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My 2008 Mazda 6 2.3L air conditioning isn’t quite cooling like it should. I had it charged at the end of last season when the temp outside was in the low 70’s. I’m thinking that maybe it’s overcharged for 100 degree days. Does anyone know what the high and low pressures should read?

(Is a 2008 considered first generation? It’s listed on both 1st and 2nd generation.)
 

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This is the chart that I go by when dealing with a.c.. It's work great in any of the cars that I've used the chart with. While I don't have Mazda specs for the a.c., my 03 a.c. works great here in FL. I hope that it helps.
 

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Note that ambient doesn't tell the whole story; for TXV systems you need the superheat measurement. The reason is that airflow is not a constant. Further, this assumes fixed-displacement compressors, which I believe Mazda has in all their cars (common in those of Japanese and US heritage; European vehicles tend to be variable-displacement.) Variable-displacement compressors CANNOT be accurately checked; they can be WAY off in charge state and show what look like correct values but they're not. You can only charge those by weight which means you have to recover all the existing refrigerant, draw vacuum and then recharge from zero.

Whatever you do never use anything other than straight refrigerant to "top off" a system -- none of the gimmicky stuff, ever, and ESPECIALLY nothing with a "stop leak" or similar in it. The only exception is a small amount of UV dye if you're trying to find a leak, although most modern refrigerant cans have a bit of it in there anyway for leak detection purposes.

There are a number of things that can cause poor performance; the first to check is the fan(s) behind the radiator to make sure they're working properly under all conditions. Different vehicles do this differently, but a fan that is not running when it should will produce not only poor performance but eventually destroy the compressor due to excessive heat. The next possibility is a malfunctioning TXV (usually pretty easy to replace), and then you have both the evaporator which can be plugged inside the airbox (from outside-the-vehicle debris, although modern vehicles have cabin filters that usually prevent that) and both it and the condenser can have some of their tubes plugged due to an overheated system that cokes the oil. If that's happened then the entire system needs to be ripped out and replaced as it's virtually impossible to adequately flush and clear the internal tubes in parallel-flow evaporators and condensers.

If you had it charged last year and it's low now you need to find the cause. If you're lucky it's a bad O-ring or hose, but often the issue is the shaft seal on the compressor, and if so there's nothing you can do but replace it. You CANNOT just drop a new compressor in either as they all come pre-charged with oil and as such you HAVE TO flush the system to get all the existing oil out *and* any time the system is depressurized the dryer has to be replaced. The dryer's chemicals are irreversible (unlike the little dessicant things you can buy for a safe or similar) and moisture + modern refrigerants + heat = an acid, which will eat the system from the inside.

Finally it's illegal to intentionally vent refrigerants so unless the system is already empty the only legal way to do any of that is to have a shop recover the refrigerant that's in it for proper disposal.

Also, mandatory warning I always include when talking about stuff like this -- NEVER, EVER work around any refrigeration system of ANY sort without good hand AND 100% COVERAGE eye protection. If a hose fails or similar the boiling liquid will instantly freeze whatever it comes in contact with. If it gets sprayed into your eyes you're flat-out screwed and freeze burns on your skin are no fun anywhere.
 

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Again there is no accurate way to know other than to recover the refrigerant in the unit, vaccum it and then charge it by weight. If you never open it in this process to the atmosphere then you do not need to replace the dryer.

However, I'll put $20 out there that this isn't the problem. It's possible, but not all that likely. More-likely causes are:

1. FANS! Check them FIRST. They (or the controller) fail all the time and when they do cooling is severely impaired, especially at lower speeds (e.g. in traffic, etc) or when it's REALLY hot.

2. The evaporator and baffles/selectors. These are a BIATCH to get at in most cars as they're buried in the dash. Some vehicles (e.g. VWs) have foam on the blend doors from the factory to seal holes and the perimeter in same. It deteriorates and then the doors do not properly select which way the air flows (over the evaporator, over the heater core, etc.) Figuring this out is a five-alarm PITA and fixing it on my most cars is even worse; it's not uncommon to have to remove the entire dashboard to get to them. NOT fun.

3. The compressor itself. It's basically a rotary-driven piston thing (multiple pistons in bores) and when they wear out they start bypassing refrigerant and don't actually compress. If that's happened there's not much you can do other than replace it. But replacing it is an involved process because you MUST flush everything to get all the old oil and gunk out and, if there's any evidence of destruction in the hoses and such (e.g. metal flakes, etc) you have to get THAT out or the new compressor will be destroyed VERY rapidly. This is why an auto AC shop will NOT just change a compressor in most cases; they don't want the warranty callbacks and 90% of the time they'll get em if they don't change EVERYTHING, so they do (and you get the bill.)

Having the unit vacuumed out and recharged should not be very expensive; the refrigerant is maybe $30 or so, and it's a half-hour of labor or thereabouts, so call it $100 to have it recovered, sucked down under vacuum and recharged by weight. But if any of the above is involved that won't do anything useful.

Putting a set of manifold gauges on it will tell you if the system is GROSSLY out of spec but not if it's only somewhat (e.g. half-charged) so. Oh, and there's a risk to using the service ports too -- the seals deteriorate, so if you stick gauges on the system you better have a set of service ports in your toolbox (they're standardized and about $10 for both) because after 10 years or so you're odds-on that one of them will fail when you disconnect, and if you DON'T have them in-hand at the time then you're buying a dryer immediately since all the refrigerant is going to leak out when you remove the gauges!
 
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