How to Properly Set You Mustang's Idle
This is information on how to set your idle rpm. This covers year models 1986-1993, and 1994-1995. This information is from various sources, like my experience, AFM, the Ford Fuel Injection Book, other site members experiences.
Note if you have an auto trans then you need to set the idle with the car in drive.
Note if you insist on using the idle control ON with the PMS, then you need to go into the PMS and set the idle control ON before setting the idle rpm. Also, you should set the air bypass to 66%, the idle rpm in the pms to 700rpm (the lowest), and the idle timing to +15* as preliminary settings, before actually setting the idle rpm at the throttle body. These are suggestions, each car will vary.
1. The car must be at operating temps. If you can't keep it idling, drive it until it reaches full water temp.
2. Turn ON the headlights.
3. Turn OFF any accessories, like a/c, heater, defrost.
4. Unplug the IAC connector. The IAC is the small cylinder attached to the throttle body.
Note: If you unplug the IAC and the engine stalls, then you know the idle rpm was probably too low. Leave everything as is, just open-up the throttle screw a bunch so that the engine can breathe while you set the idle rpm. Restart the car, and continue with #5 below:
5. Adjust the throttle stop screw until you have the idle rpm you desire.
6. Plug the IAC connector back in.
7. Recheck the TPS voltage, since you may have changed the throttle blade position.
Make sure the TPS is right
The TPS is another issue. Generally, you want to try and get it as close to 0.97-0.98 at idle as possible. Make sure that when you open the throttle all the way it hits 4.60 volts. Now, with the advent of the 04 PMS, you can "fudge" with those numbers. If you cannot get the TPS to read under 1.00, for example, you can set the PMS to tell the EEC it is seeing idle at 1.01. You should always try to get the idle reading below 0.99, but if you cannot, then this feature is really useful. The same is true for WOT, you can use the TPS option in the PMS to set the WOT tps voltage. Say you cannot get it to read 4.6, or you want earlier WOT tuning to take effect. Then you can set the WOT to say, 4.0 volts at 30% error. There are more detailed threads on this.
Begin with a cold vehicle. The idea here is to get the car to a firm cold idle with enough air bleed capacity left in the idle circuit for IAC adjustment.
The idle stop should be set first. Back out the idle stop screw, away from the bell crank arm, until about 1/2 turn past the point where it no longer makes contact (blade fully closed). Using an 0.010" feeler guage, tighten until gauge just drags between screw and bell crank arm. Remove feeler gauge. Tighten screw exactly 1 1/2 turns. If the screw is very loose, put a drop of loc-tite or silicone on it, so it doesn't work out of adjustment.
Now remove the connector to the Idle Air Controller (IAC) just on the other side of the throttle body. Start the car and allow vehicle to warm for 2 minutes. Give a small "blip" to let it settle. If it is having a hard time staying running you may have to get an assistant until you can get to the front of the car. Now open or close the air bleed screw (CCW opens) next to the IAC until the car idles at 575 to 600 rpm. For guys with aftermarket cams and an EEC tuner, you might want to idle a bit more briskly, say 650 to 675.
Obviously, this rpm range is by what the car and driver wants...ie, no set idle speed, whatever works for YOU.
Turn off the car. Now count the number of turns clockwise to close on the idle air bleed screw. If it falls between 1/2 and 2, it's okay, now reverse it out the same number of turns. Log the number somewhere in case you need it for the future. Reconnect the IAC. You are done.
If the air bleed screw is above 2 turns, it's a good idea to tighten the idle stop screw another 1/2 turn, then repeat the idle setting. If it is below 1/2 turn, then loosen the idle stop screw by 1/4 to 1/2 a turn, and repeat the idle setting. Be sure to put another drop of silicone RTV on the stop screw if it was disturbed. Reconnect the IAC. You are done.
Another variation for the 94-95 cars is to reset the idle in the procedure listed above and get the car to idle as low as possible with idle control OFF and the IAC unplugged. (around 600-650 rpms). Then plug the IAC back in, turn off the engine and unhook the negative battery post for 20 minutes. Then hook everything back up and put the desired settings in the PMS for idle control ON. Start with the percentage around 40% in the PMS air bypass setting and work from there. Once you set the desired idle RPM in the PMS, save and start the car. After a minute or two it should be able to control the idle.
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