Run away idle

junkyardjeff · August 09, 2019, 10:18:48 PM

I have a 95 F150 with a 5.8 that the idle has been gradually getting higher and the shift points changing and now will not shift ,code scanner said TPS not in range so I plugged in another and still the same but no CEL. Had to disconnect the IAC so I could move it so where do I start looking.

kb426 · August 10, 2019, 06:49:01 AM

If the iac is real old, you may find the plunger coated with carbon. Cleaning it could help. If the iac is wore out, there is a slim chance that it isn't moving properly. Those are stepper motors that move very small amounts and very accurately when working properly. Next on the list is a vacuum leak somewhere. The tps could have an effect on the trans not shifting correctly. If you take a dc voltmeter and use 2 small needles for probes, by checking the right 2 of the three wires, with the key on and engine off, you should have right at 1 volt. If you have more than that, the tps still could be having a problem. Is there a chance that the idle screw on the throttle body has come loose and run in from it's original position?

kb426 · August 10, 2019, 06:50:27 AM

If the iac is real old, you may find the plunger coated with carbon. Cleaning it could help. If the iac is wore out, there is a slim chance that it isn't moving properly. Those are stepper motors that move very small amounts and very accurately when working properly. Next on the list is a vacuum leak somewhere. The tps could have an effect on the trans not shifting correctly. If you take a dc voltmeter and use 2 small needles for probes, by checking the right 2 of the three wires, with the key on and engine off, you should have right at 1 volt. If you have more than that, the tps still could be having a problem. Is there a chance that the idle screw on the throttle body has come loose and run in from it's original position? You need to disconnect the battery for a while after changing sensors so the ecu can reset also.

junkyardjeff · August 10, 2019, 08:18:03 AM

This truck sits alot so I will check for chewed wires,it seemed to be worse after I moved the wires from the TPS.

junkyardjeff · August 10, 2019, 08:34:52 AM

I would like to get a wiring diagram and shop manuals but those on Ebay selling them think they are gold,this will probably be my last vehicle with a computer controlled trans and will be going back to one with a carb.

junkyardjeff · August 10, 2019, 03:14:50 PM

Disconnected TPS and idle came down.

kb426 · August 10, 2019, 06:42:17 PM

I'm reasonably sure that your truck is eec iv. This is pre OBD2. The code reader you are using doesn't connect to newer obd2 connections? You can also unplug the iac and see if the engine dies when warm. Don't be afraid to replace wiring to the tps if you can consistently wiggly the wiring and get erratic results. Do you see any signs of animal damage anywhere before the wiring goes into the larger loom? Have you crawled under the truck to check the wiring to the trans? Maybe an animal chewed up the wiring to the trans and has shorted across 2 wires. I don't know everything about eod trans but I'm having trouble thinking the tps would stop the trans from shifting. I think you need to look a little further. Don't be frustrated. When you get past the learning, you will be more than happy with efi.

junkyardjeff · August 10, 2019, 10:55:02 PM

No animal damage could be found,I found a second TPS in the garage and hooked it up and a normal idle. The one TPS came off a engine out of a wrecked truck and now might have found the cause of the wreck as the idle with that TPS was higher then the one that is on my truck. New sensors are going on tomorrow morning,the TPS and the used coolant sensor that I put on in the spring will be replaced.

junkyardjeff · August 14, 2019, 11:03:02 PM

TPS was the problem.

kb426 · August 15, 2019, 08:43:27 AM

Congratulations.