Greetings!
Firstly, the vast quantities of knowledge available on this forum have been wildly helpful, thank you. Decided to join and contribute now that, as Skynard said, "I know a little." I'm currently experiencing a headache with my 1989 F150 5.0 M5OD 2WD. I will do my best to describe the issue, and hopefully you can identify something I'm missing. I hate to write an essay here, but I've already traced the usual suspects and want you to have the information I already have.

The problem: Truck will rev itself up to about 2500-3000 rpm and stay there, as if the throttle is depressed. This will persist until the truck is turned off, and restarted. It will then take between 2-10 mins to re-exhibit this behavior.

Info/symptoms: I purchased this vehicle from a mechanic who told me sometimes it idled high and "All it needs is a new IAC." The truck was cheap, low miles, low rot (for a NJ truck) and came with a new clutch kit installed. It starts like a new truck and doesn't look half bad. I test drove the truck pre-purchase with the IAC unplugged and it ran half okay. During my first week of ownership, I plugged in the "bad" IAC and the truck ran great during my small commute to work (-10 miles). Then one day as I was pulling into a gas station it began its self rev high idle. I unplugged the IAC at the station, drove up the block to a parts store and picked up a new IAC and a battery. Got the truck home, scoured the forums for test procedures for the IAC. When removed, the IAC looked clean and opened and closed freely with a pick. I multi-metered 20.5kOHM out of it though, and from what I've read on here, it should read between 7-14kohm. New IAC read 10.5kohm. Installed the IAC/Battery, cycled the headlights / brake pedal / ignition a few times, let it sit for 30 mins, the whole magic procedure. 3 days went by and it did it again, no codes.
Didn't want to break out the parts cannon again so tested the TPS next. 5V in, good ground, but reference voltage read .2v closed. Bad TPS, cool. Since I was taking the throttle body off for this, (which I ensured was clean and that the butterflies moved smoothly) I also snagged a new coolant temperature sender (on the hope that maybe it was faulty and telling the computer to raise rpms for cold start procedure?) Installed both,"clocked" the TPS to .97v reference, (though I've read that anything between .6-1v is also okay?) did the magic battery procedure to let it relearn sensors. Ran even better than it did with only new IAC. Noticed about this time that the plastic shaft on the throttle cable was binding on the spring above the throttle body, old yellow plastic had cracked. Could be a contributor, so replaced the throttle cable, noticed when hooking it up to gas pedal arm the ferrule on the cable would get caught on the rubber floor liner, great, trimmed that back a few inches and the throttle cable feels perfectly smooth when depressed and released via the pedal in the cab.
Drove it for a day and it revved itself up again. $%#&. Got home, tested my TPS and IAC again. Within spec. Jumping the eec connector for codes flashes a code 11, the Innova obd1 reader recommended here reads out a code "10 or 11". All systems okay. I consulted the Haynes manual, PCV maybe? Sure that's fine, threw one of those at the truck too. Still revs high on its own. For Christmas week I had no time to work on the truck and no choice but to drive it to work and back. I noticed that right before it high idles, it will drop about 100rpm, (ex. slowing down to a red light, rpm at 1500, clutch in to neutral, coasting idle at 900, idle drops to about 750-800, check engine light flashes once and then goes away, idle returns to 900 and then over the course of about 3-4 seconds idle will climb up to about 2500-3000.) Turn the truck off, start it back up and I'd get a few more minutes of driving before it did it again, the same every time, rpm drop, check engine blip, high idle. If I do not turn the truck off, it will stay at 3k rpm. Tapping the throttle does not bring the idle back to normal, it will go up to 3500-4k and then return to 3k until restarted. (In review, I realize I have neglected to mention I also tested fuel pressure at the rail at normal idle, and I used a smoke machine / starter fluid method to check around the engine bay for vacuum leaks, the lines look goof, no noticeable leaks, and fuel pressure is good aswell.)

Current theories/plan:The check engine flash / rpm drop has me thinking that the computer is losing power temporarily, logging a new baseline for tps idle voltage, then opening the IAC bringing the rpm up. Perhaps? Its the best I've got right now... I have the next three days off of work so I plan on testing/checking all the grounds, ensuring continuity from one end of each ground wire to its destination, pulling the computer to check pins on plug for corrosion, open the case to check for leaky capacitors, and testing the eec and fuel pump relays. The wiring on the truck is old, but un-molested and clean. Terminals on the battery are good, as is the battery itself. Most of everything I've read about failing computers presents symptoms near opposite to the ones I am experiencing. Odd huh? Unfortunately, its 30 degrees here on the coast and the wind is 20+mph right now, I much prefer chasing electrical headaches when its warm and sunny, ha.

Thank you very kindly for reading this post, and in advance for any advice you may have. Subford, if you're out there, your posts and comments have been instrumental in getting this far, thank you! I am under the impression I have all my bases covered, anything I might be missing? Cheers.

 

A picture of the truck for those interested aswell.

 

I would take a look at the computer capacitors anyways if it were me. You’ve checked all grounds?

She does look clean!

 

Well all the grounds are good, pulled the computer to find a leaky capacitor. Gonna throw a computer at it now and see what that does for the truck, I'll update this again when I get the replacement computer installed. Keeping my fingers crossed that ends up being my solution..

 

That’s been leaking for a minute! Hopefully it will solve the problem, can’t hurt!

 

So.
New computer is installed. Reman unit from Crock-auto. Cracked it open and the "Reman" is just the replacement of the bad caps.. The rest of the computer is identical to my factory unit. Thinking about keeping it and eating the core charge to swap the caps out myself and then maintain the original unit. Dunno yet.

First drive was great! Until it wasn't. Did the same thing, idle skyrockets up to about 3k. Damn. This time though I watched it log and save codes via the Mil.

Pulled em. Codes are
#29, VSS problem.
#33, EGR did not open/ did not open intermittently
#63, TPS low voltage.

Sonofa. So I went and tested the TPS AGAIN. 5V in, .97v reference, tested on the harness for the TPS, not using battery for ground. Smooth sweep to 4.5v WOT
What am I missing here?? Could the signal reference wire from TPS be shorting intermittently somewhere between the TPS and the EEC? Do I have a TPS that tests good cold, hot, but then malfunctions midway through my driving??? Am I barking up the wrong tree and is my EGR somehow blasting my idle to the moon? Any ideas welcome.
Gonna go have a beer. 11 am, have all day to poke this bear.
I have ANOTHER spare TPS I'm going to throw into the truck.

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"Remanufactured"

 

Thats rough. Your making good progress though. Learning more about the rig. The reman could've been fine before you installed it and it could be getting to much voltage over feeding the computer making that capacitor explode or it was an older reman and it was weak. You might need to do a power check at all points on the computer test if anything is getting over fed or under fed leading to wiring or charging system. I've never had issues so far because I haven't driven my 90 enough to get an issue like this.

 

Board with the black cap is a tested good "Reman" unit before I installed it. Board in my December postings is the "bad" unit with the original blown capacitors. The new computer did fix the issue I had where I wasn't getting any codes back and it wasn't logging any. I'm very hesitant to think it's a bad computer considering how much better the truck feels running and driving before it high idles.

 

Try checking for Vacuum leaks. Your EGR could be leaking out of its plunger very common for older EGR's to leak. And those old lines like to crack and split. Try using a smoke machine to check for leaks in the system.

 

Ooooo there's one. Crack in EGR leaks air into the intake intermittently as plunger opens?
Turning the EGR into a big IAC. I'll poke around that some, thank you!

 

I've seen it happen out of some Youtube videos like junkyard digs with his late 90's 5.8 when he smoke checked his manifold the EGR was pouring out the holes. But there still is a code for electrical for your TPS and VSS could be sending something to the computer telling it to raise to idle in order to not stall the engine. Speed sensor check that on the transfer case or trans if it is a 4wd should be on the transfer case. The low voltage could be due to charging system acting up check what voltage the battery is while running. You might still have some gremlins lurking around that rig. But the EGR could be leaking and lines. And your brake booster could be leaking as well. If you unplug the line to the booster and plug it with your finger and it idles down then your booster is bad.

 

If you don’t have access to a smoke machine, you can use something as thin as a coke
can, cut a piece, and make a temp “block off plate between the EGR and the intake. If it doesn’t do it again, found that problem! As for the tps, did you have the battery disconnected while redoing the computer?

That replacement cap area looks suspect. I would ohm it to make sure it’s good. Inspect the entire board!

 

Okay, back in the house and warming up for a few minuets. I have re-tested and re-checked almost everything. I cut my "egr block-off plate" ala 90project5.0's advice out of an Arizona can, not a coke, so perhaps that is playing into effect but absolutely zero change with the egr removed from the situation. Still will skyrocket idle for no reason. I tested the TPS again KOEOff, 5v VREF, .97v signal, KOERunning I got the same .97v signal but 4.05v VREF. What's up with that voltage? I have one of those Bluetooth adapter cig-lighter plug-ins for my radio, it displays Voltage too, truck always charges while driving at 13.9-14.4V depending on additional load, headlights, wipers, heat, etc. While idling "normally" between 7-900 rpms I tested voltage at the battery with no electrical loads and got 14.4v. Charging system good. I wiggled the daylights out of any harness, connector, vacuum line I could get my hands on aswell and observed no changes. I do not have a smoke tester yet, but I again went around the motor and sprayed some starting fluid on every vacuum line, connection, and intake-related-gasket I could find. No change in idle rpms. I unplugged the fat vacuum line to the brake booster as dentside lover advised, rpms raised up to about 1500, but when I plugged the line with my thumb they dropped back down to proper idle. So, booster is functioning as it should?
I re-tested the IAC for resistance and found it to be exactly where it was when I installed it at 10.5kOHM, I do not have a power probe to watch the shaft open and close but I can still move it with a screwdriver as freely as I could when I installed it last month.
So frustrated, I continue to try and google these symptoms and keep getting "Oh you've got a bad TPS, you didn't clock it to the right voltage when you installed it" etc etc. Its testing within specs!! I do not know what else I could possibly be missing. I have one more brand new TPS in the garage I stocked up on just in case that I will install, and clock to .97v again. I really hate throwing a part at it and hoping it fixes it when all my testing is confirming the individual part is a non-issue. The code calls for TPS low voltage, but my meter says all is well? I'm trying to find a wiring diagram to determine which connector the TPS runs through before it meets the main harness for the EEC. Maybe loose/corroded connection there?? If you know where that connector is, or if you can share me a beautiful diagram to assist please let me know.
Its getting dark here, Im running out of AAA's for the headlamp

 

I would clock it closer to .9 if it were me. Take that as you want. Did you remove the probes and place them back and verify your ground probe was strong. The computer is what sends that voltage out, but everything that gets the 5v reference uses that same wire I believe! A lot of tracing… Did you wash the tea off the can first?? Joking! It did its job in telling you that the EGR didn’t cause that to happen. So it’s most likely the IAC. Not saying it’s faulty, but the computer commands it to do its job. Not saying the computer is bad, but it’s possible. Something in that loop is the suspect. You’re getting EGR codes, so there’s a problem obviously, but it’s not for that I don’t believe. I don’t think any signal from it tells the computer anything that would affect the IAC. If I’m wrong, I’ll find out soon.