Dave

sounds like you are going to conquer it.did you see the video in your PM box at Y.E.com.this is more or less how it should behave when idling down,even with a cold engine.

DEET

With the throttle body just about wide open, the TPS reads 88%.

I tried adjusting it and it will only go to 18% at rest.

So I completely removed the TPS to see what it would read at rest.

With the TPS completely disconnected from the throttle body, it is reading... 10% open.


I don't know if the TPS is "bad" (although it is new).

I can't make it read any lower.

Dave

adjust your idle screw,hook up a digital VOM to the signal return and V Ref wires (i think,i am very rusty) then adjust the screw on the throttle body.the aftermarket parts these days come out of China and the quality often leaves very much to be desired so the chance of a defect is very high.i would use a genuine Motorcraft part on that one for sure.

DEET

I'm going to measure the voltage next...

DEET

Adjusting the screw won't change anything. When the TPS is completely disconnected from the throttle body it still reads 10%.

Dave

speaking of idle screw,how to set base idle.
unplug the IAC Valve then adjust the idle screw with the engine warm to about 600 rpm,check the TPS voltage and make sure it never goes past .99 volts at closed throttle, .6 to .8 is what you want.

Dave

either have a wiring problem or bad PCM.

one other thing,remove the TPS from the throttle body,plug it into the harness and then check the voltage.it is possible something went wrong when installing it.did you check the voltage at the TPS with a DVOM before?

Dave

here is a video i stumbled across,instead of using body ground for ground use the black wire for the ground as it is the ground from the PCM.the 3 wires on this is the Vref wire,the signal return wire and the ground.you would want to hook up to the green and black wires on a 1990 Mustang.the 351 in your truck even though it is OBD2 is similar to the OBD1 is on the Mustang.OBD is short for On Board Diagnostic.the later ones this capability is enhanced and monitors more functions that OBD1 does.this one is Supercharged and this is why he sets the voltage that high,a naturally aspirated near stock or stock engine does not require this to be set that high.this will cause drivabillity problems done this way though.

DEET

If I unplug the TPS, the scan tool reads 0.0%

Dave

yeah,there is something wrong with the TPS or something went wrong in the installation.with the TPS removed from the throttle body and plugged in into the harness what does it read?

i go by process of elimination and is one of the best ways to diagnose problems i found.parts changing will bite you where it hurts.

good move the way you did there BTW.