So my car broke down due to a transmission cooler line leak. Had it towed to a shop where they diagnosed the issue, replaced the cooler lines and at their recommendation replaced the radiator. I pick up the car and after about 15 minutes of driving realize the A/C is just not blowing cold. I call to ask if they would have evacuated the refrigerant to install the new radiator. Guy at the shop says they're 2 separate systems and wouldn't have done that. Suspiciously the schrader valve caps are also missing. I'm not one to argue so I try to diagnose the issue myself. Notice the AC clutch isn't engaging. Hot wire the relay and it clicks. Cross my fingers and hope it's a faulty relay switch. Not the problem. Order a manifold gauge set & the entire system is at zero PSI. So either there's a leak or somebody removed the refrigerant. I pull a vacuum and it maintained at the -30psi for an hour so either it's a really slow leak OR somebody removed the refrigerant, realized they may not have needed to do that, and then forgot to add it back in because well why would the refrigerant be missing. Anyway at this point I might as well finish the job but would greatly appreciate some sanity checks here before I go ahead.

  1. Do I need to add any oil to the system? I have no idea how I'd determine the amount that would need to be added and without a "no you're good" or a "just add x and you'll be good" I'm going to take this into a (different) shop.

  2. I haven't ruled out the possibility of a very slow leak. The transmission cooler lines were leaking and the car is not the newest. I figure adding some UV dye would help catch any leaks and in that case I take it into the shop.

  3. A lot of refrigerant comes with stop leak. I've been seeing mixed opinions online about this stuff and want to get some feedback on whether it's worthwhile to use. My 08 Ford Ranger (4L v6) calls for 2lb 1oz -> .94kg -> 33oz so I need 3 12oz cans. Any reason to make 1 of them a stop leak can? Strong feelings here? Right now I'm defaulting to not using it.

  4. My plan is to pull a vacuum. Add refrigerant to the low side while the car is running w/ Max AC and basically aim for getting 33oz of the theoretical 36 oz I'm purchasing into the system with some UV dye all while trying to maintain the suggested PSI on the r134a refrigerant chart. I've got a kitchen scale to measure the quantity added. I've also read some stuff about making sure the freon doesn't enter the compressor as a liquid. I imagine if I just make sure the low side pressure stays close to 30 I should avoid that issue. Can always adjust how open the low side valve is to slow it down if it's building too fast. From what I'm seeing on the internet at 60 F you need to stay below 57 PSI & 70 F you want to stay below 71 PSI so rule of thumb is keep it below the ambient air temp in F. Anyone here that can speak to this part of the process?

Thanks in advance and apologies for the length.

  • Welcome to Motor Vehicle Maintenance & Repair! If the system holds a -30 for an hour, you really don't need to put dye or stop leak in it. If it will hold the vacuum without issue, it's not going to leak when you pressurize your system, so putting the dye and stop-leak in it isn't going help you much. I'm not positive about the PAG oil, so will leave that to the AC experts we have here. Aug 28, 2023 at 14:59

2 Answers 2


You should never put stop leak in an AC system. It can and probably will damage and contaminate your system. The dyes are recommended, especially on older systems. They may eventually leak so it makes it easier to find the source later when it does. And it is harmless. The oil is the tricky part because there is no way to measure how much is in the system. The only way it can leave the system is through leaks or physical removal. You lose very little oil with a small slow leak and adding oil is usually not done. But on a rapid leak there will be more oil expelled, so many technicians will add a half an ounce but not over an ounce when this happens. It is suspected but not confirmed that you may have had a rapid leak so a half ounce is recommended.

  • What about for situations as the OP is suggesting probably happened where the refrigerant was purposely removed? Is that the same as rapid leak? Aug 28, 2023 at 19:36
  • If it was purposely removed properly with an evac machine, it's controlled enough that a minimal amount of oil is lost. There's some lubricants in the refrigerant to help. So adding oil is not necessary. According to the OP the shop denies evacuating the system. That would lead me to believe that there was an accidental leak. It appears that the point of the leak was repaired because a leak was not detected with a vacuum test.
    – Jupiter
    Aug 29, 2023 at 2:12
  • No leaks have been repaired to my knowledge. The fact that the vacuum test passed seem to rule out the possibility of a rapid leak leaving me with the conclusion it was intentionally removed Aug 29, 2023 at 2:18
  • @Jupiter - Just because they deny it, doesn't mean they didn't do it. It seems the most likely scenario. Why do I say this? If there wasn't a leak before and there's no leak now (evidenced by being able to draw a vacuum), as well as the possibility of a mechanic making a mistake and not wanting to own up to it, it seems highly likely. I don't know who would have fixed the leak (or maybe you're saying it's coincidental), because if the shop didn't evac the system, I doubt they are going to fix a leak. Aug 29, 2023 at 9:59
  • All just assumptions of course, but the vehicle went in the shop with a working system. It had some pressure. If it was a preexisting coincidental leak it would have some pressure left in it when returned to the owner. So I'm assuming that the leak happened in the shop. If they intentionally evacuated, they wouldn't deny it. So I'm assuming that they may have unintentionally caused a leak maybe at the condenser and fixed the leak to cover their tracks and didn't charge the system because they may not have had the proper equipment.
    – Jupiter
    Aug 29, 2023 at 11:58

1-Was ac working perfectly in your '08 Ford Ranger before the radiator was replaced? (no mention and I ask the stupid questions to be clear) All that follows is based on a perfectly working ac system and 'suddenly, somehow, ac became inoperative suggesting a questionable repair shop possibly attempting to generate more business from you.

2-Presuming ac was working fine before repairs, not working at all after repairs, connecting gauges revealed an empty system and evacuating shows vacuum holding after several minutes, was the vacuum pump shut off, valves closed to monitor system vacuum? (some diyers unfamiliar with evacuation procedures leave pump running and incorrectly declare a perfect vacuum).

If correct evacuation procedures were performed and vacuum gauge readings don't display needle creeping towards zero after pump and valves are closed, the corollary of a vacuum holding suggests pressure will hold. Presuming refrigerant was surreptitiously removed, oil is not needed to refill this system. Over oiling can be detrimental; leaving less room for refrigerant and inadvertently creating higher operating pressures. Whenever a system is opened for repairs, service manuals describe parts replaced with specific oil amounts added into the part (condenser coil, evap coil, compressor) to balance the oil removed with a like amount replaced. If this step is forgotten, pag oil is added after evacuating the system since a vacuum will draw in air, moisture, dirt, oil, refrigerant and dye. Either a canister of pag oil with a refrigerant charge is used or a special refillable canister with hose and valve is filled with oil and refrigerant for injection into the low side service valve. Oil circulates as a mist but when injected into the compressor may create a hydraulic lock; manually rotating compressor shaft to distribute oil prior to refilling with refrigerant.

In all repairs, an evacuated system leaves a vacuum for immediate introduction of refrigerant into low and high side if time is of the essence otherwise the high side gauge valve remains closed for the remainder of refilling procedures to ensure safety against high pressure incidents when compressors are operating. Whether using small 12oz cans or 15lb/30lb canisters of refrigerant, the vacuum will suck in at least 8 ozs immediately if not the entire contents of a 12oz can. Avoiding injection of liquid refrigerant simply means keeping the canister valve upright. When turned upside down with can valve below the canister, liquid refrigerant is fed for quicker refilling (into high and low sides). However, 12oz cans warmed in a bucket of warm water will transfer warmth as refrigerant is fed into a system and canister begins to develop frost on the can (refrigeration at work). Swirling the first can in warm water will empty it in less than a minute or two. 8-12 ozs should be enough to allow compressor operation, needed to inject more refrigerant (into the low side ONLY). Connecting each canister requires purging the hose line feeding refrigerant into the gauge set so air isn't introduced into the system. A brief release of refrigerant at a fitting purges the line. Keep the canister upright, swirling the can in warm water and allow pressurized refrigerant (approximately 70-90 psi) to be drawn into the suction/low side of the compressor. The second can will take a little longer to empty as more refrigerant is added into the system. Raising engine rpm to around 1200 or so will allow higher compressor speed to draw refrigerant from cans. If you shut off the feed valve on the gauge set, you'll see low side pressures drop to actual values. High side pressures rise as engine rpm rises and should never exceed 275+ psi at any time. The second 12 oz can should begin to allow cooled air from vents (blower left at medium speed). A full charge should result in restoring ac back to factory conditions.

Dye is factory added in GM systems using r134a. I don't know if Ford does this. If not, adding a can of refrigerant and dye will not harm a system but can be messy when connecting/disconnecting fittings. Never use any sealer as this simply contaminates a system and usually a hail Mary effort at cheap fixes that fails more often than not. Any reputable dealer, repair shop and knowledgeable diyer will never use sealer since it contaminates equipment.

  • Thank you this was informative. To answer the first two points the Ac was working perfectly until I got it back. And the vacuum pump was turned off after shutting the gauge valves. My thought is adding some dye can't hurt in the event a future leak develops. Will stay away from the stop leak. Aug 29, 2023 at 2:18
  • Some of us share invaluable info to keep money in our pockets. Ac systems was never rocket science but does require knowledge no different from anyone willing to rebuild or repair engines. Hopefully you restore your ac back to factory condition.
    – F Dryer
    Aug 29, 2023 at 10:58

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