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The computer, harness and engine are from the 89. Its has a T18 4spd trans.
Before switching the IAC, the truck would start a little slow and idle around 750 but had a bad lope. It would eventually smooth out as it warmed up. After putting the 89 IAC on, it start right up and idled smooth around 1200, but the idle increased instead of decreased. After it warmed up, unplugging the IAC brought the idle down around 750-1000 but still smooth. I talked to the guy that put the engine in he did the swap in 2017. He drove the truck and it ran fine. there has been 1 other owner since then. The TPS was replaced in July of 2020. THe plugs, wires and cap/rotor are all new.
I use the Innova 3145 for these early Fords with OBD.
I am concerned that you do not have the computer you think you do. The best way to know for sure is to pull it out and check the numbers on the label. You can post a picture of the label and folks on the forum can tell you what you have.
Ok The temperature finally went above freezing and started melting the nasty white stuff, turning everything to mud. I haven't been able to look at the computer. I did unplug the TPS and put a meter on it. I got about 4500 OHMS closed and 780 wide open. I did make sure there was 5 volt with the key on also. I have read other posts about back probing it still plugged in but that will have to be another day. Is this an accurate way to check the TPS? These readings are out of range if I can believe the other posts I've read. Could it be out of adjustment or bad? The previous owner had it replaced but I'm sure it was an aftermarket and it was put on at Tire Discounters so who knows if it was put on correctly. Thoughts?
Back probing is the way to go to see what is going on with parts hooked up when you don't have a break out box, which most folks don't. The back of pretty much all the connectors where the wire enters the connector, is a soft rubber grommet. Be very careful either way, don't damage the grommet, and don't open up the contacts when probing from the front.
The TPS can go bad. You can test some things with a meter, resistance and voltage signal. An analog meter can help you to see a clean signal sweep range. I don't have the specs in front of me, but RLA or Subford could help you with the normal specs.
They can be installed incorrectly, mostly with the tangs not properly aligned to the throttle body shaft. on the 5.0 and 5.8 the best thing is to remove the whole throttle body so you can see that everything is installed and working the way it should.
They can be adjusted a taste usually, but it shouldn't be necessary as long as the throttle body stop screw has not moved from the stock position.
I back probed the TPS. I have 1.0V closed and 4.4V wide open. The voltage transition was smooth and didn't change as I tapped on the TPS. So the OHMS check out of spec, the voltage seems to be right at the limits. I have a brand new Motorcraft TPS on hand. For under $50, is it worth changing? I also found another potential issue. Someone mentioned the screw on the throttle body. I was looking to see if it showed any sign of being moved and found out you can turn it easily with your fingers so I have to believe it has been moved. Is there supposed to be something on it to lock it in place?
The set screw is to ensure the throttle blades do not close too far and get stuck in the bore. The factory typically put Loctite or similar on it to discourage tampering. If the new TPS nets no change and you feel someone has messed with it we can walk you through the process to reset it. It is not uncommon for it to be tampered with in an attempt to correct idle/stall issues.
Is it a matter of setting it so that the voltage is correct in the throttle closed position?
No. The throttle stop screw needs to be adjusted so the throttlebody blades do not close too far in the bore and jam. I tend to do this visually. There is a Ford TSB out there that has you adjust the screw to a base idle, IAC unplugged, ~500 RPM for a V8. That may get you by but if there is a vacuum leak you will be trying to adjust it out which will result in the blades closing too far which results in them jamming in the bore.
UPDATE...I know its been some time but here is the current status of this old truck. The IAC wasn't working. When you unplugged it while its running, nothing happened. It has been replaced with a Motorcraft IAC. The TPS is also a new Motorcraft and the voltage checks in range now when back probed. The truck still idled around 1200 when starting and then climes to 1900 after a few minutes. I got my hands on a throttle body from another truck. The screw shows no sign of tampering. It made absolutely no difference. I checked the computer and it its from a 1992 truck with a 5.8 / manual transmission, not from an 89 like the previous owner had said. There is no sign of a vacuum leak. The intake had a little coolant leak so I put all new gaskets on the top. About the only thing left that I can think of is either the ECM or the harness. I'm looking for a pin out chart to see if a harness for a 4.9 is different than a 5.8. I know the injector harness was changed but I doubt they swapped the whole harness. Any other ideas? I'm running out of things to check. I haven't driven it in a while but there is no check engine light from idling.
Thanks
Unplug the IAC, does the idle RPM drop to ~400-500 RPM or the engine die with the new IAC?
If the idle RPM drops as described above something electronic is causing the computer to increase the idle RPM. Could be a faulty engine temperature or air charge temperature sensor or simply a bad computer. Has Code 126 been resolved? If not that can be another cause of your issues.
The new IAC appears to be working. Idle drops when unplugged. Air charge sensor is new. 126 code came after driving. No check engine light since new intake gaskets. Another MAP sensor was plugged in and made no difference. I'm thinking it is the computer or the harness. I am trying to find the difference in a 4.9 harness and a 5.8 harness. I am looking for another ecm to plug in to see if it goes away.
Pull out the computer then open it up to look for signs of leaking capacitors as well. I do not have a diagram for connector C101 for a 1992 model year truck. Many of the signals and wiring from the computer to the engine go through that connector. You could compare the diagrams of the engine controls for V8 versus 6 cylinder trucks to see if there is a mismatch on pin-outs.
