AC system issue

[Pangus]

Hello,
I need some help with my 2006 Z71 Tahoe with rear air.

A month or so ago the AC system started to randomly blow hot but seemed to blow cold again once under moderate braking. I checked under the hood and found that I had very low coolant due to a weeping crack under the upper house connector. So I thought the evaporator in the grill was just getting too hot. But the AC system continued to randomly blow hot after coolant was added. I borrowed an AC gauge and the low side pressure was up around 90. The AC compressor was short cycling every 30 secs. So, I replaced the low pressure switch. It didn't help. I thought, well, maybe there's no suction from the compressor and since AC compressors aren't ridiculous in price and my Tahoe has 244,000 on it, id just replace it for peace of mind. Had Pep Boys evacuate the system for me to work on it.

So I replaced the compressor and orifice tube. The orifice had a couple bits of black plastic on the screen but nothing to indicate a catastrophic failure. I replaced the condenser too. All new seals, and vacuum test showed no leaks. However, once recharged they couldn't get it to blow colder than 60 degrees and the low side pressure was still 60psi. They thought the AC compressor might be "weak" but after a few minutes of driving it went blowing hot. Then, after parking, and turning around to take it back, then fan inside made a sound like it was "finally falling into place". And it blew cold constantly, 48degrees, the whole way home. I checked temp at home.

The next morning I drove kids to school, ice cold air. But that afternoon, started ice cold then blew humid hot off and on with cold. After 10 minutes, the compressor short cycles every 30 seconds, so I know there's a crazy pressure on the low side again. But I've replaced everything.

I am second guessing the oiling. The compressor came with 8 ounces in it, and the book says rear air should add 3 ounces. So, since I replaced the condenser, I thought the oil would be good, but I wonder if there was more in the lines than I thought. Would too much oil cause the pressure build up?

Also, I didn't replace the rear expansion valve, which I think I should have. But would that cause the pressure to build on front lines?

 

[OneofFew]

Frankly I think you could have saved yourself time and money having it professionally checked out. Having had an indication that there is a blower motor/ A/C system control issue, it is possible you are seeing high low side pressures because the system is freezing up. But who knows without hooking up a gauge set- nobody.

 

[Pangus]

Frankly I think you could have saved yourself time and money having it professionally checked out. Having had an indication that there is a blower motor/ A/C system control issue, it is possible you are seeing high low side pressures because the system is freezing up. But who knows without hooking up a gauge set- nobody.

I’m not following. There’s no fan issue or sign of freezing up. The condenser has condensation on it like all ac systems but not freezing over. And with new components it is demonstrating the same issue of not relieving low pressure side. The compressor keeps kicking on for 3 secs and off for 30.

It was put on gauges, and they said it appears everything is right, but can’t figure out why the low side is still so high.

 

[OneofFew]

deleted for clarity

 

[OneofFew]

"2006 Z71 Tahoe with rear air.

A month or so ago the AC system started to randomly blow hot but seemed to blow cold again once under moderate braking. Indicates a A/c system control issue
I checked under the hood and found that I had very low coolant due to a weeping crack under the upper house connector. So I thought the evaporator in the grill was just getting too hot. But the AC system continued to randomly blow hot after coolant was added. I borrowed an AC gauge and the low side pressure was up around 90. What is the high side pressure?
The AC compressor was short cycling every 30 secs. So, I replaced the low pressure switch. It didn't help. I thought, well, maybe there's no suction from the compressor and since AC compressors aren't ridiculous in price and my Tahoe has 244,000 on it, id just replace it for peace of mind. Had Pep Boys evacuate the system for me to work on it.

So I replaced the compressor and orifice tube. The orifice had a couple bits of black plastic on the screen but nothing to indicate a catastrophic failure. I replaced the condenser too. All new seals, and vacuum test showed no leaks. However, once recharged they couldn't get it to blow colder than 60 degrees and the low side pressure was still 60psi. They thought the ACcompressor might be "weak" but after a few minutes of driving it went blowing hot. Then, after parking, and turning around to take it back, then fan inside made a sound like it was "finally falling into place". The fan made a noise and the A/c temp changed... Also indicates an A/C control system issue.
And it blew cold constantly, 48degrees, the whole way home. I checked temp at home.

The next morning I drove kids to school, ice cold air. But that afternoon, started ice cold then blew humid hot off and on with cold. After 10 minutes, the compressor short cycles every 30 seconds, so I know there's a crazy pressure on the low side again. But I've replaced everything. SOunds like the evap is freezing up causing the high pressure switch to trip, but we will never know without hooking up a gauge to the high side.

I am second guessing the oiling. The compressor came with 8 ounces in it, and the book says rear air should add 3 ounces. So, since I replaced the condenser, I thought the oil would be good, but I wonder if there was more in the lines than I thought. Would too much oil cause the pressure build up?"
Too much oil would be evident in low and high side reading. but would not be accompanied with evap freeze up

Also, I didn't replace the rear expansion valve, which I think I should have. But would that cause the pressure to build on front lines?"
I doubt it, but not 100% sure
As you see several of your symptoms point to a control issue. Of course everything is guess work without hooking up a gauge set so we know both the low AND High side pressures.
You did replace the accumulator/dryer also, correct?