I've got a problem I haven't been able to fix entirely since I got the car. I've already replaced the air idle control valve, and the map sensor. When the engine is cold it idles high, from 3-4k rpm. Sometimes it tries to sit at 1500, but it sounds a bit wrong before jumping back up. It's not affected by the temperature outside, and starts the same way when it's below 0°F. Once the engine has warmed up enough for the temperature gauge needle to start moving, it idles calmly at 1500. It's the 84 4 speed SE with the 2.5l i4 iron duke

quote Originally posted by Saggy: When the engine is cold it idles high, from 3-4k rpm. Once the engine has warmed up... it idles calmly at 1500.

Your engine is idling much too high when it's warm as well.

I suspect you've either got a large vacuum leak (which would include the idle stop screw being messed with)... and/or your tach is reading much too high.

Have you verified the RPM using something other than the dash tach?

[This message has been edited by Patrick (edited 09-07-2021).]

Had a similar issue with my 88 GT. Ended up being a crack in the EGR tube on the back side so I couldn't see it. I never heard it because the exhaust was so dang loud...after having the muffler patched I was able to isolate the leak. I capped it off at both ends with a cut up can/gasket. Blocking these off made a massive difference. She idles at 900 now. I just have it blocked off while I am waiting for a new egr tube from Rodney Dickmans as she is my daily and I need her running.

I belive the rpm gauge is reading correctly. I'll see if I can find any egr issues. I have done a smoke test, so I know there are no longer any vacuum leaks.

quote Originally posted by Forrest: Had a similar issue with my 88 GT. Ended up being a crack in the EGR tube on the back side so I couldn't see it. I never heard it because the exhaust was so dang loud...after having the muffler patched I was able to isolate the leak. I capped it off at both ends with a cut up can/gasket. Blocking these off made a massive difference. She idles at 900 now. I just have it blocked off while I am waiting for a new egr tube from Rodney Dickmans as she is my daily and I need her running.

I just realized you said 88 gt, I have the 2.5l i4 engine.

quote Originally posted by Patrick: Your engine is idling much too high when it's warm as well. I suspect you've either got a large vacuum leak (which would include the idle stop screw being messed with)... and/or your tach is reading much too high. Have you verified the RPM using something other than the dash tach?

I have not verified it with anything but the engine does sound like it's running at 1500. The rpm gauge seems accurate. I'll see if I can test it here pretty soon. I poked around my vacuum lines, and I have done a smoke test in the past, so I don't think it's a vacuum leak. Are there a pile of sensors I have yet to dive into that could help? I have also noticed I don't seem to have a rev limiter, is that normal?

A high idle is a pretty common issue, and one that has been discussed in this forum many many times over the years. Look over these threads Here and Here to try and narrow your focus.

quote Originally posted by Patrick: A high idle is a pretty common issue, and one that has been discussed in this forum many many times over the years. Look over these threads Here and Here to try and narrow your focus.

That is awesome, I didn't know you could search through old threads. Thank you!