Q. I added several cans of refrigerant to my car’s A/C system, but it still isn’t cooling very well. Do you think I need a new compressor?

A. It’s hard to say without doing some diagnostic checks. First, check to make sure the compressor clutch is cycling on and off while the engine is running, and that the drive belt is tight and is not slipping. If the compressor appears to be working normally, hook up an A/C gauge set to the high- and low-pressure fittings on the A/C system. The gauge readings can tell you if the system has too much refrigerant (overcharged), not enough refrigerant (undercharged), if there’s air in the system, if there’s a blockage in the system, or if the compressor has a problem. Internal wear or valve leakage inside a compressor can reduce its pumping efficiency, which will reduce cooling.

  If an A/C system is leaking refrigerant, eventually the refrigerant will get so low that the system no longer provides much, if any, cooling. Most A/C systems have a low-pressure cut-off switch that prevents the compressor clutch from engaging if the refrigerant charge is too low to protect the compressor from possible damage. Refrigerant helps circulate compressor oil in the system, so if the refrigerant is low, the compressor can be starved for lubrication. With no  lubrication, it can overheat and seize.

  Assuming your system is fully charged with refrigerant and the compressor is engaging (cycling on and off while the engine is running), your poor cooling performance problem could be caused by air in the system. When refrigerant leaks out, air can also seep in. The same thing happens when a hose or other A/C component is disconnected to replace a part. Air rushes into the plumbing.

  Air in an A/C system is bad because (1) it displaces refrigerant and reduces the cooling efficiency of the system, and (2) it contains moisture which can react with the refrigerant to create corrosive acids and sludge that can damage the compressor and other parts. Air contamination also makes the compressor work harder and run hotter, which can lead to premature failure — especially if the system is also low on compressor oil (which it often is because oil leaks out with the refrigerant). Air can also make a compressor noisy.

Q. So if I have a refrigerant leak, how do I fix it?
A. First you have to find the leak(s). This can be done with an electronic leak detector, or by adding a dose of ultraviolet dye to the refrigerant, or by looking for greasy oily residue around hose connections or the compressor.

  The o-rings inside quick-lock couplings on hose and pipe connections often leak, as does the compressor shaft seal. Leaks caused by internal corrosion can also occur in the evaporator, while the condenser is vulnerable to stone damage and other road hazards.

  Once the leak has been found, the refrigerant needs to be removed and recovered from the A/C system so repairs can be made. Various types of A/C sealer products are available that can plug small leaks temporarily. But some of these products can gum up system components or A/C service equipment if used incorrectly.

  After the leak has been fixed, the system must be purged to get rid of all air and moisture. This requires a special vacuum pump. A high vacuum should be maintained on the system for 30 to 60 minutes to make sure it has been thoroughly evacuated before it is recharged with refrigerant. If the A/C system fails to hold vacuum, it is still leaking and needs additional repairs.

  Recharge the system with the proper amount of refrigerant and the specified amount and type of compressor oil for the application. There are different viscosities of PAG oil, and it is important to use the one that matches the lubrication requirements of the compressor.