I know this topic has been covered I just wanted a few more opinions. I am working on a 92 F150 w/ 5.0 high idle has reared its head again. I was able to pull the DTC codes last night and got a 23 and 31. From the info I have found so far the 23 is TPS sensor out of range, that is the only sensor we have not replaced and I will replace it. The 31 is related to EVP, I understand the canister purge probably isn't working but that shouldn't affect how it runs correct?

Thanks for any insight.

BTW the IAC is getting full voltage all the time and the IAC is new, just took it apart last night and cleaned it even though it wasn't dirty.

 

The DTC for the TPS can cause a high idle. Only thought's on that is if the idle set screw has been adjusted from it's calibrated point or else the TPS is faulty. Backprobe the green wire on the TPS and check to see if you are within .80-1.0 volts. I have seen some mention of it being as low as .56 and properly run. If, with key in ON/START position, it is above 1.0 volts or lower than .50, then it's either faulty or need to set the idle screw properly. Only adjust the screw if it has been adjusted from it's mark from the factory. No adjustment should be necessary if it hasn't been tampered with. While backprobing, watch for a smooth and steady increase in voltage from low to high. It shouldn't be lower than 3+ volts at WOT.

Best way to backprobe that wire is simply use a seamstress pin to lightly pierce the wire. When finished, be sure to put a dab of silicone on the puncture to prevent moisture from getting through and into the circuit.

Note: You can run the black lead of your DVOM to the neg battery post and use the red lead to test the circuit with the pin.

Note: A steady increase in voltage should be present when actuating the throttle linkage from resting position of the throttle plate to WOT.

 

Thanks for the very thorough explanation. I had read about checking the voltage but not which to probe. I will check that out asap.

Thanks again

 

Ford started to transition to 3 digit codes in 1991. Your 1992 could very well display 3 digit codes. How did you retrieve them?

 

I retrieved the codes by using the jumper method and counted the MIL flashes. I read that the trucks with AOD tranny still had two digit codes, this truck has an AOD.

 

My truck has the 2-digit as well. But RLA noted a good point. Another member posted about a 93 with 2-digit codes but his was really 3.

 

That is the extra tidbit of information I was looking for. Generally speaking the trucks with E4ODs were the first to start using 3 digit codes, but there are exceptions. I always questions someone who state they have 2 digit codes on trucks of this era. More than likely yours is displaying 2 digit codes.

 

I also have a 92 F150, with the 5.0 L, and E4OD that had an intermittent high idle issue. Finally got around to pulling codes, and got a consistent 121 (TPS out of range). Using my DMM, found that my green wire on the TPS was at 2.02 VDC closed. Also found that when actuating the throttle plate, voltage increase on green output wire was smooth, except that it consistently would go from 3.9 VDC to just a "blip" of 6.0 and then right back to 4.0, and smoothly up to 4.76.

Went to AutoZone and bought one of their "Limited Life Time" TPS for $36.99. Removed the whole TB and installed the new TPS. It reads .89 VDC closed, and smoothly up to 4.76.

Disconnected the battery for 15 minutes + after install, and then took truck for test/re-learn drive.

WOW! While the truck always ran well, (except for idle sometimes, though not often, being high), but had always wondered a bit about transmission shift points being higher than I've read. Now idle is smooth. On cold start, idles up for short time, then drops to normal range. Shift points are now as I expected them, including going into OD around 40 mph depending on throttle input obviously. No CEL now. I'll pull codes again in a couple of days of running the truck.

I hope this may help you with your high idle. I had cleaned IAC, checked for VAC leaks, etc, without results. The TPS was the fix. Hardest part of changing it was getting the plug on the old one disconnected, and dealing with the coolant lines going to the TB. By coincidence, just did this today. I'm a happy man now!Good Luck with your problem!

 

Awesome results! Good to hear the news!

 

Thanks for the reply, I hope to install the TPS on my truck today or tomorrow.

 

I installed the new TPS last night, took less than an hour even though one of the throttle body bolts would not come out. I did not check the voltage since the TPS is not adjustable without modification and I didn't have time to do that. It fired right up and idles nice now. It doesn't have a tach but I would guess it idles at about 700RPM.

 

Congratulations! Glad to know it fixed your problem.

 

The truck runs much better but gas mileage is much worse, it dropped from 15mpg to 12mpg, any idea why this could be? I did get new tires but that is a big difference.

 

Stock height and width tires?

 

They are 31-10.50s but I accounted for the difference in height in the mileage calculation. We will see what the next tank does, maybe it was an anomaly. I was really impressed witht the 15 it was getting, that could have been because it was running too lean all the time.