GMC Troubleshooting Questions? Ask a Mechanic for Answers
The first thing to do should be to check the refrigerant pressure levels. Just slightly low freon on these GM 5.3l engines will make it only blow cold at higher RPM. You will be able to tell if the freon is low based on the AC high and low side pressures.
The next thing to check would be to make sure the orifice tube isn't clogged. Normally a clogged orifice tube would cause it to not blow as cold at high RPM, but occasionally these engines will only have enough compressor pressure to push past the clogged orifice tube at higher RPM.
Next, when the engine is running at an idle and the AC isn't very hot, try spraying the condenser to see if the AC suddenly goes cold. If it does after spraying the condenser then the condenser is likely clogged or defective.
Sorry, that wasn't clear- spray it with regular water. All you are doing essentially is cooling it off. Sometimes, especially this time of year, the condenser is also plugged up with cotton from cottonwood trees, dust, dirt, etc depending on the part of the country you live in. If you notice a lot od debris of any kind washing off of the condenser while you spray it off then that might be the cause of the problem as well.
I need to call it a night. I'll check in with you tomorrow to see how the testing/cleaning goes and if you need additional info.
Let me know if you still need help with this.
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