poa system questions

[gregs 67]

new to the forum and i have some questions i could use a little help with. i am working on a 1967 Chevrolet El Camino. i am converting from R12 to 134a. Background : The car has the A6 compressor with a poa valve, and txv. i have taken the system apart completely. i have flushed the evaporator, condenser and have cleaned the original hoses. i bought a rebuilt A6 compressor, new txv, new receiver dryer, and had the poa sent out and rebuilt and calibrated for 134a refrigerant. I have installed everything, I vacuumed the system down and let it sit two days. no leaks all good. i put 4 12 oz cans of 134a into the system. so the system is still slightly low. my discharge air is 80 degrees F (bad) low side 28 high side 175-200 ambient temp 92 degrees. i am not sure where to start from here. what should i be looking for?

[JohnHere]

Although conversions don't cool as well as the original R-12 and the R-134a charge might still be a little low, with pressures like those, you should be getting some amount of cooling, not 80-degree air. It sounds to me like a lot of heat is mixing in someplace. With a vehicle that old, the blend-door foam seals, and gaskets around the evaporator and heater core, probably are shot unless you already replaced them. If not, I'd start there.

I don't believe the '67 El Camino has a hot-water valve. If it doesn't, the heater core will always be hot (assuming the engine is running and warmed up), with the blend door controlling the discharge air temp. Lots of air leakage around the door could cause the problem you're experiencing.

Check the temperature control cable as well. Although you're moving the temperature lever on the dash to max cool, the cable might just be sliding back and forth and not actually moving the blend door itself to shut-off the heat. Ensure that the setscrew is tight where the cable wire meets its pivot point on the blend-door arm and that the cable's spiral sheath is secured down there in its clip.

It sounds like everything else you did to the refrigeration system is spot on.

[gregs 67]

checked all of the items you mentioned and even disconnected the hot water hoses (bypassed heater core completely) cables are closing the doors maybe minor leakage but nothing to write home about. looking deeper into it when it is running the poa is making a fog horn sound. i tried tapping on it bit it dont seem to fix the noise. it is acting like maybe the poa is stuck open ? or maybe partially open allowing the refrigerant to just flow through. TXV seems to be working as it is warm all along the lines to it and receiver dryer is the same temp. the sight glass shows bubbles flashing but i would expect the way it looks with it a little low on charge. after the TXV the line is cold and the poa is frosting over all the way to the compressor.

[bohica2xo]

If you have a foamy sight glass then your TXV has poor quality liquid. Get the rest of the charge in it first.

Once you have enough refrigerant in it, test at both ends of the POA range:

At 1500+ engine RPM, with the cabin fan motor disconnected. Your POA should hold the evaporator pressure with no airflow.

At 1500+ engine RPM with the cabin fan on highest speed, both doors open. Run for 1 minute to stabilize, & record the pressures while the engine speed is above 1500 RPM.

.

[gregs 67]

recovered the system and took the poa out to bench test it. found that the poa is stuck open. not all the way but somewhere in between. so that explains the fog horn sound and the high evap temps. i also tested the txv just to be sure it was ok. seems to be working as expected. open when bulb is warm and closed when it is cold. i dont know what could have happened to the poa, i took everything out of the car and bought a full gallon of flushing fluid to clean it. I really tried my best to be sure it was clean and sanitary. but i guess stuff still happens. so from here i guess i will need to send the poa back and see if they can do anything for it and if not have it rebuilt again. mean time i will be starting the flushing routine all over again just to be sure i didn't miss anything else.

Thanks for your help with this and i will post again when i get things put back together again.

This is my second R12 to 134a conversion, the first one went perfect. 38 degree evaporator temp and it freezes you out of the car. it was not a poa system though. it was a 1984 Toyota Supra and i did the same thing, tore everything out, cleaned everything and replace the compressor etc. still working great to this day. so i am sure i will get this 67 El Camino working just as well.

[JohnHere]

Since you pretty much looked at everything else, I, too, was about to suggest that you take a closer look at that POA valve. But you already discovered a latent problem with it. Great!

After all your work on this system, I hope that the rebuilder will at least send you another POA at no additional charge...one that actually works. Let us know how you make out with that, as well as the final outcome.

[gregs 67]

well it took a month to get the poa shipped to Florida for inspection and repair. the poa was bad and i had to wait for them to rebuild a new one for me. i reflushed the entire system, just to be sure, drained all of the pag oil out of the compressor and made sure it was clean. new receiver/dryer again tested the txv. it is working proper. open when hot closed when evap is cold. ok all is good, put in the new pag oil and vacuumed the system down. ran vacuum for two hours and let it set for 2 days. no leaks all good. i put in 3 pounds of 134a and was watching things happen, all is good evap temp down to 40 degrees. some bubbles in the sight glass but not bad may be slightly low. outside temp was 90 degrees low pressure 40 high pressure 290 which is a little high. that probably explains the bubbles in the dryer. it looks good so i am going to stop where it is and just wait and see. blowing 40 degree air works for me it only takes about a minute to cool down the el camino cab. so i guess my advice to anyone making this type of conversion is, clean everything super clean and then clean everything again. and once more just to be sure.

[TheBandit]

What kind of flush are you using to clean out the system and the POA valve?

I take it the POA must have been stuck in a partially open position as opposed to fully open since only a partial position would restrict flow out of the evaporator and lead to higher evap pressure / temps. When you were having this problem did you check the low side pressure at the POA valve service port or at the suction line to the compressor? You said you had a low side pressure of 28psi - I'm guessing that was not at the POA service port?

[gregs 67]

there are only two service ports on this system, low pressure is on the poa and the high pressure is on the in line muffler there are no other service ports to access. so to answer your question yes that pressure was at the poa service port. the flush is called johnsons Freeze. it is compatible with r-12 and 134a systems. the stuff works great, very flammable but leaves everything super clean and removes all of the oils with it. it leaves no residue after clean up.

[TheBandit]

Thank you for the reply Gary! My thinking was that if the pressure at the POA service port was at 28psig the evaporator should have been around 32F and you should have had some cooling even if the POA valve was stuck. Maybe the reference pressure back to the TXV was out of whack. Whatever it was I'm glad you got it worked out.

Thanks also for the reply on the flush! Did you flush your evaporator while it was on the car? If so could you tell me how you did it? I was thinking about attaching some clear hose where the TXV would attach and pushing the flush from the top/exit backwards through the system, then blowing it out with compressed air.