1. Fuel Delivery:
* Fuel Pressure: The most common culprit. The '86 5.0 uses a fuel pressure regulator. You need a fuel pressure gauge to verify you're getting the correct pressure at idle. Too low, and it won't idle. Too high, and it can flood. Consult your '86 5.0 specifications for the correct pressure.
* Fuel Filter: A clogged filter restricts fuel flow, especially at idle when demand is lower. Replace it.
* Fuel Pump: A weak fuel pump may not provide sufficient pressure at idle.
2. Ignition System:
* Distributor Timing: Improper timing can severely affect idle. A timing light is essential here. You'll need to set the initial timing according to the '86 5.0 specifications. This is crucial.
* Spark Plugs and Wires: Worn plugs or bad wires can cause misfires, preventing a smooth idle. Replace them.
* Ignition Control Module (ICM): If you're using the original '86 computer and ignition system, a faulty ICM can disrupt spark delivery.
3. Engine Management System (EMS):
* Throttle Position Sensor (TPS): A faulty TPS provides incorrect information to the computer, affecting the idle. Testing involves checking the voltage output at different throttle positions. You might need a multimeter and the relevant service manuals.
* Idle Air Control (IAC) Valve: This valve regulates air flow at idle. A dirty or malfunctioning IAC valve is a common cause of poor idle. Cleaning it (carefully!) is a good starting point. It might need replacement.
* Mass Air Flow (MAF) Sensor (if applicable): If you're using a MAF sensor, a dirty or faulty one can cause erratic idle. Cleaning it (with MAF sensor cleaner, *not* carb cleaner) or replacement might be needed.
* Computer/ECU: While less likely, a faulty computer could be the problem, especially if other components test good.
* Vacuum Leaks: Any vacuum leak in the intake manifold, vacuum lines, or PCV system will affect the idle mixture. Inspect all vacuum lines carefully for cracks or disconnections. Use a vacuum gauge to check overall manifold vacuum.
4. Engine Mechanical Issues:
* Vacuum Advance: The distributor's vacuum advance mechanism can influence idle. Check its function.
* Compression: Low compression in one or more cylinders will make idling difficult. A compression test is recommended.
* Valves: Burnt or sticking valves can also hinder smooth idling.
Troubleshooting Steps:
1. Start with the basics: Check fuel pressure, spark, and obvious vacuum leaks.
2. Inspect and clean: Clean the IAC valve, MAF sensor (if applicable), and throttle body.
3. Check the timing: Use a timing light to verify the distributor timing is correct.
4. Use a multimeter: Test the TPS and other sensors' voltage outputs.
5. Consult service manuals: Obtain service manuals for both the '68 Mustang and the '86 5.0 to understand the specifics of each system.
Important Note: Because this is an engine swap, the wiring harness and engine control system are likely custom or adapted. Tracing the wiring to ensure everything is connected correctly and using the correct sensors/components for your setup is absolutely crucial. If you're not comfortable with automotive electrical work, seek help from a qualified mechanic specializing in classic Mustangs or engine swaps. This is a complex issue that can't always be diagnosed from a description alone.
