idling way too high

i have a 2.3L 88 mustang no turbo.

my idle is really high 15-1600 rpm. i followed the directions in the chilton manual which said to warm up the car to operating temperature, park, turn off engine and disconnect the radiator cooling fan and the air bypass control from the electricity, turn on the car, rev the car at 2000 rpm for 2 min, then adjust the idle to the recommended 850 rpm and leave it idling for a bit. turn off the engine reconnect the cooling fan and the air bypass control to the electricity. then turn on the car and the idle should be fixed.

the haynes manual said that trottle body engines dont need to have the idle adjusted

unfortunately the idle does not come down, as soon as the air bypass is reconnected and the engine restarted the idle remains high again at 15-1600rpm once it settles down. at start the car begins at 2000rpm.

i have repeated the process and have even tried it with the cooling fan connected, and again with both the cooling fan and airbypass connected, but each time the idle is not stabilizing when the airbypass is connected. and i have checked for vacuum leaks, and all the hoses look fine.

any suggestions on how to fix the idle would be appreciated.

btw the car is naturally aspirated

Sounds like your having the same problem I'm having. Its your throttle body. The return spring is worn out. I replaced TPS, IAC, checked for vacuum leaks, ect ect before I temp. fixed the problem with a chevy throttle return spring.