merchauser

PO converted my AC from R12 to R134a. no problem with AC blowing nice and cold. my rear evaporator was blocked off and I am going to repair/replace.
will drain oil from evap, measure and replace, but not certain about what oil is most compatible, with what may be trace remains of original mineral oil

internet searches explain that PAG is the best oil for R134a system, but is not compatible with mineral oil. ester oil appears to be compatible, but it is less clear
about POE oil

what would be the best oil to add?

bureau13

I've always heard POE listed as the oil that should be used in converted systems. I have not heard it referred to as "ester oil" in this context, but it may be the same thing?

dr bob

A good conversion includes a label that offers charge and oil info on it. For some reason, many of the most popular "DIY" conversion "kits" included a PAG charge in R-134a. The Castrol kit snaps to mind, a comprehensive collection of conversion parts that had charge port adapters, a can tapper, and the oil charge canister with nothing else. Little wonder so many were reporting failed equipment and poor cabin cooling after their "conversion".

The oil you have in there now is probably OK if it has lived this long with no equipment failures. If you were starting from an R12 system, you'd flush the mineral oil out prior to resealing and re-hosing. If there's any risk of residual mineral oil, POE (poly olester) gets along better with that residue. Meanwhile, ND recommends their own version of PAG in their compressors. For your already-converted system, getting the same oil (PAG or POE) back in their saves you a system flush. You'll be adding oil to make up for losses when you change the drier, Good Idea to use the same stuff for that.

I used POE in my now 20+ year old (spring 1998) R-134a conversion, no issues with very continuous use in the Los Angeles area for a decade and a half before coming here.

Christopher Zach

I recommend Double End Capped PAG. Flush the system, get as much of the old oil out as possible. DEC PAG does not break down as much as normal PAG and seems to work well both in my 928S and 944S.

dr bob

Expanding on any option that includes a different oil than you have in there now:

You MUST flush the old oil residue out of the system. That includes the condenser and the evaporator separately, and each of the connecting hose and pipe sections.

The drier has a dessicant bag that's normally saturated with oil and moisture. The whole drier gets replaced.

The compressor has a small reservoir that needs to be drained, flushed, and the new oil put in. The compressor comes out of the car, and the oil is drained out via the refrigerant ports. Rotate the compressor by hand to get as much out as possible. Measure what comes out for curiosity, compare with WSM compressor charge number. . Then refill with some new oil, rotate the compressor by hand to circulate the oil, and drain. Repeat this a few times to make sure that there's virtually no old oil, then put the correct amount of oil for the whole system back in the compressor. I like this method over a solvent flush through the compressor, because there's no risk of leaving solvent in the compressor with new oil. The system vacuuming will remove any cleaning solvent left in the other sections, but won't get it out if it's mixed with compressor oil. Reassemble the system with new o-rings, then vacuum-check for leaks. If no leaks, apply vacuum at least overnight, longer if the weather is cool. Then, BEFORE YOU START THE ENGINE THE FIRST TIME, rotate the compressor in place by hand to clear any oil that might be in a cylinder. a couple dozen hand rotations is enough. Do this immediately before you run the compressor with the engine, and you'll let the compressor pump oil mist through the system as it runs. That mist will gather in the drier and the bottom of the evaporator, and the rest will circulate as mist through the system whenever it runs. That leaves the correct amount of oil in the compressor for normal operation.

Mixing oils is a problem, especially if there is any mineral oil residue from R12 service. PAG particularly will turn into a jelly wherever it meets mineral oil. That ties up the oil so it can't lubricate, plus it likes to coat and clog system components so they don't work at full efficiency. POE (poly olester or "Ester") gets along much better with mineral oil, passing over residue as lubricating mist without making the jellied mess. It won't go through a drier that's soaked with mineral oil. In fact, modern replacement driers have an improved dessicant that's made for use with R-134a oils. Normal practice is to replace the drier anyway any time the system is opened, so the oil-in-the-old-drier problem is somewhat moot. Just be sure to add oil to the system for the new drier whenever you install one.

Christopher Zach

Amazing post Dr. Bob. This is pretty much what I did with my 928S when I replaced the compressor (was leaking through the front) and totally cleaned out the lines, new compressor, evaporator and condenser. One thing that always made me wonder: When you get a new or rebuilt compressor from a Porsche reseller, is it filled with PAG, POE, or R12 ester oils? by default (I just dumped mine and refilled it with DEC PAG)

Just checking: Is that correct? I seem to recall that PAG goes gooey if it comes in contact with R12 gas, but we're pulling that from the car. I don't think small amounts are a problem. POE has the advantage of working with either R12 or R134 (so if you switch to 134 then get bored you can switch to R12 with a simple evac/replace) *but* the big problem with POE is that it turns into an acid with water, so if your system gets water you wind up with a rotted condenser (happened on my 944S the first time I rebuilt the AC). Capped PAG is PAG that has been modified to be more chemically stable, thus less chance for acid with water (and perhaps less reactivity with mineral oil).

Source:
Kawaguchi Y. (1998) "The Performance of End Capped PAG as a Refrigeration Oil for HFC134a". Purdue University, retrieved from: http://docs.lib.purdue.edu/cgi/viewc...0&context=icec

Hilton

I just had my 87 S4's AC restored using r134a. The important part from the above postings is you must flush all the oil from the system, as the oils don't mix and will cause the mayonnaise-like sludge which blocks up the TX valve.

To flush, remove the Tx valve, and the compressor, and then flush the loop in sections using some clear PVC tubing and an AC flush gun. First with mineral spirits to capture the oil, then denatured alcohol to remove the remaining mineral spirits residue. For my car, a good quality vac pump with a ballast was used, which allows it to vent vapour directly, but even then the oil had to be changed in the vac pump once as it picked up all the vapourised spirits.

New compressor from Roger (drained and filled with PAG46 per factory), and its been ice-cold in >35 degree C, 95% humidity days over summer..

merchauser

thanks all for the information; a lot to process. my AC has been working great ice cold, and as mentioned, PO had done an R134a conversion. from various websites,
POE or ester oil (same thing) is most recommended for conversions because it IS compatible with trace amounts of mineral oil as well as R12.

since my AC was cold, and drier was replaced in 2016, I don't plan on starting over with the oil. I think if the conversion worked for the past 3 years, then whatever method
was done has been adequate.

I removed the rear AC cover today and discovered that the evap had been capped on both sides of the evap side, so I took it out.


plan was to drain whatever oil was in there, measure and replace, now I guess will just add a few ounces to the new one, and reinstall?