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2001 CR-V low refrigerant advice and thoughts sought


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JimmyC
Novice

Sep 9, 2020, 10:01 AM

Post #1 of 12 (1793 views)
  post locked   2001 CR-V low refrigerant advice and thoughts sought  

Hi. Thanks for reading my message. I'm new here and would appreciate peoples' thoughts. I've got a 2001 CR-V. It used to be (before 2017) a daily driver but, in 2017, it was stored. It's now used at my farm as a utility car, but it still runs very well and could be a daily driver. I'm interested in seeing if I can revive the AC which was working fine before the car was stored, but which is not working now. I'm also interested in not screwing things up! In the years before it was stored, the car had a SLOW AC refrigerant leak. It had to be topped up just once (around 2014, I think) because it wasn't blowing cold anymore. I used a kit to top it up which had a refrigerant bottle which contained oil and a leak stop. The refrigerant was just a touch low at that time --- though nowhere near zero, and the compressor was cycling perfectly --- and the top-up restored the cold AC perfectly. Since being stored, that slow leak once again saw the refrigerant leak out slowly. This year, on removal from storage, I noticed the compressor wasn't cycling in at all. I did a static/cold pressure check on the low side port of the ac and, although I neglected to write down exactly what I saw there, I believe there was a residual charge of about 20-30 PSI on that side. Again, definitely not zero, so no air or moisture should have gotten into the system during storage. My hypothesis is that the compressor is not cycling on because the pressure switch says "not enough refrigerant". I added a full 12 oz can of straight r134a over days, slowly letting the contents diffuse through the system while watching the static pressure rise. Fill a little, wait 12 hours, come back, read static pressure, add a little more... repeat. At this point the static pressure in the system is around 50 psi and the compressor still isn't cycling on. I'm not surprised, though, because static cold pressure in my other cars (I measured to compare) is around 70 PSI, so I'm probably still below the pressure switch minimum threshold to let the compressor cycle on. I likely need to add more refrigerant. However, at this point, I'm saying to myself, slow down, maybe you've lost a lot of oil, too! That question brings me here.
I suspect the very, very slow leak in this system is at the low-side schrader valve. It's one of the physically highest points in the system. I suspect this because when I did the top-up years ago, I removed the cap on that side and heard a small "pffft". Same this time after removing the vehicle from storage. There should be no pressure (above atmosphere) between that cap and the schrader valve. Hearing that pfft tells me there's material escaping from the schrader that this cap is covering. Now, seeing as that value is physically a high point in the system, and assuming that my hypothesis is correct that this is where the leak is, have I lost oil, too? Does the oil settle at low spots in these systems so that if I'm leaking at that schrader, I'm only leaking r134a and not oil?

I assume I could get a syringe-type PAG oil injector and add an amount of oil to this presently-pressurized system, but do I need to do that, or am I fine to add a little more r134a out of a second 12 oz can? A reference I found online indicated that my system takes 25 oz of r134a and 4.5 oz of PAG 46. If all I've lost is r134a through the schrader, I should have lots of oil in the system still. If I've lost r134a AND oil, I'd better be happy I've not had the compressor cycle in yet!
Keep in mind, this is just a fun academic exercise for me. This isn't my daily car, and I don't want to spend lots of money here. I've only got a low-side gauge and don't want to rip this system apart, but it would be fun to get the AC running again. I also really don't want to destroy anything, either. I'd love everyone's thoughts!
Thanks for your time!


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Sep 9, 2020, 10:29 AM

Post #2 of 12 (1783 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

You are so far off on your logic and understanding of refrigeration systems that you should just stop while you have not destroyed anything yet.

First off, the static pressure reading has absolutely nothing to do with the amount of refrigerant in the system. As soon as you reach base pressure which is usually less than a pound, nothing changes in pressure whether it has one pound or ten pounds in it. Temperature is the only thing that changes that.

You also cannot accurately "top off" a system as a system cannot be charged according to pressures.
The ONLY way to correctly charge a system is to completely evacuate the system, vacuum it @ 29" of vacuum for 30 minutes and then recharge the system with the exact specified weight of charge.

If you are not equipped or prepared to do that, then you should just leave it alone before you do damage.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



JimmyC
Novice

Sep 9, 2020, 11:00 AM

Post #3 of 12 (1777 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

OK, thanks for the advice and the response. I definitely feel humbled. So, if I were to do as you suggest, does the vacuum pump pull out the oil, too?


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Sep 9, 2020, 11:05 AM

Post #4 of 12 (1773 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

No, it does not. Keeping the system under vacuum lowers the boiling point of water down to room temp so the purpose of it is to boil off any moisture in the system.

As far as oil, you just have to guess at how much was lost. I usually add 2 ounces to a routine recharge.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



JimmyC
Novice

Sep 9, 2020, 11:27 AM

Post #5 of 12 (1768 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

OK, thanks again. So back to my original idea: If my leak of refrigerant was at the schrader valve, have I lost oil AND referigerant, or mostly just referigerant? Please go gently on me: I don't need to be humbled! I'm the first guy to say I have a pretty limited understanding of what's going on. I am asking, after all! That's why I'm asking someone who knows a lot more than me!!


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Sep 9, 2020, 11:34 AM

Post #6 of 12 (1762 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

Oil mixes with the refrigerant.
It's not the location of the leak that matters, it's the speed of the leak. The faster the refrigerant leaks, the more oil it will take with it. All leaks will take some amount of oil with it. The oil settles into various components when the system is shut down.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



JimmyC
Novice

Sep 9, 2020, 11:58 AM

Post #7 of 12 (1756 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

OK, so if my leak appears to be very slow, and the car has stood unused for a long time, it's possibly safe to conclude, from what you've told me, that I haven't lost a lot of oil. That's good news.

Now, if I look online, I can find a vapour pressure chart for r134a which tells me that at about 70 F (I'm around that today) the vapour pressure of r134a is about 71 PSI. I understand this to mean that if I have a sealed container at 70 F and a little bit of liquid referigerant in that container, and a little bit of vapour refrigerant, a pressure gauge placed on the container would read 71 PSI.

Taking this a little further, if I have another closed container at the same temperature, but a gauge attached to it reads less than 71 PSI, I must be able to conclude that there's gas-phase refrigerant in that container, but no liquid-phase refrigerant.

Do I understand this correctly?


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Sep 9, 2020, 12:08 PM

Post #8 of 12 (1752 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

No, you do not.
The only thing beside temperature that will change the pressure is air in the refrigerant which usually only happens in a recovery machine sucking in air from a leak in the car's system. That's why recovery machines have a gauge for tank pressure to know if there is air in the tank. The air will rise to the top and can be bled off after the tank has sat for a while.

You are really over thinking this pressure stuff. The static pressure is not that important to what you are doing.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



JimmyC
Novice

Sep 9, 2020, 12:21 PM

Post #9 of 12 (1746 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

OK, if I may ask, where's my mistake in my understanding of vapour pressure? Please trust me: I'm not asking to be a pain. I really am trying to understand. You mentioned earlier that my understanding was really poor, so I'm trying to fix that.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Sep 9, 2020, 12:31 PM

Post #10 of 12 (1742 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

I've already answered this question a couple times. You are fixating on something that is just not important in fixing your car. The static pressure doesn't tell you much of anything and the info is of no value is repairing your car.
There is no difference in vapor or liquid pressure. A small amount of gas is at the top and the liquid is at the bottom.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



JimmyC
Novice

Sep 9, 2020, 1:57 PM

Post #11 of 12 (1731 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

In fact, in principle, the static pressure could tell you everything about a car's refrigerant level if the oil level is reasonably close to specifications. If the system is evacuated, and then the proper amount of charge and oil is added as per your aforementioned procedure, and if that charge does not cause the pressure in the system to exceed the vapour pressure of the refrigerant, then a total system volume can be calculated (minus the oil volume) at a given temperature using the static pressure measured after the admission of the correct charge amount. That can be deduced using the ideal gas law PV=nRT. Knowing the system volume will allow you to deduce the level of refrigerant in a system forevermore simply by measuring the system static pressure and temperature, assuming the oil level has remained approximately constant between measurements. The volume will vary from car to car, by model and year, of course, but if, as I am, one is working on only a single car, it's all one needs.

Adding charge to a fully evacuated system will increase the static pressure of the system in proportion to the amount of charge added. Again, this is described by the ideal gas law. Since V, R, and T are constant, then

P=n(RT/V),

where n represents the amount of refrigerant. This will happen until the vapour pressure is reached, after which point an amount of refrigerant liquid will condense in the system and another relationship applies. However, below the vapour pressure, one could plot a relationship of static pressure versus refrigerant added to arrive at a determination of the system volume. As already stated, one could then work backwards with the calculated volume to determine the amount of charge simply by measuring the static pressure.

I'm happy to be corrected if there's an error in my reasoning.


Let's move on to pressure switches.

Is it true that there is a pressure switch in the AC system which prevents the compressor from activating if the pressure at that switch is below a certain threshold?


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Sep 9, 2020, 1:59 PM

Post #12 of 12 (1727 views)
  post locked   Re: 2001 CR-V low refrigerant advice and thoughts sought  

OK, we're done.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.







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