Having problems with your 1991 Ford F150 ?

High idle when warm

\015 91 4.9L\015\012truck runs normal when first started. After full running temp is reached idle increases to the point that the truck will shutter going into gear from park and will get up to 20mph on its own. Someone has already backed adjuster screw out completly. I blocked off IAC valve, not as bad but still much to high. Blocked off vacum lines one at a time, no difference, sprayed T-Body cleaner on intake gaskets, no change in idle. When decelerating the engine will run up and done two or three times like its not sure what to do?\015 Posted / edited by AnonymousUser on : 17-09-2018

Answers :

The first check you should do to solve this issue is a KOER and KOEO. More commonly known as "pulling codes". I have linked the instructions on how to "pull codes". In a nut-shell you need to use a length of wire or paperclip to "ground" a pin on your self test connector (under the hood, driver side). Then, you get in the truck and turn on the key and count the blinks of the check engine light (CEL). This is called a KOEO (key on, engine off) test. For the KOER (key on, engine running) you will need to warm the vehicle to operating temperature then go through series of "press the brake, turn the wheel, press the OD switch etc..." and it will test all of your sensors. Then, it will blink some codes at you once this process is complete. It might be annoying at first, until you get the steps memorized. However, once you've figured out how it works--it'll be a life-saver. Don't replace a component without out doing a KOEO or KOER, first; or your just wasting money. Hope this helps

http://oldfuelinjection.com/?p=13
Sounds like the throttle posititon sensor
Repair Help & Product Troubleshooting for 1991 Ford F150

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