85 3000 rpm

[nchsinstructor]

I have a 85 mustang, I replaced the engine with a used engine. The idle speed goes to 3000 rpms shortly after starting, stays there until the pedal is tapped then falls to 1000 rpm for a few seconds then right back up there. I have replaced the tps, and idle motor assembly, i have tested the computer and there are no on demand or keep alive codes. cant get it to run the koer running test because after 6 cylinder id the engine goes to 3000 rpm. I do not have a break out box. I dont know how the car ran before because it set for 4 years with a bad engine. it does not matter if the egr is un plugged it does the same thing. also Map seems to be functioning. Any suggestions would be helpful.

Read more: idle on my 95 mustang is 3000 rpm no vacume leak... - JustAnswer

 

[MustangLX-5.0]

is this an 85 or a 95? On that link it says 95. Not much info on the link as it wasn't very descriptive.

Are you sure there are no vacuum leaks? Have you done a vac test with a gauge connected to a manifold vac line? A normal running engine should be within 19 - 23 Hg. Anything lower than that wouls indicate either a vac leak in the manifold or leaky cylinder heads. The reading should fluctuate downward if you tap the throttle but then should return after idle speed remains. Where did you purchase the IAC sensor at? If it was Autozone, then you probably got a defective sensor. Try this as a test to rule the sensor out. Remove the Idle air sensor, then use something solid to plug the holes that remain. Something that wont get sucked into the holes but will prevent any vac from escaping. Start the engine. You may have to adjust the idle screw accordingly. If the engine idles much lower...700 - 1500 then you have a defective IAC. Ignore the codes that result from the removal of the IAC.