2001 Suburban with rear air AC Low Side High High Side Low

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99-02nut

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New to me 2001 Suburban A/C issue has me stumped and don't want to just throw parts at it. Bought it in dead of winter; subsequently discovered that it had the patented GM Leaking High Side Test Port. Replaced that, and also replaced orifice tube as a matter of course while I had it open. I have a nice Robinair vacuum pump and gauge set, and I pulled a good vacuum for a long time and verified that it held before recharging with correct amount of R134a - not an expert in this stuff, but it also isn't my first rodeo.

It has front and rear AC with manual controls for both. It has brand new cabin air filters and I shop vac'd the outside of the evaporator as best I could without disassembling the housing.

With rear air off, compressor cycles frequently and dash air blows warner than ambient. With rear air on, rear blows cold, compressor cycles less (sometimes clutch will just stay in for as long as I watch it, other times it still cycles occasionally - seemed normal for ambient conditions), and front blows warm. Sometimes front will blow cool when driving and high rpms (like driving in manual third above 2,000).

With gauges on it on a not-particularly-good day for testing (ambient around 65 degrees), low side pressure is way too high (70-90 psi) and high side is either OK or too low depending on RPM and compressor on or off - high side went as low as 50 psi and as high as about 130 while low side was between 70 and 90.

It also equalizes very slowly after engine shut down.

Pretty sure it is not a blend door/actuator issue because changing those settings on heat changes the temp from the vents like it should.

Compressor has to be working, since rear blows cold.

Not a clogged orifice tube because I just changed that, and the old one was clean with no visible debris on the screen. I suppose the new one could have been bad out of the box - had that happen several times lately on other parts, but it was an ACDelco orifice tube.

My working theory is that the front evaporator has an internal clog reducing the flow of refrigerant - with rear fan off, rear expansion valve isn't flowing any refrigerant and the clog in the front evaporator isn't flowing either so compressor cycles; when rear fan is on, rear expansion valve opens, refrigerant flows through rear evaporator, and that lets compressor stay on; low-side test port is after the orifice tube on the line that goes to front evaporator and doesn't see flow/pressure drop to rear; exposed portion of line between orifice tube and front evaporator is cold/sweating while front air blowing warm (with rear air on high).

Things that may cut against my theory but that I have decided are unrelated:

1. This engine runs 5-10 degrees warmer than any other 5.3 of the era that I have encountered. It has a brand new ACDelco radiator in it, so I have chalked this up to temp sensor/thermostat variation, but it could have a weak fan clutch (feels fine spinning by hand).

2. I cannot hear the compressor clutch kick in from the driver's seat (again unusual in my experience on these trucks), but laying under it watching it while someone else hits the ac button, it seems to snap right in and start turning, and I can hear a distinct click from right next to it.

Hoping someone will chime in that I'm an idiot and that it is actually the easy-to-get-to turbo encabulator, but confirmation that my "clogged front evaporator" theory is sound before I tear the @#$%ed dash apart would also be appreciated...
 

waveryd

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Sounds like a reasonable theory to me.

Any knowledge of the system history such as:

*oil level?
*condenser replacement?

I have seen a pickup condenser used in an SUV which also has an orifice tube but it's in the condenser. This is fine unless there was a compressor failure which will clog the orifice tube and most techs forget to check that.
 
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99-02nut

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I don't have any history on it.

Seems like condenser issue should be affecting both front and rear since it is a common component, but I also can't figure out how front evaporator would have gotten clogged without there being visible contamination on the orifice tube I changed.

I don't have any experience with the sealer junk other than knowing not to use it - are there passages in the evaporator that are smaller than others in the system such that they could have been preferentially clogged by the sealer (if used - I have no idea if any has ever been put in it and I certainly didn't put any in it)?
 

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