I recently fixed a leak in my car's AC system. Before I charged the system with fresh refrigerant after a complete evacuation, I almost forgot to add some PAG oil to the system.

Which leads me to my question. If I did somehow forget to add the compressor oil and the AC system is now fully charged, would I be able to the oil in without first recovering the refrigerant then doing a complete re-evacuation and recharge? I've seen some people using oil injectors but it isn't clear if it can be used on a fully-charged system.

Thanks in advance.

  • You can buy a small can of pag oil charge with r134A, connect this with r134a gauges and push in the oil charge.
    – Moab
    Jul 16, 2018 at 19:24
  • As @Moab stated, but you can't do it on a fully-charged system. If you overcharge the system, you run the risk of blowing seals and causing other damage. Jul 16, 2018 at 22:13
  • Oil charge only has 2oz of freon, just enough to push the oil in and not add any freon to the system.
    – Moab
    Jul 17, 2018 at 16:14

2 Answers 2


You can use an oil injector on the low-side port while the car is running, fully-charged. That's the "suction" side of the system. Assuming you only changed a seal or hose, you don't need more than a 1/4oz (or whatever you pulled out). If by compressor oil, you mean you changed the compressor and forgot to put oil in it, I'd start over.

  • 1
    Thank you all. Valuable info going to my knowledge base of DIY auto repairs/service. Really appreciate the helpfulness of folks here.
    – Kading
    Jul 17, 2018 at 13:05
  • @Kading You're welcome. Remember that PAG is soluble in R-134A. As such you don't need to get any strange chemical in the AC system when you flush; just use R-134A with a closed loop flushing system, a few minutes for each system section. Remember you only want to remove any oil left in the system; if you needed to flush the swarf of a failed compressor away i would have told you to replace the condenser (and the TXV too if dirty) and any muffler on the lines too.
    – Al_
    Jul 17, 2018 at 17:04

With AC compressor's lubrication there's no such thing as educated guessing. Even more if your compressor is of the vane or scroll type. Lack of lubrication, or, on the other extreme, hydrolocking, kills these things.

You have to recover the refrigerant charge, remove the compressor and flush the system (you haven't experienced a compressor seizure, so you will flush just to remove any leftover oil and this will allow you to flush the condenser too, and the closed loop refrigerant flush allowed by an RRR station with the proper flush kit could be enough, but it's still good policy to replace the receiver dryer each time the system is open).

Then you'll have to follow any compressor installation instructions you have, but it's generally take the compressor, drain any oil from it and measure and then insert the system's oil charge inside (if you have a drain plug, remove the drain plug and drain and then refill oil of the proper ISO VG from there, otherwise pour it in the suction port and rotate the clutch hub so that it gets drawn inside; double end capped PAG is the best option since you won't have to worry about moisture) then install the compressor on the car and rotate the compressor's shaft (rotate pulley if clutchless, rotate clutch hub if with clutch) by hand for a lot of times (20 full turns to be fully sure, though 10 might be enough), so that any excess oil gets discharged in the high pressure line and won't hydrolock the compressor upon startup. Then thoroughly vacuum, and finally charge the system up with the required weight of refrigerant (recommended: liquid in the high side, by weight, with compressor obviously off). Again, turn that compressor shaft by hand for 10-20 more times to be fully sure the compressor won't hydrolock (better be safe than sorry). After you're done let the system rest for 10 minutes and then start the engine, be sure it's in idle and then engage the AC compressor and let it run for 5 minutes keeping the engine in idle. Compressor installed, properly lubricated and ran-in, with a full refrigerant charge. Now enjoy your ice cold AC.

By all means, don't let the engine turn the compressor without any oil inside it.

That's the proper way of doing things and not risking compressor seizure due to "too much" or "too little" of anything.

The oil injector thing is to be used only to replenish whatever came out during refrigerant evacuation. That's also what the canned oil charges available in the USA are meant for.

Hint: If you haven't got an RRR (Refrigerant recovery & recycling) station, let someone else who has one (and is licensed) do the job, otherwise you can mess things up pretty easily. By all means forget about refrigerant cans and canned oil charges.

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