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1990 Miata AC Problem

I have a 1990 Mazda Miata, and the AC is not working. It has been switched over to R134a. I took it to a mechanic to have it recharged a few weeks ago. He said that one of the o-rings was cracked and so it was leaking around that. He filled it up and put some stop leak in it and for awhile it seemed to work fine. Then it stopped blowing cold air. I took it to a jiffy lube just to have the level of coolant (correct term?) checked. The person there told me that it was very low and was leaking around the valve stems. I tightened the valve stems and re-filled it. After 24 hours it had stopped blowing cold air again. I got a price quote on a new compressor and it is about $500.

So - my question is this - should I just put a new compressor in or is there something cheaper I could try first? I’m not incredibly knowledgable about cars, but I am willing to try doing simple things myself.

Stop leak is not an effective repair for a cracked O-ring. I doubt you need a new compressor, you just need to fix the leaks, correctly.

Keith -
Okay, that is good to know. Thanks! I assumed a broken o-ring would mean the whole compressor needed replaced. Can I detect all the leaks on my own?

The tools and equipment needed to find and repair and recharge your AC will cost more than the price of a competent shop doing the work. But if you wish, buy the tools and equipment and give it a shot. Then you’ll be ready when future problems arise.

When the AC was converted from R12 to R134a, was the oil in the AC system switched out?


I have no idea. What problems could arise if it wasn’t?

As a follow-up question: How much should I expect to pay if it is the O-ring? And is there a way to pin-point if the o-ring is the problem?

Lack of oil in a compressor can wipe out the compressor. Anytime refrigerant is lost from a leak there will be some oil loss so generally during a recharge it’s a good idea to add an ounce or two.

There are several ways of checking for leaks. Dye and soapy water are a couple but my preference is with an electronic sniffer.
There is no way of determining repair costs until any and all leaks are found. Most leaks involving O-rings are not that major. If a compressor shaft seal is leaking then it may be time to consider a new compressor based on the age of the car.
A DIYer can patchwork things but a shop that is expected to stand behind a job should not be cutting corners.

Don’t assume the compressor needs to be replaced at this point. It all depends on what is actually leaking.

 "I took it to a jiffy lube" 

Please never take your car to any quick lube place.  Don't even think about it.  They are well known for their problems.  

Most people would be far better served by taking their car to a local independent mechanic. 

Personally I recommend taking your car to a real A/C place.  Often they are radiator shops as well.

R12 AC systems use mineral oil as a lubricant. R134a AC systems use a PAG or ester oil as a lubricant. If the AC system is converted from R12 to R134a and the oil isn’t converted over the mineral oil won’t mix with the R134a refrigerant so it’s carried throughout the AC system. When this happens the seals used throughout the AC system don’t get lubricated with oil and they dry out crack and leak.


Many cars had kits you could install if you wanted to use the newer R134a. The kit included all new seals. On some vehicles this was very expensive (over $400). Others…Not so expensive. Some manufacturers saw it coming and designed their new AC system accordingly so when the switch was made the existing seals worked find. Other manufacturers didn’t even have a kit available. You had to replace the whole AC system.

Since your AC works good when charged, the compressor is good. A shop will charge your system using a special dye that is visible under a UV (black) light. Then the appropriate seals can be replaced.

Any vehicle can be converted to R134a, it only takes a set of port adapters. That doesn’t mean that its going to work very well, but it can be done to ANY vehicle. I converted an 86 Toyota Tercel and it worked fine. The compressor had over 300k miles on it when the vehicle was totaled and still was working. The conversion was done 3 years and about 60k miles earlier here in the mid south.

As for the “valve stems” leaking, that is a real problem with those conversion kits that just install a port adapter over the R12 ports. They have an epoxy on the threads so its a one shot deal. Screw this up and there is no fix. If this is done right, then it is not uncommon for the schreader valves to leak after a few uses. I would replace them.

As for any DIY, it takes a lot of equipment. You should be pulling out all the refrigerant with a recovery system before working on the system. Any shop that just refilled the system without check for leaks violated EPA laws and could loose their license for servicing AC systems. They are not allowed to fill a system with known or suspected leaks.

Find a good shop and get this done right or turn this into a true sports car and remove the AC altogether. I never saw an MG or Triumph with AC back in the day.