First, check the temperature of the air coming out of the vents, it should be below about 36-38 degrees with the fan on ‘high’, and close the same temperature at all air outlets. car detail I don’t recall if the water supply to the heater is cut off on these models or not when the A/C is on, if it should be, check the water on/off valve. It should shut the water off completely, even a bit of hot water will overwhelm the A/C. If the system does not include a water valve, the problem may lie in the sealing of the moveable flap in the A/C air system. If it doesn’t seal completely, heat from the heater will also overcome the A/C. If the problem is in the flap, it is usually easier (and more effective) to just cut the out of the engine side of the heater hose and put in a domestic valve. Old Ford pickups and Ford Fiestas have a simple cab le operated valve you can insert in the line.
Have a qualified A/C service man check the high and low side pressures, and if they are low, add R-12 if the system was originally filled with it. Most ‘drop in’ replacements are not as efficient, and R-134 is decidedly worse, and requires more power as well.
Pressures are very dependent on temperatures, especially the high side, and to much freon will not cool as well as, and may be damage the compressor. If the high side is too high, spray a mist of water on the condenser, the pressures should drop AND the cool air get considerably cooler.
Check the airflow through the radiator/condenser assembly, leaves, dirt and other detrious material will greatly reduce airflow, and it is hard to get dust and fine material out. I use Castrol Super Clean and a special angled water wand, although you can make one out of a small piece of tubing bent on the end and soldered to an old hose coupling.
It goes without saying that the belt should be tight, the fan clutch in good shape, and all the fan and radiator shrouds in place. Quite often the loss of seals around the hood and damage to the valence will spoil the airflow. Ditto damage to any insulation that has fallen off the cool side hoses under the hood or air leaks around the evaporator core.
With a infared temp sensor check the difference between the ‘in’ and the ‘out’ connections have a temperature difference of at least 50 degrees F, otherwise the air flow amount is too low, if so, the water spray will make the unit cool.
High pressures may also be a result of a clogged filter or orfice, again, a qualified A/C technician can test, if the unit has been repaired or charged, or just plain old, the filter may be clogged.
If major components require replacement, a good (I must stress that there is a lot of difference between ‘one who services A/C’s and a ‘Good’ technician that can analyze and correct problems) can find and fit the more modern compressors and condensors of greater capacity. Not only will you get better cooling, it takes less power to run.
Additionally, if you have more than adequate cooling you can run the fuel line and the cool side return freon line together in a heat exchanger (V-12 Jaguars have one) to cool the fuel. It gives more power and greatly raises the effective octane rating.