After I start up my 97 LX450 the air conditioning runs perfectly for 15 -20 minutes and then it stops blowing cold. For a while it would stop in a shorter period of time and I noticed the temperature gauge was a little elevated. I made sure the overflow tank was filled and now it holds a normal temperature.

I think it must be temperature related. Why would the ac stop blowing cold at normal operating temperature? The ac occasionally comes back on for a while if the trip is long enough.

 

Sounds like your A/C compressor may need a little attention, or possibly replaced. The coolant system has nothing to do with the A/C.

 

Actually, the LX450 will shut-down the AC if the engine temperature is elevated pass the "safe range." It is an attempt to not put additional load on the engine. The factory temperature gauge is near useless and likes to read in the middle, all the time. Get a scan-gauge or it is possible to modify the OEM gauge to be more "sensitive."

The cooling system on the LX450 can be troublesome if not maintainted. I would check a few items: if the fan clutch is working properly, if the radiator has sludge/contaminants in it (common), or possible blown head gasket can cause overheating. Be sure to only use the OEM Red coolant as well. If your radiator hoses are original, be sure to replace the rear heater hose (nicknamed PHH as in Pesky Heater Hose), it is not so easy to access. If the condition of the radiator is not good (passages clogged), replace the radiator.