Long time lurker, first time poster. This website has been so helpful with little things here and there I've done to my Ex over the years, but I finally ran into a problem I can't exactly solve.

So, I'm not new to wrenching, but I know where there is gaps in my knowledge and one of those is pretty much anything with the voodoo magic we humans call electrical. I know it either works or it doesn't but trying to narrow down electrical concerns has always been a shortcoming of mine. I can connect wire from here to there and know enough to not kill myself or let the smoke out (most of the time). With that said, I acquired a new to me, 2nd Ex, (V10, 4wd, '03) about a month ago and took care of all of the low hanging mechanical issues, but one that has been on the back of my mind is how high the truck revs at start up cold or hot. Cold im used to, but when its hot it still revs up over 2000-2500 and high idles for 10 or 15 seconds before settling to \~800rpm. My first thought was maybe the IAC, but it doesn't show any other symptoms of a failing valve. Then while driving I noticed that it shifts a little more firm, and at higher speeds than my 2000 v10. Additionally when coasting down a hill it seems to not want to go into fuel cut off. And randomly it will actually accelerate down hill. Then coming to stops it will down shift and pull ever so slightly. Once stopped it will pull like im giving it throttle and then after maybe 3 seconds go back to a normal idle. That last bit is very random. Fast forward to last weekend and I installed an android head unit and finally got the torque app running. I have it connected via USB (same dongel I used for forscan). While I havent monitored it with forscan yet (need go renew the license), I noticed a few things that maybe made me point towards the TPS. First was the instantaneous mpg gauge and current fuel consumption through torque. While I know this is mostly unreliable, while coasting down hill at around 55-60 mph, the mpg gauge only goes to about 35mpg and fuel consumption shows about 1.9 gallons per hour. My assumption is that under fuel cut off coasting down a hill that the mpg should be maxing the gauge and the consumption should be all but zero. So I look at my TPS gauge and it reads about 21.5% at idle and 94% at wot (engine off) and my short/long term fuel trims are trying to lean things out to the tune of -5-7%. While this all seems normal, under any throttle while accelerating or cruising the fuel trims are near 0 where I expect them. Since TPS's are cheap, i replaced it yesterday and noticed i have to rotate it before i install it, like im preloading it. As far as I know the truck has no tune, but it has a k&n kit on it and previous owners have installed everything from subs to car alarms and remote starters so who knows what else they've messed with.. i checked the throttle butterflies and cleaned them. There is no binding and the plates fully close on the stop set screw, which does to look like its been messed with.

Do any of you think im on to something? Where can I start chasing issues? Everything seems to be functioning as designed and there are no real driveability issues outside the very random pulling under braking/decel. This one kind of has me stumped and I don't want to go messing with that idle set screw. I guess I could swap the throttle body from my 2000 to test, but I hate to caniblize my 00' if there might be something obvious to someone here. Sorry for the long read. Appreciate everything.

 

the early 2V motors used to have an issue where the IAC and TPS would essentially get dirty and hang up. It was talked about in the Mustang forums a ton back in the day. Basically clean and ensure the gasket wasn't leaking. I remember people doing small mods as well, but it's been 15+ years since and I don't remember clearly these days lol.

also, by chance is your AC defaulting to the defrost position? These engines are extremely prone to vacuum leaks. And a decent vacuum leak will cause issues like you describe. Maybe do a tune-up as well. I had a misfire that wouldn't trigger CEL, drove me crazy and caused erratic trans issues for almost a year until I got lucky and the parts cannon found the bad plug

 

I am having similar issue on my '91 Ranger. I replaced IAC and TPS. Ihave yet to get the idle below 1200. I have checked the TPS voltage at idle and mine is over 1.0V which is why it is high. I believe the TPS should not go beyond 0.9V at idle. I did make the mounting holes into slots to give me some adjustment but it isnt enough. Use a multi meter and connect the ground and use the red with a pin to probe the (3) wires. One should be 5V and one should be .4-.9V which would be the Signal back to the computer. I did notice on my TPS that the teeth that catch on the throttle lever could be on wrong side meaning they might need to be engaged and turned as installed to get the output right.

 

I would suspect the IAC if you are getting high RPM every time the throttlle plate is closed (foot off the gas pedal). Note the IAC must operate properly at all engine temperatures, not just cold start. IACs are easier to replace than clean (I made mine worse by cleaning). IAC is controlled by by a duty cycle signal and can't be tested with normal tools.