AC system "adding just a little"

I need help understanding something related to AC systems. I know that there is a label that shows the total weight of refrigerant for the system. If the vents blow warm and the gauges show low pressures, why is it so bad to add R134a until the pressures come up to where they should be for the ambient temp/humidity and the vent now blowing colder? I know the right way is to evacuate the system and leak check, and I have the pump and ability to do this. It seems to me that if the system is low and you add some it should be ok. Why is this bad?

Merely a band aid.

It doesn't fix the problem.

My dads "been adding a little" for a few years now, I was finally able to look at it and it was the compressor oring all along, but thanks to hundreds of dollars of freon and dye it looks like predator was sacrificed over the compressor with an overabundance of grime.

When you add, make sure it has some leak seal in it. I did this to a B2000 and it no longer leaks down (only works with minor leaks of course). If it keeps leaking down, see above.

Finding the leak and fixing it is proper. On particularly difficult/expensive systems R134 is cheap and filling it once a year is much easier than diagnosing the problem. My Porsche mechanic said not to bother until it needs coolant more than twice a year.

I have a slight leak in my van that needs about half a can of the parts store recharge kit every year or year and a half. I figure that 15-20 bucks a year is an okay deal.

Check all the schrader valves too. 90% of the time when I find a leaking AC system, it's one of those valves and it just needs to be tightened. Uses the same tool for installing the stem valves on tires (exactly the same valves as far as I recall).

I think the bigger reason that "just adding some refrigerant" is frowned upon is this: if the system has a bad enough leak that it has reached ambient pressure, then air can diffuse into the system. Any non-condensable gas (nitrogen) will totally mess up operation of the system and you won't get any significant cooling. In this case, you need to pull a good vacuum on the system before recharging. If your system still has some refrigerant pressure, then this isn't a concern.

Yes. If there is no pressure in the system, don't bother recharging.

Does all of this apply to an R12 system,too? I have a VW that's been sitting for over a decade and the system is low, but still cools a little. I assume it's just low on refrigerant but can't have much of a leak to still be functioning.

I had a case of a customer adding "just a little" for about 4 months.

We told them it needed a condenser about 4 months ago, a rock/squirrel/potatoe/something physically smashed it in and was leaking kinda slowly.

They declined and decided adding refrigerant is easier.

Until last week, even with some pressure, not cold at all. The compressor seized because of lack of oil that has been blowing more and more out as it leaked out with refrigerant. There are now metal shavings in the receiver-drier, and both front and rear expansion valves. The compressor is shot. The condenser still needs replaced. And lines/evap cores need flushed out.

So yeah.

FWIW I've never used one of those stupid cans to recharge something. I'd certainly not do it over and over again.

DeadSkunk wrote: Does all of this apply to an R12 system,too? I have a VW that's been sitting for over a decade and the system is low, but still cools a little. I assume it's just low on refrigerant but can't have much of a leak to still be functioning.

Yes. There's very little difference between R12 and R134a systems.

DeadSkunk wrote: Does all of this apply to an R12 system,too? I have a VW that's been sitting for over a decade and the system is low, but still cools a little. I assume it's just low on refrigerant but can't have much of a leak to still be functioning.

Yes, but you will of course want to use R12 or one of the replacement blends (e.g. Envirosafe).