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Refrigerant and oil – do I measure by weight or volume?

MiataPete on Wed June 12, 2013 1:02 AM User is offline

Year: 2001
Make: Mazda
Model: Miata
Engine Size: 1.8 L
Refrigerant Type: R134a
Ambient Temp: 110
Country of Origin: United States

I need to recharge the system in my Miata. I want to make sure I do it correctly.

The sticker on my hood says, “Maximum Operating Charge 0.40 Kg (14.1 oz) of R134a”. Since it specifies “Kg”, I assume that I recharge by weight … 14.1 oz of R134a. I’ll need a scale to do this, correct? And, I’ll be using two 12 oz refrigerant cans, correct? (all of one can, and part of the other … carefully measuring how much weight is removed from each can?)

But, before I start, I have to add oil … Pag 46 (I have completely dismantled and cleaned my entire system … compressor, evaporator, new expansion valve, condenser, and all lines). A number of sources list 5.07 fl oz as the correct oil capacity for my system (2001 Miata). The Mazda Service Manual states, “A/C Compressor Sealed Volume 5.07 fl oz”. I plan to add half of that volume to my compressor (working it out of the compressor by hand, before charging, to avoid hydraulic lock) and half to my receiver/drier.

So, is this all correct? 14.1 oz of R134a, by WEIGHT, and 5.07 oz Pag 46, by VOLUME, is how I should fill the system?


emsvitil on Wed June 12, 2013 1:21 AM User is offlineView users profile



GM Tech on Wed June 12, 2013 7:59 AM User is offline

Makes very little difference-- the relationship of volume to weight of refrigerant oil is 1.1 to 1.0- so either way works for me...

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

iceman2555 on Wed June 12, 2013 10:02 AM User is offlineView users profile

A suggestion to re think this repair. Add 2-3 oz to the compressor. Re install on the vehicle and then rotate the compressor. This maintains the lubricant within the compressor body or hose assembly.
Add the remainder to the new evaporator prior to installation.
This vehicle utilizes a PF Sub-cooling condenser. The rec/drier is built into the condenser. It is highly doubtful that you would be able to add this amount of lubricant into the unit. Adding a couple oz to the evap will insure oil flow back to the compressor at start up.
Question is why was it necessary to completely flush the system? What type flush was utilized with this cleaning process and how was it removed from the system to prevent residual accumulations of flush chemicals? How as the compressor 'cleaned'?
Keep in mind that your suggested recharge method is not an acceptable method to charge the system. It should be recharged utilizing a method to correctly measure the amount of refrigerant added to the system. Also the amount and method of recharge will produce a seriously undercharged system, there is no allowance for the amount of refrigerant necessary to pre fill the manifold/hose assembly. This amount is typically 3.5 to 4 oz. Measure to insure amount. Pressures are not a method to insure proper recharge rates. A 'short' charge will reduce lubricant migration thru the system and result in serious compressor damage.
Good luck, take the time to insure a complete recharge to prevent a premature compressor failure.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

mk378 on Wed June 12, 2013 11:56 AM User is offline

Charging with a big cylinder and scale is a lot more precise than using cans. It avoids the uncertainty in the losses involved in changing cans over. Late-model smaller systems such as yours need to be charged quite exactly by weight to get proper performance.

Measure oil by volume for example with a kitchen measuring cup. PAG cleans up easily with water so don't worry about using it in the kitchen again later. Avoid exposing the oil to air any more than necessary as it rapidly absorbs water.

Edited: Wed June 12, 2013 at 12:01 PM by mk378

MiataPete on Wed June 12, 2013 2:20 PM User is offline

I’ve already got the evaporator re-assembled, with the new expansion valve, and it’s all reinstalled in the car. It is not a new unit … I flushed/cleaned the old unit, which was in perfect condition. I used “A/C Pro Flush & Clean”, which I purchased at Autozone, blew it out with compressor air, baked it in the sun, and blew it out again and again … then baked it some more, and blew it out again (same process on the condenser).

Since the condenser is completely apart, and out of the car, I thought I could add part of the oil to the rec/drier. I was planning to add it in, when I replaced the desiccant bag. Yes, it is a small unit, but I have complete and easy access to it. Won’t that work?

I did more than just flush the system … I completely removed, and then dismantled it … every single part … the entire compressor, too (it is back together now, with all new seals).

What happened to my system? A very LONG story … basically nothing. I added some R134a, and subsequently blew the pressure relief valve at the back of the compressor. Internally, my system was completely clean … no debris, clean oil, no blown seals or o-rings, no damage to the compressor, no leakage at the shaft seal. The compressor is now rebuilt with new gaskets, o-rings and shaft seal.

Since Mazda specifies “Maximum Operating Charge 14.1 oz”, how can this be too little R134a? I thought that the label on the hood was the “bible”. If I weigh the refrigerant going into the system … weigh it carefully … am I not measuring it correctly? Doesn’t the manufacturer (Mazda) take into account hose/line volumes when stating Maximum Operating Charge?

iceman2555 on Wed June 12, 2013 5:59 PM User is offlineView users profile

Yes, Mazda knows the correct charge for their vehicles and if it is 14.1 oz, then this is the correct charge rate. However, the posted procedure may not produce this same charge rate. Often some refrigerant remains in the cans, some is lost when cans are changed. The suggested addition of refrigerant was not an adjustment to the factory charge rate, it is the amount of refrigerant your service manifold/hoses require to pre charge. Typically a block type manifold with 72 inch hoses will require an additional 4 oz of refrigerant to pre fill. Modern AC service equipment programming allows for the refrigerant necessary to pre charge the hoses. The posted procedure or lack of understanding could result in a seriously undercharged system. This would drastically effect lubricant movement thru the system and result in serious compressor damage or complete failure. A suggestion would be to bit the bullet and have the system professionally recharged.

Secondly, the flush utilized contains approximately 65-75% odorless mineral spirits. This material can not be adequately removed from the system. Traces will remain on the internal surfaces of the components and then be removed by the refrigerant as it migrates thru the system. This residual amount can effect the desiccant material in the new 'drier sock'. This material should be removed from the system utilizing a different flush chemical. Mineral spirits is a good cleaner...just should not be used in an AC system.

Adding 2-3 oz of lubricant to the drier of this condenser may not be the best method. This is tremendous amount of liquid to be inserting into the rec/drier cylinder. The sub cooling section of this condenser has the least amount of flow area in the condenser and it is possible that the lubricant could result in a flow restriction. Keep in mind that most of the lube added to the compressor will be ejected thru the discharge hose and into the condenser also.

Good luck !

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

MiataPete on Thu June 13, 2013 1:04 AM User is offline

Thanks, iceman2555!

By the time I got your recent reply I had already guessed that you were referring to gauges/manifold/hoses … I didn’t think about it long enough, before I started to type. Sorry about that. And, yes, I can easily see how accuracy can be lost. A reply by mk378 also mentioned the problem with tracking weight when changing cans.

So, two questions …
First … is it a waste of my time, and money, to try to recharge the system myself? Is there ANY chance I can do it accurately, since I can only find 12 oz cans available here in the US? (though, I can certainly get the right amount of Pag 46 oil into the system … in the right places, in the right amounts, and worked thru the compressor to avoid hydraulic lock)

Second … you mentioned trace mineral oil from the flush. The stuff I used states it contains, “a very light grade Polyol Ester based fluid” (which is an oil, soluble in R134a), and “synthetic Alkylanted Arly cleaning compounds”. The label states, “cleans R134a systems”, “ideal to retrofit from R12 to R134a” (which I am not doing), and “compatible with PAG, Ester, and Mineral Oils”. Are you sure that this product is a bad flush? If so, then what should I use? I’ll have to pull the evaporator back out, disassemble everything and re-flush … including the parts not yet reinstalled (condenser, and all fluid lines and hoses). Can I simply flush with R134a, and assume that it will carry the trace residue out?

Thanks for your help,

emsvitil on Thu June 13, 2013 2:32 AM User is offlineView users profile

You weigh the can (with the tap on it) before and after to find out how much actually left the can.

Awhile back I sacrificed some R134a from a can to figure out how much is lost by charging the various hoses and manifold gauge by measuring the weight of the can before and after each step. Don't have the figures handy as I'm out of town, but it wasn't anywhere near 4 ozs........


HECAT on Thu June 13, 2013 8:49 PM User is offline

The flush probably carries a 486 or 487 part number and says "Contains a very light grade of Polyol Ester based fluid with synthetic Alkylanted Aryl cleaning compounds." The MSDS says the chemical components are "proprietary", that means you don't need to know, just trust us.

First I think you have worked hard to remove and clean all the pieces you have wanted to reuse and given your "extra" effort I do believe the trace residue oils will be of little to no concern. But I do want to explain what you have read on the label; or more like what you are not supposed to read. Marketing 101.

Polyol Ester can be used as another term for crude oil and its many derivatives. Using it here makes you believe it is POE oil compatible. What it really means is a cheap and oily petroleum distillate base that is going to leave a lot of oily residue being but its OK wink wink. I know for a fact a higher quality and less oily petroleum distillate base stock can (is) or could be used as a component of A/C flush. Even if it was proven to be chemically compatible, wasn't the whole flushing idea about removing the oil?

The synthetic Alkylanted Aryl is another play on words (do a google search) and probably is something like heptane to kick up the solvency of the cheap base stock. Makes the stuff smell like it must be a bad ass cleaner.

All of the higher boiling point solvents offered for A/C flushing will leave some oily residues even after proper drying efforts, the key is to use one that leaves very little trace and has a proven track record. How much is left behind comes from the chemistry and short cutting the removal/purge process;how much is left behind and how compatible it is within the newly operating system is the million dollar question.


HECAT: You support the Forum when you consider for your a/c parts.


wptski on Thu June 13, 2013 10:19 PM User is offline

I had thought that ester oil and POE oil were one in the same but it appears not. A Wikipedia on ester oil states that it has a solvent-like quality where it will absorb mineral oil residue.

Edited: Thu June 13, 2013 at 10:22 PM by wptski

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