a/c oil under the hood

ruf-porsche · ruf-porsche's Garage

If you see a little streak of a/c oil under your hood, don't worry about a seal failure in your compressor. Some info I found on the net thanks to Jim Sims.

The first thing to recognize is that refrigeration compressors DO NOT use rubber lip seals. Instead, they use what are commonly called "face" seals. A face seal is basically two finely machined surfaces which use the surface tension of some type of liquid (usually oil or water) to create a barrier to gas and liquid flow. On side of the seal is usually made of a softer material such as carbon or bronze and the other side is generally steel or iron. The two surfaces fit together very tightly and, in the presence of liquid, make a very reliable and long-lasting seal.

It is important to know that face type seals have important advantages and disadvantages when compared to the better know rubber "lip" seals. These are:

1. Face seals require oil to seal -
Without a good coating of oil between the two faces, the seals will leak. This is why it is a good idea to run your car air conditioner periodically even in winter. By running the compressor a few minutes every month or so you re-establish that a coating of oil between the seals. Without that oil, the seals will leak. If you are going to leave you compressor unused for more that a month or two, make sure you either (a) have someone turn it on periodically) or, (b) close the compressor shutoff valves to prevent gas from leaking out of the rest of the system should a leak develop.

2. A small amount of oil/gas leakage is normal -
Face seals do not create a perfect seal. Realize that some leakage is normal. One of the oldest tricks in the refrigeration technician "scam" book is to place an electronic leak detector right up against the shaft seal. When the alarm goes off, the technician explains that the seal is "bad" and needs to be replaced. The fact is that these seals very rarely fail and generally last the entire 20-30 years that the rest of the compressor will. Also, realize that it is normal to have a small amount of oil "sling" out from around the shaft. It does this in your car air conditioner as well. You just haven't noticed it because car air conditioning compressor have a built-in "wick" to retain it in one area. again, we are not talking about massive oil leaks here. Just remember that a little oil does not mean that your seal is failing.

I think when they refer to seals drying up because of not using the a/c they are really referring to the face seal of the compressor.

Bob's Flat-Six · Bob's Flat-Six's Garage

Ruf, Thanks for the great info.

david914 · 08-02-2003, 05:57 AM

Ditto! Thanks for the info! I had been wondering about that since I had noticed that even with a new compressor.

ruf-porsche · ruf-porsche's Garage

Yeah I was thinking about finally changing out the old Yorky to a Seiki Seiko or Sanden compressor because while cleaning the car and engine bay I notice a light film of oil above the a/c compressor. But the system is working great and I really hate to mess with it until I really need to, so I will just keep an eye on the hood to see if the oil deposit increase.

Jupiter · 08-02-2003, 08:39 AM

Great info, I was wondering where that little oil came from anyway. Won't worry anymore

GeorgeW · 10-09-2003, 05:46 AM

Hi,
Please let me have your thoughts:

My 86 coupe seems to have a compressor seal problem. It slings a lot of oil on the uderside of the engine lid, a thick blackish layer, after only running the a/c for about 1 hr.

The PO had the compressor fixed, I have paper work, but the problem persists. Before I take it to a pro wrench I would appreciate any thoughts you may have. The compressor is a York and time certs were installed in the compressor bracket mounting the compressor. There is a slight cam oil line leak, but I think the 'sling' is not motor oil.

Could the 'seal' be bad eventhough there maybe no rubber seal ?

Jim Sims · 10-09-2003, 09:15 AM

The face seal can go bad; it depends on the compressor bearings to maintain it's running clearances and hence it's ability to seal. If a seal begins leaking because the compressor bearings are worn then a replacement seal will also soon fail if the underlying problem (such has worn bearings) is not addressed. The new seal installation could have also been improperly done. This issue has to be faced when using rebuilt components; are they rebuilt properly or just "patched"? Jim