Nemesis_5.0

I have a 1993 Mustang GT with a 5.0 which I swapped out engine and transmission with a 1990 GT. Everything has been hooked up fine and when I went to start it for the first time it started up good but when I let off the clutch, the RPM suddenly shot up to 2000 and stays there. As soon as I depress the clutch and hold it down it goes away? Has anybody run into this issue before? Thanks in advance.

mjr46

are all sensors hooked up?? 5 spd or auto?? if 5 spd ...is the nuetral switch hooked up on top corner of the tranny??

Nemesis_5.0

Yes sir all the sensors are hooked up. It is a 5spd and is that sensor the one that's near the top of the tranny close to the gear box? If so then i have read on another forum where a guy disconnected it and it ran smooth with the clutch disengaged, but then it would rev up to 2000 when the clutch was engaged, which is opposite of what it is doing right now. For him though it worked fine he said he just left it disconnected.

aode08

When you adjust the idle with the screw, do you reset the computer?


Anytime you adjust the idle screw, you must unplug the IAC and pull the SPOUT connector out. This locks timing and disables the IAC. Set your idle down to 600-800RPM and then unplug the battery for 30 mins to reset the computer.

Hook everything back up, verify TPS voltage is between 0.5-1.1V (if it's a Ford TPS it should be, aftermarket might require adjustment to the 0.5-1.1 range....you don't need to do that 0.99v BS..all a myth)

Restart car and let idle for 2 mins, and then 2 mins with all accessories on.


If you try to back the idle screw down, all it does it cause the IAC to open more. Usually if you start messing with the TPS and IAC and idle screw without a reset, it causes high idle issues.

Nemesis_5.0

Before i swapped out engines, the replacement engine was running great, no problems at all. The high idle started after i installed it in the '93. Do you think the IAC could have gone bad in between swaps?

Nemesis_5.0

Still having problems with this guys, any new ideas? I have checked the TPS sensor to see what the voltage is and it's reading .60 volts on the green wire. I checked the red wire and am getting around 5 volts. I checked the black wire and I'm getting 2 volts. From what I have read the black wire should be a ground correct?

dawson1112

you should not see any voltage to the black wire. The green wire is the feedback to the eec , the red is the reference from the eec , the black is ground, and is a shared ground with various other sensors. It sounds like you have a short to ground some place.

Nemesis_5.0

Thanks for the reply, I guess next step is to start tracing out wire and checking with a meter. Another thing is, i tried a new tps sensor and then re-checked the green wire and im getting 2.4 volts? Is this correct or could this be something to do with the black wire being shorted?

dawson1112

Your getting 2.4 volts because you need to set the tps voltage by turning it. The tps has slotted holes so you can adjust the output voltage at idle to around .9

Loosen the 2 screws that hold it in place and using your volt meter turn it so your reading drops to .9v and then tighten it back up.

tomdicaraus

According to Transmission Experts the problem is due to Vacuum leak or incorrect throttle/tps settings.