Charging an AC system - Do I need oil too?

Both of my son’s cars (1996 Explorer and 1998 Taurus) need their AC recharge. In both cases, they slowly lost refrigerant (over 2 plus years). I’ve charged systems before but am wondering if you need to put one of those cans of R134 mixed with oil when you’re recharging, or just the R134 can by itself ?? I’m assuming that both have small leaks because the last time they were charged was a few years ago (by a mechanic).

Yes, it’s a good idea to add a bit of oil. Whenever refrigerant is lost oil is also lost with it as the oil under pressure becomes a fine mist.

Sometimes you may see an oil/dirt residue that has accumulated at the spot of a leak due to this oil mist.

If I was recharging my system, I would use the refrigerant with oil in it. However, I wouldn’t recharge the system without finding and fixing the leak first.

I have recharged using a stop leak additive for a slow leak. I had to recharge every year and now it is 2-3 year intervals.

It might also be a good idea to pull a vacuum before refilling.

Even if it was a small or slow leak, I would still get it fixed. Recharging the system doesn’t address the contamination that is getting into the system.

It is so hard to figure out how much to put back in. Perhaps OK4550 has noticed, do you think the oil residue left by the oil used in an R12 system is easier to see that the oil residue left by a R134A system? several techs I worked with say the oil trail is not so obvious with the R134A systems.

For this OP ,you should look at new driers also.

The oil mist is not that noticeable but over time the dust in the air (plenty of that in OK) settles into the oil residue and can make it pretty noticeable.

I wouldn’t bother adding oil unless you have a major leak and/or have had to top it off many times. Most of the oil is likely still in the system. I wouldn’t bother with those oil charge cans either—I’m no expert, but I think over adding oil would be almost as harmful as not having enough, and I doubt you’ve lost much.

You usually need to add oil only when major service is done. It’s a sealed system and it doesn’t use oil like a car engine does.

It’s doubtful that contamination is getting into the system—the system is under pressure and is leaking out. The fact that it is under pressure makes it pretty hard for anything to get in unless you lose all pressure.

I would go with the oil. It’s probably leaked out with the R-134, but I would try to find the leak beforehand (you can buy a dye tracer kit for relatively cheap, plus it makes a fun weekend project). Also, the oil can help to deal with some of the moisture in the system. I would be willing to bet that the compressor’s life shortened because some moisture is bound to have found its way into the line; the oil will help to somewhat counteract that. Also, you may want to consider replacing the drying bulb.


Oblivion is correct…95% percent of the oil stays trapped in the compressor. With very slow leaks, very little if any oil escapes…The ONLY way to safely charge an A/C system is with a SET of gauges so you can monitor BOTH the high and the low side as you are adding refrigerant…

The “just dump in a can or two and hope for the best” method is really asking for trouble…

What I was really getting at was that it seems that the oil residue from R12 systems attracts more dust than R134A systems, just a unscientific observation

What happens when you shut it off and the pressure drops? The dropping pressure doesn’t suck in any contaminants?

How can a leaky system be considered a “sealed system?” Aren’t those things mutually exclusive?

Oil loss due to refrigerant leaks is what causes the compressor to go bad or even seize up or explode in severe cases.

Look at the bottom of an aged compressor and ask what’s causing the grunge on the bottom of it in the case of a shaft seal leak, which is the most common leak.

It drops from about 250 psi on the high side and about 40 psi on the low to about 60 throughout the system. It never draws a vacuum

Hi @davidmorales8219 :
I found a recent link of yours on another site where you also replied to an old thread with:

  1. A well written reply
  2. A link to the site with the same text as your reply.

Is there a reason your replies include both the same text as the obdadvisor site and then a link to it?

Thank you.

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Me thinks Spam …

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I’d like to hear the explanation for that load of BS.

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