Model: Statesman deville
Engine Size: 308 ci
Refrigerant Type: 134a
Hi guys, I have read all the old post's on adjusting poa valves , but very difficult to get any additional info here in australia.My question is that i was told to adjust the poa valve pressure on the low side down to 24 psi for 134a on this vehicle as it will produce a far better cooling and will not freeze due to radiant heat from engine.I have it all apart and test results showed 28 psi which was original factory R 12 setting. The poa valve looks the same as most of the 60's and 70's GM cars in the states. From what it seems on here though most say to set it down as far as 26 psi and no lower due to possible freezing, however is it possible that freezing may not occur on this model vehicle due to excessive engine bay heat near by the evaporator given its compact design and constant heat transfer from the fan when the air is on. This is a first time post and your thoughts on this would be really appreciated as i cant find anyone here that has experience with older air in gmh cars. We often get very hot temps of over 100 f here so I am looking to get the best result possible. Many thanks.
The saturated temperature for R134A at 28psi and 26psi is maybe 3 degrees (~33F vs 30F). Ice buildup is mainly caused by a large temperature difference between return (entering) air, and the saturated temperature. The larger the difference, the more moisture will condense on the cold evaporator coil, and may freeze if the temperature of that coil is below freezing. So it really isn't going to do much if you're going to lower your POA valve setting by 2 PSI, in terms of getting better cooling, or preventing ice buildup. It depends more on ambient temperature and humidity.
You would get better results by cleaning your condenser and evaporator well. Especially if you're using the same condenser from before your car was R134A retrofitted. A cleaner condenser will be more efficient, and produce better subcooling. More subcooled liquid = more saturation quality in your evap = more utilization of your refrigerant's latent heat capacity = better cooling. Especially if your evaporator is also nice and clean and can transfer the heat out of the air without any losses.
If I wanted to get the most out of the system, I would insulate the liquid line after the condenser and up to the metering device, to prevent heat absorption from a high ambient. I would also see if I could insulate the evaporator coil, or put up a foil barrier to try and lessen the radiative heat from the engine bay - that way, most of the heat absorbed by the refrigerant comes from the air I want cooled. I would also insulate the suction line from the evap back to the compressor to avoid the addition of more superheat from the engine bay. If all this fails to provide results you want, you could lower the POA. If your POA was indeed factory set at 28psi, the sat temp of R12 in the evap would've been below freezing anyway (~30F). However keep in mind all original system components are designed for R12 and will not perform the same with R134A.
If, for some reason, your coil ices up and air can't flow over it, you will build up liquid within your evaporator. A POA will act as a restriction, since it limits compressor suction. And the lower you have that setting, the more restrictive it is. It will act as an orifice and you will begin flashing off at that point before the compressor. You will see the POA valve frost up, and the evaporator will stay warm.
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To test the POA on the vehicle, disconnect the blower motor lead. The low side should drop to 26 PSI for r134. If it stays at 26,connect the blower motor lead again. Go to the procedure section and search my article on testing Chrysler EPR valves. Check the TXV. If the ambient is 100*, your vent temps should be 65*ish give or take. Temps depend upon vehicle color, size of windows, etc. Test rpm 1500, max recirculate, fan in front of condenser.
Thanks guy's for all the information so far.I have followed the advice in keeping the poa valve set to just above freezing and insulating both the suction and high pressure lines. The evaporator and condenser have now been cleaned, and also fitted two thermos with high cfm ratings to move more air over the condenser given the fact i'm changing over to R134a. I have also had to retro fit a new compressor, sanden SD7H15,drier ,hoses and fittings, hopefully it will deliver a high level of performance. After carrying out some tests on the txv as mentioned i found it to be faulty. Lucky i was able to source a new R134a replacement here in Australia. The only issue now before charging the system is sourcing some new small o rings for the oil bleed and liquid line on the poa valve as everywhere here in Aus seems to have ones that are slightly too big. As this old GM poa is rare here maybe i will have to source them from the US.
You can always look into kits to eliminate the POA valve. It converts it to a straight through adapter, and incorporates a low pressure cutout. Seems to be popular, I've never installed one myself.
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If you can get the POA system working, you'll be miles ahead of any POA eliminator
I will stick to the poa valve for now as i already have it back in the car and it did seem to work and adjust up well when tested.I noticed when it was all apart the coil had Harrison written on it ,hopefully these were known an efficient coil.I'm really hoping the system delivers ice cold air for when summer returns here. After i source and fit the new small o rings in the poa i have to get a licenced technician to charge it, as Australia they have strict laws on refrigerants.I know there is a lot of variables here but any ideas on what vent temps i might see on a moderately warm day, say 90f ?
vent temps on the original r12 system was 48 to 52*
Agree. The last thing you'd want to do is get rid of the POA valve. A constantly running compressor will give you the best cooling performance hands down.
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