A/C not cooling

One of the first things I would do would be to put a set of A/C gauges on it and test the high and low side pressures to see what the state of charge is. Those numbers would also give you an idea if it was the charge or if it's a worn compressor by the way the pressures change. Those would be the most suspect parts as the problem occurred gradually. Or to verify that the charge isn't the problem do a full recovery and re-charge by weight. It should take between 1.3 and 1.5 pounds of 134A.
https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

From what you posted I suspect it is either low charge level or a worn compressor. Both would cause the same type of delayed loss of cooling.

Thanks so much for the prompt and helpful reply, Steve. I have been trying to get ahold of some A/C gauges. I will get back to you when I am able to recharge the system and check the pressures.

You can usually get them at the places that do the loan a tool routine, which is sort of a gimmick anyway as most have you pay the full price of the tool, then refund the money when you return it, but it can be good if you don't think you will ever use a tool more than a couple times. Let us know what you find once you have a set on hand.

Thank you so much. You 2CarPros are a Godsend.

Thanks again! Still waiting for a chance to get to the auto parts store for the vacuum gauge loan. Would it be a good idea to replace the expansion valve while I am at it?

Not yet. I wouldn't do anything until you can do some testing. No reason to spend money on parts you may not need.

Great. Thanks!

Hi Steve. I was able to get ahold of vacuum gauges... assuming rented gauge is correct, pressure is low, low side reads 8 PSI and high side reads 2 PSI. The clutch is not energized so I cannot tell if the compressor is compressing. Can I check the compressor at this low charge momentarily before I purge the system? can I safely bypass the low pressure switch and where is it located?

Air temperatures here are 80' (Hawaii).
Miswrote the gauge readings. Reading 28 PSI low and 25 PSI high. I charged with a little r134 to try to get the compressor to kick in. no joy.
Thanks again for your work.

Okay, you have a very low charge. You should see a pressure like those in the chart with the compressor working. Static pressures should be slightly higher than the temperature when you are testing, So if it's say 75 degrees you should see a static pressure around 78 PSI. That would be with the compressor off. If it's 90 degrees it would be about 95 PSI.

The AC is working great! Thank you for your excellent help!

It sounds like you have some leakage, now that you know the charging methods I would put some UV dye in it and then go hunting for it. You can also look for oil staining from the oil in the system. The dye makes it much easier though. Beyond that it's great to hear you have it working.

Possible, but the vacuum held for an hour at -28 PSI, and I got good readings when I put the r134 into the system.
Went for a drive to return the vacuum and guages, and the A/C worked well, but quit about ,1/2 hour into the drive. Stopped for groceries and it was working again. The cooling fan works, the car is not overheating, and the blower pushes plenty air, something was signalling the compressor clutch to disengage.
Any ideas?

What were the pressures at when you finished? When it shuts off then starts working after setting I would look at the system pressures while running, that somewhat suggests a higher than correct charge as the system will shut off if over pressure than may start working once the pressure has equalized.

225 PSI high side and around 60 low side.. holding steady for 30 minutes with vehicle idling. Pressures dropped when I occasionally turned off the A/C, but I didn't check the exact final static pressures.Weather temperature here was around 80.

I can borrow the guages again if need be.

For 80 that's about 15 PSI high and could be the issue if the high pressure cut out is a bit aged. The only way to know for sure would be to run it until it shuts off and then see what the pressures are when it shuts down. Attached are the numbers the switch should operate at. The increasing numbers are where the switch cycles So on the low side the switch turns on at a pressure higher than about 25 PSI and off at about the same The high side will shut off the A/C at a pressure above about 400 PSI and as the pressure drops it turns back on between 200-250. So a weak switch could easily be turning the system off at 350 or so, and it comes back when it drops back below that. But I would check the pressures simply because running them high can easily damage the system.

Okay, thanks! I will get the guages again because it will be good to know. Are you thinking maybe the electrical component of the switch, or likely a sensor replacement that will require another vacuum of the system?

I would check the pressures first, it may be that they just need to be lowered a bit to make it work properly. There is also the possibility that it was working too well and iced up the evaporator core, that would trigger the temperature sensor on the core to shut it off until it thawed out. I'm leaning more toward the pressure switch but want you to know the other items that could act that way.

Okay. Thanks. It will be a couple of days before I can get the car long enough to check the pressures. I will let you know then. In the meantime, we will not use the A/C..
I am pretty perplexed because outwardly, the system is performing similarly to when it was definitely short on refrigerant. Intermittently, and never really getting ice cold..

Thanks again for your help.

Overcharge and undercharge both act very similar depending on how close they are to the correct numbers. The only way to see what is going on is to run the pressures and then diagnose what those and the symptoms are telling you is wrong.

Understood. Thanks again. I will get back to you with the PSI's

STEVE W. is one of our best! Please let us know what you find. We are interested to see what it is.

I absolutely agree and value his help! The Xterra was needed for my partner's work the last few days, so I haven't been able to get the guages and get back on it. I will let you know what I find asap. Thanks again!

Back with the guages. the A/C has pretty much been not working all week. haven't been trying to run it much.. just testing it now and again. The car is warm from an hour drive and the A/C is turned on full and blowing warm air.. compressor clutch is not engaging. guages read 18 PSI low and 25 PSI high side.

Air temperature is around 78F.

It sounds like it has a pressure leak then. The pressures are showing no charge again and it had to go someplace. I would add some dye and then start looking. I've seen evaporators leak under pressure but not under vacuum, same with o-rings and seals. I've used both plain air and nitrogen to avoid using refrigerant and just using the pressure of those to push the dye around, then hunt for the dye stain.

Thanks so much for the fast reply! I have learned so much from you. I would not have thought that I could lose PSI under pressure but not under vacuum... great point. I will get some dye in the morning.

Yep, pretty common in A/C and other systems as well. The way the seals work they can be drawn tighter under vacuum while they still leak under pressure. Or some odd ones like a very corroded condenser core on a Ford a while back, it would leak like a screen door under pressure, but under a vacuum the corrosion would draw into the pitting and flakes and seal.
Similar issue with the o-rings, They get squished into an oval with flat sides, under pressure the sides will act like one way valves and let pressure out, then when you apply a vacuum they will draw into the split line between parts and seal up. Can be frustrating at times. That is why I generally test under both as a vacuum can give you a false result.

That makes a lot of sense. Wish me luck in finding the leak! I am assuming that there are special o-rings for resistance to the chemicals in refrigerant.

Yes, but you can usually get kits that include all of the needed ones for a particular vehicle. This is a picture of the kit they sell for your Nissan, lot's of places that can leak. Plus the compressor and other hard parts. It's why I would start with the dye first and then go hunting. Another trick to finding the leak, use compressed air or similar to pressurize the system, then mix up some dish soap and water and spray it all over the parts and look for the bubbles.

Sounds good. thanks again for your quick reply.. I will go to the parts store in the morning.

Okay. Got the dye and added it to the system. Right now I am circulating it a little longer because the first time I added for pressure, and circulated for 15 minutes, I could see no leakage with the UV light and a mirror to see under fittings. The only place that I have not looked is the aft side of the firewall. I am a little suspicious of the low pressure Schrader valve. . Maybe I need to put a 'balloon' on that. The high pressure is 200 PSI and low 45 PSI. A/C is working.

A/C is working but is not real cold.

Those pressures are high so it likely won't work great, but you should be able to find the leak. A place to look for the interior pieces is the A/C drain under the car around the firewall, any moisture or oil can get there and drip, and you would see it on the rubber duck bill or drain tube. Sometimes it can make you want to burn the car trying to dig out the leak, some of my worst ones were tucked in the dash or a crack on the bottom of one of the lines and you couldn't see it because the dye would come out and drip off. Found it when I stuck some clean cardboard under it overnight. Be sure to cycle the A/C on and off as well, that will let things relax and possibly show up better. Have had many valves leak, a balloon or even just putting some soap in there will show those.
One of my secret weapons is the cheap bubble solution the dollar stores sell. That stuff really bubbles up with even a tiny leak and it's ready to go in the bottle.

While waiting, I took out the glove box and the A/C air filters to have a look at the expansion valve fittings and at least the right hand side of the evaporator. There is no sign of the dye in there. Pressures are remaining steady. the car is idling in the shade. When I rev the engine in neutral, high pressure side goes to 210 and low drops to 22.

Just saw your last reply. Thank for the tips. I made some bubble solution earlier but was afraid to use it on the Schrader site because I thought that I might contaminate the system.

Okay, those pressures are better. Some leaks can be a nightmare to find, especially when you are looking for something the size of a sewing needle. I have faith that you will find it, It may take a bit but any system that drops to the pressures that one was when you started is leaking somewhere.

If the pressure is coming out it won't hurt anything and will only wash the dye away. Then when you get it repaired and pull a vacuum it will remove anything from the system anyway.

Okay, thanks again! I feel very fortunate to have your help.