift the throttle on my 02 the truck takes forever to slow down. I suppose it is the high gearing.

 

It's the IAC doing that if the throttle is released. As a test, disconnect your IAC and see how fast it slows down.

 

There is way more to it than "high gearing", like what transmission you have and what gear it's in as well as the computer-controlled idle control system.