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POA Not cooling well? Pages: 12

Big V. on Wed June 25, 2003 2:04 PM User is offline

Year: 1971
Make: Buick
Model: Riviera
Engine Size: 455
Refrigerant Type: R134A
Ambient Temp: 87
Pressure Low: 32
Pressure High: 225

I need some advice ! I have a 1971 Riviera GS in great shape... As a matter of course, not knowing the A/C history and wanting it to cool well this summer...although the system worked so-so, I replaced the compressor, the expansion valve, the reciever/dryer and later, the POA valve. I cleaned the condensor (although it looked clear) and checked the evaporator for any possible blockages. I also checked to make sure all the dash controls worked properly, temp door, etc. I replaced the heater valve and verified it shuts off as required. I flushed the cooling system and replaced the anti freeze and replaced the fan clutch. The radiator is new and I have no overheating problems.
I flushed the A/C system, hoses, and condensor, replaced the O rings, evacuated the system and checked for any leaks...none were found and then charged the system with R134a. (3.5 pounds worth seems to give the right pressures). I installed the correct amount of oil to the new compressor and system.
In other words, the system is about all new here and I have tried everything to improve the cooling, but I don't think it does that well at all.
At 86 ambient temp....pressures at 1500 rpm are 32 and 225 and BEST vent temp at max A/C is 51, even when going down the road. When engine is allowed to idle, such as when stopping at a traffic light the vent temp quickly climbs to 61 degrees. It will only cool to 51-54 degrees when engine is rpm is 1150 rpm or higher, or while car is in motion, otherwise 61-65 degrees is at the vent. But to me, the best vent temp of 51 degrees seems high...I would hope for 45 degrees.
What can be wrong ? I have tried everything I can think of ? ...or am I expecting to much from this design....what do you think? ....and what do you think the system should perform like?
Just to make sure Ester oil OK to use? one other thing...should I expect the sight glass on the recieverdryer to clear up when full, as with R12?

help........and THANK YOU...
Big V.

Big V.

Edited: Thu August 28, 2003 at 5:08 PM by Automotive Air Conditioning Information Moderator

Mitch on Wed June 25, 2003 2:35 PM User is offline

Big V,

Check out my post in the general automotive AC questions titled "1972 POA system resurrection". It is about a 1972 El Camino, with AC that is essentially identical to yours, that I converted to 134a. You will have to go to page 2 of the topics and scroll down, or search, because there has been a lot of activity since the temps are heating up across the country.

Also check out the 1970 Vette topic just below, and the 77 Vette topic down a little ways, both on this conversion and retro forum. The 77 uses a VIR, but the VIR includes a POA.

I am really impressed with the El Camino. I am getting vent temps of 37F.

I can provide you some additional info later when I have more time.

Edited: Wed June 25, 2003 at 2:59 PM by Mitch

Big V. on Wed June 25, 2003 3:43 PM User is offline

Hi Mitch, so after reading your "POA ressurrection" and other articles you mentioned.....would I be correct in assuming that I should try adjusting the POA valve 7/32 screw counterclockwise 1/4 to 1/2 turn....recharge with refridgerant and then see how it does at the vents? and ck pressures to...

Thank You.
Big V.

Big V.

Mitch on Wed June 25, 2003 4:05 PM User is offline

This is what I would do,

Disconnect the heater hoses and jumper the engine connections. That will absolutely remove any effect from a leaky water valve and/or leaky foam seals in the evap/heater box. Even clamping the water hoses does not always guarantee no reheat.

Run the engine at 1500 rpm with the AC on and the blower motor disconnected. That will bring the low side down to the POA setting. If it is still 32 psi, it is way too high, and will need to be adjusted. If it is down around 29.5 psi, which is the nominal setting of an R12 POA, it will still help if you can adjust it down to 26-27 psi, but not as much as if you find it at 32 psi. You should be able to get the downstream half of the POA to freeze, as in my photos, while the upstream half is sweating. That tells you that the POA is regulating.

If the low side drops from your previous 32 to a lower value with the blower disconnected and no adjustments, there is a possibility that your compressor is weak.
Was it a rebuilt or new? If rebuilt, what is the brand? Make sure your compressor belt is tight enough also.

You may need to check the superheat on the TXV, but for now let's assume it is OK. What is the original R12 capacity of your system?

Let me know what you find out.

Also, try to confirm that your temp gage and low side gage are accurate. It is not unusual to find thermometers that are 5 degrees off and gages that are a couple of psi (or more) off.

Forgot to mention that the POA regulating pressure automatically rises by 1/2 psi for every 1000 ft above sea level. What is your altitude?

Edited: Wed June 25, 2003 at 4:45 PM by Mitch

Big V. on Wed June 25, 2003 6:41 PM User is offline

Mitch...I performed the tests as you stated..... bypassing the heater core made no difference in A/C performance, but I left it bypassed anyway for the tests...
At 1500rpm with the blower on ...low side pressure was about 35 psi, with the blower wire disconnected...the low side pressure was 31 and stable at 1500rpm and the downstream side of the POA valve did freeze up as you said and the upstream side of the POA valve sweated.

Yes, the compressor belt is tight and you had asked about the is a rebuilt, sold by Factory Air, with a Four Seaons sticker on it. I got it at Autozone. The previous compressor was of the same type (but I don't know how old), but it gave the same pressure readings, but the high side had alot of gauge needle flutter, so that looked fishy to me and I replaced the compressor...the gauge needle does not flutter anymore.
I checked my vent temp with 2 different thermometers...both read the same, bout 55 at best today at 88 ambient temp.
So, what you think I should try removing the POA valve and give that screw a turn counterclockwise ? if so, how much?
...or do you see another way to go?

Big V. oh asked what is my is about 500 ft above.....Dallas, TX.
you also asked about original system capacity......I have seen a picture of an original sticker that said 4.5 lbs...that seems like a lot to me, but it's got to be at least 3.75

Big V.

Edited: Wed June 25, 2003 at 6:53 PM by Big V.

Mitch on Wed June 25, 2003 7:13 PM User is offline

It's finally warm here today - 85F I am heading out for test drive on the Camino with the gages in the cabin. I'll report when I get back.

Big V. on Wed June 25, 2003 10:50 PM User is offline

Let me know how your test drive goes Mitch and please read my last entry and tell me what you think ???
Thank You
Big V.

Big V.

Mitch on Wed June 25, 2003 11:04 PM User is offline

This is the story on the 72 at 85F ambient. Running down the road at 60, the low side was 26-27 and the high side was 135, which is really good. Vent temp was 40F on recirc and next to fastest blower speed. I was cool as a cucumber. I stopped and popped the hood and the POA was half frozen.

When stopped at 600 rpm idle, the compressor couldn't hold the 27 psi and low side went up to 30s and vent temps to high 40s. High side went up to about 200-225. Currently this vehicle has a flex fan which is 3 inches too small and is in too deep into the shroud.

This vehicle also has an unknown quality compressor. I think that there are 2, maybe 3 problems at idle. One is the fan which is not flowing enough air. Two, I believe that the compressor is weak. I am going to try a brand new A6 and see what the difference is, and three, I measured the superheat of the TXV and it looks a little low. I measured about 4 degrees and the norm is 5-15 degrees. The TXV has an adjustment for superheat and who knows what may have been done to it in the last 31 years.

V, you will have to get the POA setting down to at least 27 for best performance. A quarter to half turn may do it. it was reported on the Pontiac forum that 1/8 turn CCW is 2 psi lower. You also may have a weak compressor. The low side drop from 35 psi to 31 psi with the blower off indicates the compressor may be weak. Four Seasons has just about the worst reputation of anyone rebuilding compressors. The name alone makes it suspect. If you are serious about the AC working the best it can, I would get a new A6 - less than $300.

You probably should check your TXV superheat also. Charles just retroed a 1985 BMW and his new TXV was defective out of the box. Pressure was hunting all over the place. You can check out his saga by searching on "author" Charles and "phrase" wandering and also on Charles and expansion valve. Make sure you check the boxes to search in all forums.

It's never easy working on AC - old or new. Too many things can't be proven for sure.

Lots of big GM cars in the 60s and seventies took over 4# of R12. I have a couple of old A6s that show 4.125 pounds capacity on the nameplate.

You also need to verify your evacuation procedure to make sure you are not getting any air into the system during charging. NickD is supposed to write up a foolproof procedure soon. We can go over a good procedure later. What is the rating on your vac pump?

Edited: Thu June 26, 2003 at 8:09 AM by Mitch

Big V. on Thu June 26, 2003 3:35 AM User is offline

Thanks for the great info late tonight Mitch....I was hoping to have a plan to act on in the morning.....
I'm going to adjust the POA screw 1/2 turn counterclockwise, recharge and see what I get....I sure hope it helps!

If my results get close to what your getting in your 72 El Camino ...40* on the road and high 40's at idle.....I would be a very happy camper !! It sure would be a lot better than 54 and 64 respectively...Yuk!!

The POA valve"adjustment" has just got to be the's all that's left to "mess with" !

Maybe the "new" rebuilt 4-seasons compressor isn't the best, (I don't know), but I did pay $118.00 for it, so I can live with it for a while and be happy if I can get close to duplicating what you have done with the '72 El Camino.

You asked about my vacuum pump...ya, ....I know it's not the won't pull to 30" but it will pull to 25" and it's stood by me and provided good results on the other Family cars over the years...I don't think it's a "big" factor...I hope it's not in your eyes & experience ?

As far as this (TXV superheat) device you've been mentioning....that's the (Thermostatic Expansion Valve), right ? if so, I think it is OK, my old one gave the exact same reading and performed the same way..I just changed it out to be sure it was clean and good.

Big V.

Big V.

Edited: Thu June 26, 2003 at 3:54 AM by Big V.

Mitch on Thu June 26, 2003 8:21 AM User is offline

Big V,

25" is a little light on the vac pump. Barely acceptable for R12. 134a needs a better vacuum especially on a retrofit. The GM manuals for R12 systems always said you will need 26-28" vacuum. My vac pump will get down to 29.9.

Remember that the POA has no effect at all unless the compressor is trying to get the evap below the valve setting. So, if the valve setting is 27 psi and the compressor can only hack 31 psi low side under the prevailing conditions, then both sides of the POA will be 31 and the POA will be wide open.

I have had some experience with Four Seasons and most has been bad. Some are very noisy and obviously defective, but others can be quiet and just not seem to get the job done. Unfotunately, they seem to have most of the market - probably because they are so cheap.

Yes, TXV is thermostatic expansion valve.

Edited: Thu June 26, 2003 at 8:22 AM by Mitch

Dougflas on Thu June 26, 2003 8:11 PM User is offline

You guys sure got me thinking back 30 years. I have a POA tester and a couple of Ford POA's. I may even have a GM POA. Tomorrow if I get a chance I'll play with a POA on the bench and let you know how I make out. The tester I have allows me to use shop air for testing. I also have a tester for the EPR's . Haven't used them in years. Just as a reminder, be careful of the oil bleed line plugging up.

The POA systems back then were perfectly normal with 48 degree and higher vent temps. 1972 Chevelle specs were: RPM 2000 100* air at condenser input 210 to 220 head pressure 28 to 31 at POA vents were 44 to 47.

Edited: Thu June 26, 2003 at 8:24 PM by Dougflas

Mitch on Thu June 26, 2003 8:59 PM User is offline


I am sure you are correct about the 1972 Chevelle specs of 44-47 @100F because my 1970 manual has exactly the same numbers; however, the 1970 manual shows vent temps of 38-43 for the Impala, also @100F. Not sure why there would be such a big difference. That is on high blower speed and I usually check mine at med-high because that is where it usually runs.

My real life experience supports the Impala numbers because I bought a 69 model new on August 1, 1969 and still have it. I have driven it almost 400,000 miles and vent temps are in the 30s whenever it is 100F or less.

Interesting that you have a POA tester. I was trying to come up with a design for one using shop air so that I wouldn't have to install and charge everytime I needed to check one. Let us know what the details are.

It was 95F here today and I took the 72 El Camino for another test ride. At 40 mph on inside air, next to highest blower speed I was getting vents of 42F. At 60 mph, the vents were 44F, but I was getting a lot of outside air in because the vacuum canister is missing and the windows are not sealing well, especially the one I have my gage hoses running thru.

At idle, the cooling fell off drastically because of the wrong fan and improper fit in the shroud. I also believe that the compressor is no good at the higher head pressures.

I will post more numbers later but sitting at 750 idle, the high side starting climbing past 350 with 56F vents. I hit the condenser with water and the high side dropped like a rock to 130 and the vents at 750 idle were 39F !!! in the sun. Even the POA started to freeze on the outlet side at idle. That tells you something about the benefit of keeping the high side down.

I also believe that the compressor works fine at low head pressure, but gives up when the going gets tough. I called the owner and the best we can tell, the compressor is a Four Seasons - Yuk! I am going to try a brand new one, but I need to get the fan fixed first.

I also just found out that I was never truly on inside air because the flapper in the plenum is broken. This model uses two flappers, one in the plenum, one in the kick panel. I could hear the noise change when the kick panel flapper moved, but couldn't tell that the plenum flapper was still open. That is why the vents went up, the faster I went.

The joys of a 30 year old vehicle!!

Edited: Thu June 26, 2003 at 9:26 PM by Mitch

Big V. on Thu June 26, 2003 10:06 PM User is offline

Mitch My Man...You have done it. You found my "Silver Bullet", "Wooden Stake"...whatever you want to call it !!
Your magic suggestion of giving that internal 7/32 screw inside the POA a CCW turn of 1/2 rotation has solved and fixed all my A/C problems !! It makes R134a work like it ought to.

Throw away all the GM manuals saying that POA's are not adjustable !!

As you will summarize...I had replaced the compressor, the receiver/dryer, the TXV and the POA as well (found one on ebay and thought I'd see if it would help, but it acted the same as the one I had). I had flushed and replaced the oil as well. In other words my system was basically new. I had checked all under dash controls, water valve, cooling sytem, fan etc. ...all OK.

As of yesterday, my problem was this....that on an 80 to 85 degree day my system would only cool to 50 to 54 degrees at BEST at the vent and then the most annnoying thing was that when returning to idle, or just pulling up to a traffic light...the vent temp would quickly climb to 60-64 degrees. My system low side pressure was 35-45 at idle (depending) and 31 at 1500 rpm..or higher rpm

Today...the ONLY change I did was remove the hose on one side of the POA and reach in with my handy 7/32 socket and precisely turn that "magic" screw 1/2 turn CCW...then I evacuated
(with my so-so 25" pull vac pump) and recharged.
I installed the same amount of R134a that I had previously. (The system calls for 4.5 lbs of R12 (72 oz), so I installed about 65 oz of R134a, or 90%)

Anyway...the POA adjustment made all the system now MAINTAINS 27PSI ON THE LOW SIDE REGARDLESS OF ENGINE RPM. !!
It cools well whether I'm idling, or moving down the road.

The best vent temp I got was 39 and whether idling, or at higher rpm, the vent temp ranged from 40-44
with the A/C set on recirculate at max blower (max blower when set to recirculate is automatic in my 71 Riviera).
Vent temps with the "regular" fresh air A/C setting were about 45-48 degrees. The ambient temp today was about 82.

I am impressed and delighted !!!

So I guess my "new" rebuilt 4-seasons compressor is OK and doing it's job...glad for that ! ...huh?
It seems very able to sustain that 27 psi low side pressure. I checked it several times after driving.

I noted that like your 72 El Camino...that the POA frosts up real icey on occasion as it needs to on the downstream side of the valve...the rest of the time it's very sweaty all the way up the hose to the compressor.

Oh, by the high side pressure after charging and stabilizing was at 200 not bad at all !

My only remaining question at this point that I have is in regards to the sight window on the reciever/dryer.....Should I expect it to clear up using R134a. I put a few more ounces in, but it never clears doesn't have big bubbles, just a smooth white foam, which sometimes, but not often, approaches clarity. Do you think I'm still undercharged ? I tried to minimize any losses during canister changes and let each can escape into the system as much as possible...dipping the can in hot water till the pressure dropped off...
What do you think about that ?


In all the 26 years I've had Riviera's ....I 've never seen such a wonderful "repairing effect" caused by turning a small screw 1/2 turn ! a part that the manufacturer has flatly stated could not be adjusted.

Big V.

Big V.

Edited: Thu June 26, 2003 at 10:47 PM by Big V.

Mitch on Thu June 26, 2003 10:48 PM User is offline

Sounds great! You need to thank 8lugthug too. If you search on Catalina, he first brought up the adjustment possibility on his 1970 Cat although the procedure he quoted was to loosen a locknut which the original valves don't have. Today, I found an o-ring under the spring that the adjusting screw goes thru.

Thank Tim too. This is a great forum to share info on.

After I added 91% of R12 weight to the 72, I had to add a few more ounces to clear the glass. I still get an occasional bubble under certain conditions and I am still trying to decide whether to add any more.

I also noticed that the compressor was fine up to about 200 psi and then performance started to fall off.

On the road at 40 mph, the high side was 135, and at 60 mph, the high side was 150 psi. Under those conditions, low side was steady at about 26-27. Cooling was great even with about half outside air bleeding in at 92-94F. The gage was over by the pass door and my eyes aren't that good to tell the exact number.

Always glad to hear a success story!!!!

Big V. on Thu June 26, 2003 11:14 PM User is offline

Of course !...Thanks to Tim and thanks to 8lugthug...very very much !

Big V.

Big V.

Dougflas on Thu June 26, 2003 11:36 PM User is offline

I noticed that you asked about altitude. I am under the impression that POA's (PILOT -operated absolute throttling valves) are not affected by altitude. The older STV's were affected by altitude but not POA's because POA's have a bellows that contain a nearly perfect vacuum.

Mitch on Fri June 27, 2003 12:40 AM User is offline

This is from the 1969 Chassis service manual for Chevrolet vehicles:

"The only check for proper POA operation is to check the suction pressure at the valve as during a performance test. The POA valve is an absolute valve and will provide different gauge readings based on the altitude where the readings are being taken. Correct gauge reading is 29.5 psig. Gauge readings will be one-half psi higher for each additional 1000 ft of elevation. If a valve gives improper gauge readings, it must be replaced since it is not repairable or adjustable."

I like the part about being non-adjustable. Now we know better, but I believed that for 34 years.

STV valves had a vacuum diaphragm that would raise the setting a few psi when the heater cable was moved off of full cold.

Big V. on Fri June 27, 2003 2:22 AM User is offline

I just wanted to point out that indeed, despite what GM said, either by policy, or it's written service manuals....about it's POA being non adjustable......I applaud the actual hardware of the POA valve. It just took a "jeannie" to uncork the simple truth of the adjustable screw and what to do with it, but the capability was there....a gift from the design past.........
I suppose, in the distant land of 30 + years ago when R12 freon ruled the cooling world, such a policy of non adjustment of the POA made sense (just replace it) to policy making engineer/s. Perhaps it was a warranty issue, or consistency issue, who knows.
But indeed the truth comes out when needed and we need it now and the design is still good just took some looking into and some risk taking for someone to try and "dink" around with it to figure it out.

Hey. it works...and it is adjustable! one can argue with that !

If anyone is can see my 1971 Riviera GS (that now has good operating airconditioning)

You guys stay "cool"........

Big V.

Big V.

Dougflas on Fri June 27, 2003 4:04 PM User is offline

Ok, here's my results on bench testing a POA. I used a Ford type which is basically a GM type with the oil bleed and low pressure ports in a different physical location. Using shop air of at least 60 lbs. I was able to adjust the low pressure from 20 to 29. (I may have been able to go further but I didn't see any sense in that). You have to be careful as the lock nut affects the adjustment. I can't wait for someone to ask me to convert an older system now.

By the way...the tester is rather simple to make. All you need is a fitting to screw into the input(to simulate the evap). put an air nipple on it. Then take you manifold, put the low pressure hose on the low pressure port of the POA. Put the supply hose on the input of the fitting you made. Then take the high pressure hose and connect it to shop air supply of 60 lbs. Open the manifold. Listen to the POA to "click" and stablize. Read the low pressure guage.

Mitch on Fri June 27, 2003 8:20 PM User is offline

Cool, I'll have to try it.

Here's a good business opportunity for someone. Collect a few hundred or so of the millions of POAs made that are now in junkyards all over the country. Test them and reset them for either R12 or 134a and sell them to those who need them.

The Old Air parts company has brand new, out of production, POAs for $650.00 !!!!! when they can get them!!

Who needs an expensive new one when most 30 year old ones are still working just fine.

Edited: Fri June 27, 2003 at 8:21 PM by Mitch

Big V. on Sat June 28, 2003 9:56 AM User is offline

Ya, really, no doubt .....
The extra one I got on ebay (having the exact same part number) I got for $135 and was supossedly 'NOS", not sure if that was true, but it did clear up that occasional wierd, but apparently harmless noise the other one made sometimes either at turning the A/C on, or once in a while sounding like about a 10 second long loud duck-call.."Waaaahhhhhhhh - uuuuu -Waaaaaaaaaahh.

Big V.

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