I have a 1990 f-150 that is running rich at idle and low speeds, but clears out at higher rpms. I think that the fuel pressure regulator is good, because I doesnt seem to be putting fuel into the vacuum line. This is an inline 300 6 cyl.
Probable causes of an engine running rich are as follows: 1. Excessive fuel pressure can be cause by restricted fuel return line
2. Defective Oxygen Sensors.
3. Leaking injectors and fuel pressure regulator.
4. Coolant temperature sensor out of calibration.
5. Restricted air intake system.