Need help. Having trouble with idle on my 96 f150 5.8l 4x4 supercab. If I just touch the throttle it doesn't close up all the way. I have to physically push it closed to get it to idle down. Or if I am inside the truck I have to hit the gas pedal hard to get the the throttle to spring back closed. I have replaced and tested TPS and IAC. Checked vacuum lines and replaced the one to EGR and the black one to canister. Fixed pcv valve line also. I am stumped. I can't remember when exaxlctly it happened as I have been working on the truck. I have put Shorty headers on and a cat back exhaust. My truck doesn't have the air pump or anything like that. Any thoughts would be greatly appreciated as I am frustrated beyond all end. Thanks in advance..

 

My 92 does the same thing. I am thinking maybe wear on the butterfly pivot points but not sure maybe someone here will have a better idea.

 

It only does it with the engine running. When the engine is off it goes right back to the screw even with a light touch. That's what has me baffled. Could I have back pressure or something in the Plenum keeping the throttle from closing?

 

Weak return spring.

 

I pulled the spring around another turn and no change. Just made tougher gas pedal

 

Barring any binding linkage or other such physical deformities, the idle speed is controlled by the computer.

What does it mean when you say you "replaced and tested" the IAC and TPS?

What computer codes are being thrown?

 

Tested voltage on new TPS. replaced Iac because it was filthy. Codes I don't remember right off hand the numbers but one was the EGR code and the other two had to do with ignition. Will post numbers when I get home.

 

The codes will be shown with the most-important one(s) listed first.

Sometimes, a failure upstream will cause problems downstream, so it's a good idea to deal with them one at a time, starting with the one listed first, then clear 'em and begin pulling 'em again.

The TPS is just a mechanical rheostat, and there can be "dead spots" between the two ends.

Any idle-speed issue I've come across has always been IAC- or TPS-related so your case is interesting.

 

P1351 ignition diagnosis monitor circuit input malfunction
P1359 spark output circuit malfunction
p0401 EGR something or other

Don't know for sure what the first two pertain to, but I have EGR disconnected because truck runs like crap with it connected. By disconnected I mean vacuum line off of it and have the line plugged.

 

Go to page 31 here, see if this helps:
http://www.motorcraftservice.com/vdi...f/obdsm963.pdf

This also looks useful:
Motor Service: Pattern Failures

Finally, I found this via Googling "P1359 spark output circuit malfunction" which looks like it might be useful:
https://www.ford-trucks.com/forums/5...351-p1359.html

From what I saw, 1996 F150s/Broncos with the 351W are a One-Year Wonder and have some unique configurations....

 

What causes the IAC to make the idle high?