Hello Forum,
So last night, I vac down my AC system in my 68 big block. Tightened two fittings that alerted my sniffer to a leak at the receiver drier. I filled it with 14 ounces of R-12 and started car and it took in the rest slowly.
80 degrees in Fl last night with humidity at 84%.
Gauges read blue 50's and red 215. Vent temps at a solid 60 degrees. They came down from 80.
I'm concerned as my sniffer was picking up readings inside the compartment. Does this indicate I have a possible evaporator leak?
If I do, I'm throwing in the towel for this season and I will revisit it at another date. I have worked on this thing in my spare time for several months now and thought I meticulously dotted all I's and crossed all T's.
I'd really like to confirm this solid before re- ripping apart the evap box. I did this several years ago and cleaned it out and flushed the evap. I really do not want to go through that again if it's not needed.
I need some input.
Thank you,
Marshal

 

I slight leak in the evap would not cause an immediate cooling problem. I would be more concerned about your high suction pressure - the 50 psi suction pressure is causing your 60 degree air discharge temp. Your high side is also slightly high. I'd be looking at the expansion valve first, and also considering that you may be slightly over-charged on freon (this will cause both high and low sides to read high). Low side pressure under your stated heat load should be around 30-35 psi.

Lars

 

Lars,
Thank you for your response. What should I be looking for concerning the expansion Valve? It is new although I know this day and age that does not count for much. I can evacuate some of the refrig and bring the pressures down. I did get a little messed up trying to do a weigh in and the scale timed out on me.
Thank you,
Marshal

 

Does your system not have a sight glass in the dryer bottle..? You should charge the system until all bubbles just barely disappear.

On the expansion valve, if the sensing bulb is not in intimate contact with the outlet tube on the evap, and not well insulated, it will flood the evap with freon, raising suction pressure and eliminating almost all superheat into the suction side freon. This kills system efficiency, as does an over-charged condition.

Keep in mind that R12 has the unique property that the suction side pressure is almost equivalent to the temperature of the freon at that pressure. So if your suction pressure is around 50 psi, your evaporator temperature will be around 50 degrees, which is producing your 60 degree discharge air. When suction pressure goes to 30-35, the evap core temp runs about 35 degrees, which will produce discharge air in the low 40's. When suction pressure goes into the mid 20's or below, the evap core can freeze from the humidity in the air, which then eliminates all heat transfer in the evap, with resultant high air discharge temps. Many systems have a "freeze switch" in the evap to shut off the compressor when the core temp goes too low. Other systems use a low pressure switch on the suction side of the compressor for the same reason. Either a freeze switch or a suction pressure switch can be used to "cycle" the compressor when suction pressures, and evap temps, get too low. You have the opposite problem, and need to get your suction pressure lower...

Lars

 

Lars,
I will check the bulb placement on the expansion valve. I mounted it using the two supplied.hose clamps in the identical location as the one I took off on the evap upper pipe in the 12:00 position and used the orig. blob of putty on the pipe to re insulate.
In 1968 the compressor ran continuously so the side of the evap case has an ambient temp switch which is what I believe cuts off the compressor in the event of a evap freeze. I'm praying I just over charged it. I do have a sight glass on top of my new receiver drier. At first it looked milky and then it cleared up and I could see refrig. passing through at a fast rate.
Does that sound about right for the sight glass milky fast moving to clear?
Marshal

 

What did you evacuate the system to prior to charging? A clean condenser and good airflow across it are important to proper operation as well.

 

14 oz sounds pretty low. Just barely over 1 lb . Don't know what the factory spec is but on all my old rides (all gm) with the old monster a6 compressor not one of them required less than 3.5 lbs. Had a 71 Grandville that took 6.2 lbs. May want to double check what's required.

 

He did an initial charge of 14 oz. Then gave it "the rest slowly." But he was charging by weight and the scale re-set, so he does not know just how much freon he actually put in the system. If he's at 50 psi on the low side and over 200 on the high, he is not undercharged... An undercharged system will have very low suction pressure and lots of bubbles in the sight glass.

 

Marshal
Getting the factory a/c going can be elusive, but I think you are very close.

Your static pressure (compressor not running) should be around 85 psi when ambient temperature is around 80 degrees.
Compressor running should be about 35/160.
Your condition sounds like high low pressure and high high pressure. I believe your new expansion valve may be stuck OPEN. If it was stuck closed you would have low low pressure and low high pressure. (I got my expansion valve from Old Air. It would suck if the one you bought was defective!!)
Sight glass sounds right. The milky part you are seeing is oil mixed in with the freon. As long as the milky goes away, then you see clear bubbles flowing with velocity and then just a bit of flowing bubbles at about 2500 rpm I think you have about the right freon charge.
Can you repeat the cooling vent temp of 60 degrees when first turning on compressor??

One way to check evaporator leak is to put the sniffer right on the end of the evaporator drain hose.

Don't give up!!
VERYSOON

 

VerySoon,
Thank you first of all for taking the time to educate me on the C3 AC system.You and Lars have given me very good info. This is my first time tackling any AC system.
I left this disabled since 2011 because I did not really need it in my former State of Penna. Here in Fl my wife said to me last Feb. I wish you would fix the AC so we can enjoy this car in the summer time. So during the COVID isolation I resumed repairing the system.
Back in 2014, I removed the evaporator. flushed with 2+2 and blew out all the old oil. It was spotless. It air dried for about a week and then I reinstalled with a new expansion valve and a supposedly working POA. I resealed the evaporator case with Doc Rebuild's evaporator box seal kit.
This is where I left off.
Fast forward to retiring in FL in 2018. I decided to tackle the remainder of the system. I flushed out the condenser, installed a new receiver drier. I purchased 3 new barrier hoses and cleaned up and reused the barbed connections which are unique to 68's. I installed all new green Orings from an assortment pack I purchased from Corvette Central. I installed an Old Air Pro10gen compressor to replace the original one which I was unsure of it's status working or not working. The system blew warm when I purchased this car in 2011.
I vac. down the system for 4 hours due to the length of time it was empty. I had a vac leak somewhere so I vac down again and added to the high side liquid 1 140z can and began my search with a sniffer. I got a hit around the condenser so I checked all connections and got another quarter turn out of the fittings to the receiver drier. My follow up the sniffer was not tripped.
I started the car connected the yellow line to a 30lb jug of R12 and placed it on a scale. It weighed in at 16 lbs so I figured I had about 10lbs of refrigerant. With my gauges hooked up I had removed my vac pump and connected the jug. I opened the jug, bled the yellow line at the gauge end and slowly opened the low side only. The scale revealed I added 1.4 lbs before the 5 minute screen saver timed out on me. The sight glass was clear moving refrigerant rapidly through the receiver drier. Since she timed out I was unsure what to do now??
The system calls for 52ounces. I figured I had about 34ounces in the system if I had to estimate. 14 + 20 ounces.
The 14oz can frosted up but the jug never got cold on the outside. Does this sound normal?
As it was taking in the refrigerant the water temp gauge started to climb into the red so I shut it down. I am blessed that this big block does not over heat under normal operating conditions.
I did not have a fan in front of the condenser and the car was at idle of 750rpms with me occasionally raising the rpms to 1500-2000 range during the charging. I could not restart and add anymore refrigerant because I was afraid of the engine temp that had climbed to the edge of the red. The car normally runs in the 190-200 degree range.
This is where I stopped and consulted this forum for additional help.
I'm learning but I'm really determined to get it working on my own. It will be such an accomplishment to me to have tackled the AC.
VerySoon, I will go out and see if I can duplicate the 60 degree vent temps as the motor has had a couple days to cool off. I will check the evaporations drain hole in the underside of the evaporator case for refrigerant leak. I'm praying that if I got to have a leak it be any where but the evaporator core.
I'll report back with the results. You and Lars have both mentioned the expansion valve so I may very well have a bad replacement unit. It has been on the car since 2014. I guess it could get stuck from sitting unused for 6 years?
Thanks for listening.
Marshal

 

Here is what I would do if I were you:

First, make sure all "legs" of the system are operating without restriction. Any abnormal restriction in the system will cause an abnormal temperature change in the lines. With the compressor running, carefully touch the high pressure line running from the compressor to the condenser - it should be quite hot all the way. Touch the high pressure line coming from the condenser to the evap - it should also be pretty hot all the way, but not quite as hot. Touch the inlet and outlet lines to the receiver/drier - those lines should both be the same temperature. If the outlet line from the drier is cooler than the inlet line, your drier is plugged and acting as an expansion valve - this will cause the problem you are having. The high pressure line from the condenser all the way to the expansion valve inlet should be the same hot temp. The outlet line coming out of the expansion valve should be icy cold. Likewise, the suction line coming from the evap to the compressor should be very cold for its total run. There should be no temperature change across any components other than the condenser and the expansion valve - if you sense a line temp change across any other components (such as the receiver/drier) you can assume that component is bad.

Then, check your sight glass and verify it does not have obvious vapor bubbles in it. If it has no bubbles, I would slowly "bleed" freon out of the system until bubbles just barely start to appear. This is the low limit of your system freon charge, and should result in low pressure running in the low- to mid-30's. If your low pressure and high pressure is still high, I would then suspect an expansion valve that is not choking down and responding to the input it should be receiving from the temperature bulb. If you have an IR temp gun, you can "shoot" the line temp coming out of the expansion valve - it should be in the 30's.

Lars

 

Lars,
Thanks for taking the time to respond and for sharing your knowledge. I will attack it as you have described and report back.
It is nice to have a strategy when you are a newbie.
Marshal

 

Resurrected the long-derelict AC’s on both my ‘78s last spring. Zero complaints from my passenger this summer!
To check for leaks after re-assembly a pressure test doesn’t use any Freon and makes the leak easy to visualize.
A little 20lb. nitrogen tank and Snoop brand of soapy stuff to spray on all the connections (thanks to The Hack Mechanic Guide to Vintage AC-Rob Siegel)
Charlie

 

i did this as well..compressed air 100 psi soapy water ..if holds..then vacuum and see if it holds..then add r12...if not for r12 i would just dump it in. Good luck op! 60 better than nothing..post some pics of that big block ac

op..stupid question but you did add mineral oil right?

 

Yes 11oz of 525 mineral.
Thanks for checking on me on that one
Marshal

 

did you repair the leak prior to charging?

Sorry, read your post a second time and then seen where you repaired the leak.

Neal

 

For the record a pound is 16 oz. Does refrigerant have different weight properties?

 

According to what Ive read you weigh in the container then set the tare setting to zero out the scale.
If it doesnt time out on you after 5 min then you watch the scale till it reaches the desired charge weight.
It is measured in ounces and pounds like any other matter. A 68 takes 52 ounces.
Marshal