RezinTexas

Vehicle: 2001 Suburban with rear AC and eFan conversion with Nelson harness and Blackbear tune. I have confirmed that both fans and all relays are functioning as they should. Engine is a built 383 LS stroker that runs great with 19k miles. I'm using the factory truck style crank pulley. Here is my build thread:
https://www.performancetrucks.net/fo...2500-a-520418/

Here is the problem! My dash vent temp is 70-80F on a typical hot summer day in south Texas (95F ambient). I believe the vent temp should be 40-50 F max.

Googling says my low side pressure should be 50-55 and my high side 275-300. I’m getting 55-60 low and 225-250 high. So, the low is high, and the high is low. When I shut-off the engine, it takes about 45 seconds for the low and high sides to equalize at approximately 100 psi.

In January 2019 I did a total AC system rebuild following a previous compressor failure. I replaced all components in both front and rear circuits: compressor, condenser, both evaporators, drier, orphus tube, rear expansion valve, all seals, all sensors, and all external poppet valves. I thoroughly cleaned all hoses and tubes and re-used them. I held a vacuum in the system for 2 hours, then I added roughly 58oz refrigerant (spec is 48) and 13oz of PAG-46 oil (spec is 11). The first summer with this setup (2019), the vent temp was about what it is now – so I have no reason to believe the system is leaking or otherwise broken/degraded since the 2019 install. I believe the problem is a result of something going wrong with the install in January 2019 - but I have no idea what it could be!

Here are two possible causes I am contemplating:

1. Overcharge. I used a fruit scale and estimated that I added 58oz – 10 more than the spec of 48. However, my high side pressure is low, therefore this is not a typical symptom of overcharge? Further, "Scenario 1" at the link below suggests I might be low on charge!
https://nissens.com/Admin/Public/Dow...134A+-+ENG.pdf

2. Leaky orphus tube. I used a new tube, lubed the o-ring and was very careful during insertion, however it’s always possible the o-ring is pinched or leaking.

Any other thoughts? Help!

Many Thanks 😊

RedXray

Have you confirmed a faulty cool/heat actuator motor under the dash is not introducing heat (hot coolant) into the system skewing the vent temps.

MikeGyver

iTrader: (5)

I would start with the correct charge. +20% charge could really mess up the pressures.
Dump the charge, pull a good vacuum (it doesn't have to take long, just get a full vacuum), and weigh in the correct amount of freeze gas.
​​​​You have to have that correct first, then you will know if you need to look elsewhere.

MikeGyver

iTrader: (5)

Weird pressures especially when you have an expansion valve.
​​​​​​Having an expansion valve and an orifice tube in the same system will very much require the charge to be weighed in for proper diagnosis.
When you are testing pressures, is the rear system running at full blast?
?

1redta

my ac temperature at the vents is usually 38-40° F in NW Florida, the temp outside doesn’t matter the ac stays the same.

you knowingly overcharged the system why? I believe starting over would be the first step since your ac isn’t cooling correctly

RezinTexas

during the rebuid 18 months ago, I removed the entire HVAC box, cleaned it out and applied new foam to all the vent doors. I can't be 100% certain that they are all functioning correctly, however I know that the door actuators are at least working when you push the buttons. Any good ideas on how to troubleshoot this?

Yes, both front and rear systems are on max when I'm testing pressures. There is no shut-off to the rear circuit, both evaporators are receiving chilled refrigerant regardless of the fan on/off switch.

I'm starting to come to the same conclusion. When I did the original charge to the correct spec 48oz, the gauge pressures indicated low charge. I kept adding an ounce or two more to see what would happen, and here we are.

Thanks for the replies.

madmann26

iTrader: (7)

If the tune isn’t right, it can cause issues as well.

I had my adaptive idle off, and it wouldn’t cool. Once I turned it back on with stock values, it blows about 42 at idle.

1FastBrick

iTrader: (17)

Too much oil in the system can also cause cooling issues.

Are you charging from a bulk 30 Lb bottle or small cans?

RB04Av

Overcharge will cause poor low-speed cooling and high pressure on both sides of the system. Doesn't look like you have that.

Undercharge will cause freezing up at high speeds and low pressure on both sides. Doesn't look like that either. Freezing is caused by the very slow-moving but very cold (low pressure) vapor in the section of the evap just past the orifice getting too cold, which freezes that few inches; then the layer of frost makes a great insulator, so the very slow but very cold gas freezes the next few inches; and so on, until it's frozen from the orifice all the way back to the compressor. But the total amount of heat being transferred into the refrigerant is still negligible, in spite of it looking to the eye like it's slaying it. With the right amount of charge the refrigerant should move fast enough through there that the cold temp (ability to absorb heat from the blower air flow) is distributed all the way through the evap and doesn't get concentrated into the very first short length of tubing.

Both of the above will make the compressor run hot... it uses "waste cool" so to speak coming back from the evaporator, to cool the compressor.

Too much oil usually acts kinda like a blocked TXV or orifice tube; will usually result in too high pressure on the high side and too low on the low side, with poor cooling. Metal chips or other contamination in the system acts the same way. Moisture in the system will make it work OK for a little while, then start acting this way when it all accumulates as ice just below the orifice.

Air leak in the ducts will make it act like it's an extra 25° ambient or whatever, or sometimes like the ambient is way too low, depending on how big the leak is and exactly where. Which would be, very high or low pressures both sides, well outside of the expected range for the true ambient and amount of charge.

45 seconds to equalize seems kinda short but not unreasonable, idk how much the rear air affects that.

Low pressure on the high side and high pressure on the low points to a bad expansion valve (stuck open) or leaky orifice tube; causes too much refrigerant cycling through the system uselessly, not enough being held back and forced to evaporate through the orifice. Or, bad compressor (simply not enough "suck" and "blow"). The old R-4s were bad about that when a reed valve shredded.

If it was mine I'd be checking the TXV and orifice tube. The OT is cheeeeeep, use the parts cannon to troubleshoot, especially if you have access to a recovery machine.