I have a 1986 town car that ran to rich i installed a new iac valve an now it wont idle.

black smoke equals too rich a fuel mixture. check the engine vacuum at idle should be 20 inches of mercury if not check for stopped up catalytic converter. are there any check engine lights

change the idle air control valve.

Idle Air Control (IAC) Valve OPERATION The Idle Air Control (IAC) valve controls the engine idle speed and dashpot functions. The valve is located on the side of the throttle body. This valve allows air, determined by the Powertrain Control Module (PCM) and controlled by a duty cycle signal, to bypass the throttle plate in order to maintain the proper idle speed. Fig. 1: Typical Idle Air Control (IAC) valve which is mounted to the throttle body - cutaway view shows air bypass direction
--- REMOVAL & INSTALLATION

1. Disconnect the negative battery cable. To remove the IAC valve, first disconnect the negative battery cable, then the IAC wire harness plug
   Next, remove the two IAC valve attaching bolts . . .
   . . . and pull the valve from the intake manifold
   
   
2. Disengage the wiring harness connector from the IAC valve.
3. Remove the two retaining screws, then remove the IAC valve and discard the old gasket. To install: When installing the valve, always discard the old gasket and install it using a new one
   
   
4. Clean the IAC valve mounting surface on the throttle body of old gasket material.
5. Using a new gasket, position the IAC valve on the throttle body. Install and tighten the retaining screws to 71-106 inch lbs. (8-12 Nm).
6. Attach the wiring harness connector to the IAC valve.
7. Connect the negative battery cable.

you need to clean your idle air control valve ( or replace it) Spray carb cleaner does a nice job of cleaning it.

This tutorial is from autozone.com

OPERATION

The Idle Air Control (IAC) is a bi-directional motor driven by two coils. The purpose of the IAC valve is to control engine idle speed, while preventing stalls due to changes in the engine load. The IAC valve, mounted in the throttle body, controls bypass air around the throttle plate. The PCM controls engine idle speed by adjusting the position of the IAC motor pintle. The PCM pulses current to the IAC coils in small steps (counts) to extend the pintle into a passage in the throttle body to decrease air flow. The PCM reverses the current pulses to retract the pintle, increasing air flow. This method allows highly accurate control of idle speed and quick response to changes in engine load. If RPM is too low, the PCM will retract the IAC pintle, resulting in more air being bypassed around the throttle plate to increase RPM. If the RPM is too high, the PCM will extend the IAC pintle, allowing less air to be bypassed around the throttle plate, decreasing RPM.

Fig. Typical Idle Air Control (IAC) valve components
During idle, the proper position of the of the IAC pintle is calculated by the PCM based on the battery voltage, coolant temperature, engine load and engine RPM. If the RPM drops below a specified value, and the throttle plate is closed (TP sensor voltage is between 0.20-0.74), the PCM senses a near stall condition. The PCM will then calculate a new IAC pintle position to prevent stalls.
If the IAC valve is disconnected and reconnected with the engine running, the idle RPM will be wrong. In this case, the IAC has to be reset. The IAC resets when the ignition switch is cycled ON and then OFF . When servicing the IAC, it should be disconnected or connected with the ignition switch OFF in order to keep from having to reset the IAC valve.
The position of the IAC pintle affects engine start up and the idle characteristics of the vehicle. If the IAC pintle is fully open, too much air will be allowed into the manifold. This results in high idle speed, along with possible hard starting and a lean air/fuel ratio. A Diagnostic Trouble Code (DTC) may set. If the IAC pintle is stuck closed, too little air will be allowed in the manifold. This results in a low idle speed, along with hard starting and a rich air/fuel ratio. Again, a Diagnostic Trouble Code (DTC) may set. If the IAC pintle is stuck part way open, the idle may be high or low and will not respond to changes in engine load.

TESTING

As with most all engine control sensors used in your vehicle, a thorough and proper test can only be performed by a qualified technician using a scan tool to read the data stream from the PCM. There are a few items a non-professional should check before taking the vehicle to a qualified technician for diagnosis and repair.

Check for a poor connection at the PCM or IAC motor. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connection. Inspect for a damaged wiring harness, especially if heavy work has recently been performed (intake manifold removal, engine removal, etc.) where the throttle body has been disturbed. Inspect for a restricted air intake system. Check for a possible collapsed air intake duct, restricted air filter element or foreign objects blocking the air intake system. Inspect the throttle body. Check for objects blocking the IAC passage or throttle bore. Excessive deposits, especially on high-mileage engines, can build up in the IAC passage and on the IAC pintle. Excessive deposits can also build up on the throttle plate, so check for a sticking throttle plate. Check for vacuum leaks such as disconnected hoses, leaks at the EGR valve and EGR pipe to the intake manifold, leaks at the throttle body, faulty or incorrectly installed PCV valve, leaks at the intake manifold brake booster hose connection, etc.

REMOVAL & INSTALLATION

1. Disconnect the negative battery cable.
2. Detach the electrical connector from the IAC valve.
3. Remove the 2 attaching screws and remove the IAC valve from the engine. Remove the O-ring from the housing.

To install:

Fig. The Idle Air Control (IAC) valve is mounted on the throttle body-3.8L (VIN K) engine shown, others similar

NOTE Before installing a new IAC valve, measure the distance that the valve is extended. The measurement should be made from the motor housing to the end of the valve cone (pintle). The distance should be no greater than 1 1 / 8 inch (28mm). If the pintle is extended too far, adjustment is required, otherwise damage may occur when the IAC valve is installed. Adjust the IAC valve by compressing the pintle to achieve the correct length.

1. Use a new O-ring, install the IAC valve and tighten the retaining screws to 18 inch lbs. (2 Nm).
2. Attach the electrical connector.
3. Connect the negative battery cable.
4. The PCM will reset the IAC valve the next time the ignition switch is turned ON and then OFF .
5. Start the engine and allow to warm to operating temperature to check idle quality.

Check the IAC valve ( idle air control ) maybe dirty and sticking. Clean with intake cleaner or replace IAC valve.

Idle Air Control Valve The Idle Air Control (IAC) valve is located on the air intake plenum.




Removal & Installation
Disconnect the battery ground cable.
Disconnect the Idle Air Control (IAC) electrical connector.
Remove the bolts and the IAC valve.
Discard the gasket.
To install:
Install a new gasket and position the IAC valve.
Torque the retaining bolts to 8 ft. lbs. (11 Nm).
Attach the electrical connector.
Reconnect the battery ground cable.

U are telling me what you did as far as a repair goes but you are not clearly stating the symptoms or the actual problem, please explain what is going on here and when it happens, cold engine etc, as much detail as possible, and you should never adjust the idle speed, have u cleaned the throttle valve and the upper intake?

its located on the throttle body connected to your air intake manifold where your throttle cable connects on the throttle body there is a adjustment screw connected there

The idle air control valve is located on the back of the throttle body. It has an oval electrical connector and two torx screws holding it in. Unplug the connector, remove the two screws then lightly tap the body with a soft faced mallet to loosen it from the body. 
Once you have it out and prior to installing a new valve clean the seat inside the throttle body where the idle control fits. This must be very clean! Installation is in the reverse order. 
 To save some cash, pull the valve out and clean it with throttle body cleaner, this usually fixes the problem for quite a while.
Good luck and hope this helps.

have a ford garage check out the fuel pump don't do it yourself if you do it will be so messed up they can't even fix it trust me i did i had to scrap the car

IAC valve problem. Nissan has a updated harness (with one way diode inside of the harness) to prevent backfeeding for the IAC valve and a new IAC valve also gets installed. That's where I would look. IAC valve or "idle air control" valve.