I was driving on the highway for about 3 hrs. and I pulled up to a stop light and I had to hit the brakes harder than normal. I didn't think much of it then I looked down and my rpm's were still holding at about 1,000 or a little higher. I can still feel the truck trying to push then all of a sudden I heard a loud clunk and then the rpm's dropped and it ran normal from then on. Then about a day later I got off the highway and after slowing down I felt the same thing next thing I know without pushing the gas pedal or turning on the cruise it held speed at 30 miles an hour at about 1200 rpm. I have no idea what the problem is and I have no code. If someone can help it would be greatly appreciated!!!!

 

I drive a 2000 f150 4x4 4.6L

 

I've got two guesses. Neither may be related. First, your idle air controller acts like a throttle follower, commanding a higher idle the faster you go, when off idle. This allows the IAC to catch sudden decelleration and let the engine down gently to avoid stalls. Guess 1 is that it stuck at the position it was following your highway speed. Then clunk was when it unstuck and dropped back to where it was commanded. I would guess that it would be commanded closed to slow the idle and would have almost stalled when it came down. Did it?

My other guess is that the cruise control is commanding the higher speed and the brake switch is busted and didn't kick it off. Do your brake lights work?

Let us know.
Mark

 

No it really didn't stall or try to and yes the brake lights are working.

 

If the brake lights work, I'd bet on the IAC sticking open.

 

a loud clunk? i mean i can see the problem being an IAC, but the clunk? naw... not to my knowledge... the IAC uses 2 holes which act as a straigh pipe of air, that is constricted by a plastic/rubber plug. When that moves in to close it, the idle drops, when it opens, the idle rises... as for it being loud when it closes? i dont think so... because the only time it would close completely is when the motor is in full acceleration... if it does it at idle, the motor will cut out... and die... but the sound of it closing should never be heard inside the car... maybe when looking at it under the hood, but i cant see it being heard inside as a "loud clunk" just me opinion though ever looked inside one?

 

jrd954;

My 99 5.4 has done this a couple times on long high speed highway runs usually when I drive it from full to empty without stopping.
It seems like when you try to stop or need to slow to like 25 mph it still wants to be going 30 to 35 mph. Mine usually will stop this behavior within a few miles of slower driving. Kind of like it forgot how to idle normal, best I can put it.
I have never had any codes or problems with this other then when I have taken a couple long hauls out west and spend extended periods at high speed.

I will have to watch this thread for some possible solutions as I havent determined a cause as of yet.

Sled...

 

just a thought, when I worked at the Ford dealer we did a bunch of cruise cable replacement recalls. That was in '99 but we went through a lot of cables. might be worth looking into?! I dont know if they resolved it quickly on succeding model years or not? The recall had to do with the cable binding and causing the rpm to stay high or not drop at all.

 

I had the same problem with my truck it wouldnt idle i fixed it i locked a pair of needle nose vice grips on the throttle cable right where it comes out thru the floor board, i pulled out the cable just enough to make it idle, and used the vice grips on it as a stop, pretty smart of me? huh? i told you guys im an expert on everything