Well my truck is a 1992 F150 with the 5.8L 351w. On start up the truck high idles, then idles down to normal idle. When you first go to hit the gas pedal its like there is a wall there. Once you punch it it will not do that anymore until you shut it off and let it sit and it cools down. Now for the problem, it idles normal UNTIL you push the throttle and "break through the wall" so to speak. Then it high idles and its up and down and awful. If you shut off the truck and turn it back on the idle is a little lower but not much, and Not all the way down (don't have tachometer just know by sound) In stop and go traffic it gets worse, WAY WORSE! Stop start stop start it idles higher and higher, stop and put it in park and it is turning about 2500-3000 rpm. Mechanic told me I needed a good throttle body cleaning and a new throttle cable. Did all that myself today and let me tell you I cleaned the heck out of that thing it was so shiny! But it didn't help nothing. It almost seems like vaccuum is so hard on the throttle sucking it closed (wall) and then after you push the throttle it sucks more air? but never stops? I am so confused I don't know what to do. I will add that my coolant is low because I have a timing cover gasket that needs replaced (just haven't had funds at the moment). The shop told me that the coolant temp sensor isn't reading right because its low on coolant. but if thats so then why does it idle normal til you hit the gas pedal? New IAC, ECT, ACT, and TPS.. Help Please!

 

Sounds like the butterfly is getting stuck. Have you messed with the stop screw on the the throttle body.

 

No I haven't touched it

 

Pull the codes lets see what the computer says

 

If you disconnect the throttle cable off the throttle body, after starting the engine you should be able to reach in there and exercise the throttle body valve by hand. This should help you to identify if its a cable or throttle body problem. Let us know what you find.

 

Could it be possible that a hole in one of my cats (exhaust leak) be causing it to suck to much air (vacuum leak). I know the cats are part of the vacuum system and I do have a hole in one of the cats, I think they are the stock ones with 150k on them. Throttle cable is brand new and Like I said the throttle body is squeaky clean. I have no idea if the previous owner adjusted the throttle screw because it was running like crap when I first got it.. Just trying to give as much info to you guys as possible!

 

Im pretty sure your cats are not the problem turn the stop screw in a little sounds like the butterfly is going back too far and getting hung up the P.O. may have backed it off trying to compensate for the high idle.

 

When I first got my 91 5.8 it would not idle down under 2K.
Primary cause was a leak between the upper and lower intake. When I removed the upper intake I found the bolts loose and the gasket broken in three pieces. about 1/2 of the gasket had shifted restricting the front half of the ports about 25%. I also replaced the IAC. Now from a cold start up it will idle to about 1500RPM for about 30 seconds and then drop to btwn 900 & 1000 in neutral. In the winter it takes longer to idle down that in the summer and if you restart the engien hot it will take just afew seconds to step down.

I would say check this and for any other vacuum leaks.

ToddsSS, Benton, AR

 

Well...what I don't understand is why there is a stiff pedal... we are not talking rocket science here. There is the pedal itself.. the cable. The throttle body and its linkage. So if you disconnect the cable from the throttle body and exercise the linkage is it smooth and moving freely?

 

Throttle moves smooth and freely just stiff pedal in the mornings at cold start up. I have heard about the intake gaskets causing a vacuum leak on the 5.8. Is it hard to remove? The only thing that really confuses me is that if you cold start it and don't touch the throttle it will high idle then idle all the way down to normal. But once you hit the throttle it wont idle back down. That happens when the throttle sticks on cold start.