88 gta replacing the compressor with a
Four Seasons 58238 New GM R4 Lightweight Compressor w/Clutch
system has been open unused for probably 4 to 5 years. Can I put the compressor on and fill it with R134a?

 

I would check the orifice tube to make sure it is clean then would have the system vacuumed down to make sure no leaks first.

 

The receiver/drier has a dessicant in it to absorb moisture from the sealed system. You just exposed it to air and moisture for 5 years. At the bare minimum you need to replace it. I would flush the lines with A/C flush and compressed air. The orifice tube probably needs to be replaced, the GM white units don't really cool very well with R-134a since they were designed for R-12. What I did with mine was use a Ford Blue orifice tube off a mid 90s Ford mustang or truck. You can either buy or rent the tool to pull the old one out. They have a filter mesh on them to catch debris, and they are usually pretty nasty. The car did cool fairly well at idle and a lot better on the highway.

If you feel so inclined, you could also replace the old black O-rings with the newer style green ones. You need to add the correct amount of PAG oil to the compressor and make sure you have the right seals for the back of the compressor to fit it to the A/C line. Pull a vacuum using manifold gauges for at least 15 minutes to boil the moisture out of the system. Cut off the vacuum pump and watch to see if it holds vacuum for about 20 minutes. If it holds vacuum, then you can fill and monitor the vent temps with a thermometer. You can usually also rent the vacuum pump and gauges at the parts store too. While the engine is running, watch to see if the low pressure cycling switch is kicking on and off properly. It is located on the receiver/drier and has a little set screw in the middle that you can adjust.