1990 Mustang GT wont stay idle

[Adam frakes]

i just bought this 1990 mustang GT that would push start because it needed a new flywheel, i replaced that and went to start the engine and couldn't unless i gave it gas. After that it would cold start and come back down and drive fine, it would turn on without use of the gas pedal everytime. But it happened again and i decided to clean the TB and the IAC, then the car would still only start if i gave it gas but it wouldnt survive at idle. So i replaced the IAC and now it still only starts with the gas pedal pressed but it wont come down in RPMs and die like it used to. Im stuck please help

 

[keel]

Pull codes. Even will no CEL illuminated the computer might have stored something.
Could try base idle reset: https://lmr.com/products/how-to-set-mustang-idle

 

[AeroCoupe]

How to pull codes:

Electrical - How To Pull Codes From EEC-IV In 86- 95 5.0 Mustangs

@Noobz347 - The code dump procedure isn't in the sticky - you may want to add it... Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on. Dumping the computer diagnostic codes on 86-95 Mustangs Revised 26-July-2011. Added need to make sure the clutch is...

stangnet.com

How to do a base idle reset (Post #2 but the first post is the holy grail of getting one of these cars running and idling correctly):

Help me create the "Surging Idle Checklist"

Updated 05 July 2022 to add rough idle when cold and smooth idle when up to normal operating temperature. See the EGR tech information, section #15 for more help. Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank...

stangnet.com

 

[NooStang]

What is TPS voltage?

 

[Noobz347]

Voltage from the Throttle Position Sensor. It's the little plastic black thing sticking out of the top of your throttle body. It's got three wires coming out of it.