AC problem
 

Unfortunately I just bought a 2011 chevy equinox prior to knowing the have engine issues. The AC is also not working as well and it's starting to warm up here in lower Alabama. The compressor kicks on and at first I saw a low side pressure of 8 psi and a high side of 157 psi. After shutting her down and coming back a few minutes later I am seeing a negative pressure of 5" on the low side and a high pressure of about 68-70 psi. The compressor continues to run while this is happening. I thought the compressor would shut off after the low side dropped below 20 psi; am I wrong? I am seeing the same thing on two sets of gauges. Any idea what could be going on and why it isn't cooling at all? I have had the worst luck with vehicles lately. Thanks in advance for your help.