The idle for any 1991-94 Mercury Capri (turbo or not) should be between 800-900 RPM. with a cold engine, the RPM runs at 1500 or so, until the engine warms up. Within several minutes, it should begin to drop to 1000 and finally settle between 800-900.
While the car computer does control it to some extent, one can make manual physical adjustments to drop or raise the rpm to
the correct spec. The following all are places to modify in small increments to see how your engine reacts RPM wise.
1. The throttle cable should have flex when you press down with your finger on it. The deflection should be between 1-3mm. A tight cable with no flex when pressed is too tight. To make the cable adjustment loosen the lock nut (A) and throttle nut (B). Insert something that is no wider than 3mm between (B) and the metal support holding the cable. Tighten (B) until snug to the 3mm device. Hold the (B) nut in place as you tighten (A), the lock nut, which will lock the setting when tightened. This should create a 1-3mm longer cable so there is the same amount of flex.
A tight cable will prevent the lever that touches the Dashpot from being pushed up, which reduces idle. When the engine is hot and your idle fails to drop to 800-900, the throttle cable may be too tight.
2. The Air Adjustment Screw is like the idle screw on cars with carburators. When you turn it clockwise, less air is allowed into the throttle area on idle. Less air means lower RPMs at some point until the engine chokes. Turning it counter-clockwise has the opposite impact generally to a point.
When using this screw, make sure the engine is at 175+F, and turn in small increments to see what the RPMs do. Always remember the original setting to fall back to should things go weird. Note that whenever the car is higher than whatever RPM it normally is at for your car, it is no longer impacted by the idle settings. The idle settings only occur when the car is sitting with the engine on and no gas is being applied. When in idle mode, the throttle plate is closed, if you find that it is not, you need to adjust it by loosening the throttle sensor, which is next to the Air Idle Screw. The Throttle adjuster senses the position of the throttle plate and feeds the data to the computer.
3. If you have any vacuum leaks in hoses or other things, this tends to make the idle higher and it is possible that none of the above will have an impact. Check for vacuum leaks by spraying carb cleaner on suspected bad hoses or connections. If the engine RPM increase, you have a leak.