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Re: Ac not as cold when car warms up

Posted: Tue Nov 26, 2019 2:35 am
by Miggy
Sorry for the late replies, its been raining here so it isnt as hot. Its a bit warmer now so i decided to take the readings at 1800 rpm:

Ambient temp: 27 degrees celsius according to my cheap digital gauge, weather app on my phone says 28 degrees
Low side: just below 20 psi
High side: about 209 psi
Vent temp: 11 degrees celsius

Note: the car hasnt been running for too long when i took these readings.

Are the pressures okay? Wouldnt about 30 psi low side be optimal?

Re: Ac not as cold when car warms up

Posted: Sat Nov 30, 2019 7:30 pm
by JohnHere
For a 27C (81F) ambient, the pressures look pretty good to me. Bear in mind that the pressures will vary depending on engine and vehicle speed and hence airflow through the condenser, compressor efficiency, ambient temperature, and so on. Your 1997 Sentra has a thermistor to prevent the evaporator from frosting up. So even if the low side drops to 20 PSI or so, the thermistor will (or should) shut off the compressor as needed to prevent the evaporator from freezing and blocking airflow.

At those PSI readings and engine RPM, the system has to be cooling with vent temps typically in the 5-7C (41-45F) range. But you're seeing vent temps in the 11-16C (52-61F) range. So ambient and/or heated air has to be mixing in someplace.

I agree with earlier suggestions. Check that no air is going around the evaporator core. If that checks out okay, then try clamping off one or both heater hoses and take it for a test drive on a warm day. You don't have to clamp them off permanently, just long enough for a good test. With no hot water flowing through the heater core, leave the thermometer in the center vent while you drive and see what vent temps you get.

Re: Ac not as cold when car warms up

Posted: Sat Nov 30, 2019 9:20 pm
by Miggy
I tried eliminating the heater, but the results are still the same.

I took pressure readings again the other day: (2000rpm)
Low side: 24 psi
High side: 263 psi
Ambient temp: 31 degrees celsius according to my cheap thermometer, 30 celsius according to phone app.
I forgot to take vent temps, but i could feel that it isnt very cold.

The high side drops when i splash water on the condenser, but the cooling also seems to decrease. Might just be my imagination, forgot to get vent temp.

I asked yet another shop, they said it could be the compressor. Does it seem likely based on the pressures?

I did a search and found out that the sight glass in the reciever drier should be clear, mine is not. It looks like a stream of flowing buhbles. But i also read that you shouldnt look at the sight glass on modern cars, my cars pretty old so...

Re: Ac not as cold when car warms up

Posted: Sun Dec 01, 2019 8:17 am
by Cusser
Your compressor seems to be running fine.


Miggy wrote:I did a search and found out that the sight glass in the reciever drier should be clear, mine is not. It looks like a stream of flowing bubbles. But i also read that you shouldnt look at the sight glass on modern cars, my cars pretty old so...


Sight glass stuff is only applicable to R-12 refrigerant.

Re: Ac not as cold when car warms up

Posted: Sun Dec 01, 2019 10:34 am
by JohnHere
With the HP in the 260's at around 31C (88F) ambient, I don't think it's the compressor. Chances are that the high side would be much lower if the compressor was getting weak.

Re: Ac not as cold when car warms up

Posted: Fri Dec 06, 2019 2:08 am
by Miggy
Sorry for the late reply. after researching the past few days, ive come to the conclusion that there was air in the system causing the gauges to show a good reading but still actually be undercharged. So i thought about evacuating and recharging the system myself.

Ive run out of funds to buy a vacuum pump, but thankfully i stumbled upon this: https://www.instructables.com/id/Making ... cuum-Pump/

I turned my old fridge compressor into a vacuum pump!
I had no way to evacuate both the fridge and my cars refrigerant, so i had somebody else do that for me.

After converting my fridge compressor to a vacuum pump, i then proceeded to vacuum the ac of my car. The fridge compressor vacuum pump pulled all the way to just a tiny bit below 30 in hg. I vacuumed the system for about 2 hours.

I bought a 1kg r134a tank, then i started charging the ac. I dont have a weighing scale so i charged with pressures. Its my first time charging an ac system so i dont really know if i did it right. Here are the new pressures at 2000 rpm:
Low side: 26 psi
High side: 250 psi
Ambient temp: 31 degrees celsius
Vent temp: forgot to take, again.

I could tell that the ac is MUCH MUCH colder now, even at idle. But i have no idea if the system is undercharged or overcharged based on these pressures. I just added refrigerant until the low side read about 30 psi at idle.

Any input on these readings would be much appreciated, thanks.

Re: Ac not as cold when car warms up

Posted: Fri Dec 06, 2019 6:00 am
by Cusser
Miggy wrote:Ive run out of funds to buy a vacuum pump, but thankfully i stumbled upon this: https://www.instructables.com/id/Making ... cuum-Pump/


Autozone in warmer areas of USA has loaner vacuum pumps, I once borrowed one when 100 miles from my own. But I don't know your country or location.


Miggy wrote:I bought a 1kg r134a tank, then i started charging the ac. I dont have a weighing scale so i charged with pressures. Its my first time charging an ac system so i dont really know if i did it right. Here are the new pressures at 2000 rpm:
Low side: 26 psi
High side: 250 psi
Ambient temp: 31 degrees celsius
Vent temp: forgot to take, again.

I could tell that the ac is MUCH MUCH colder now, even at idle. But i have no idea if the system is undercharged or overcharged based on these pressures. I just added refrigerant until the low side read about 30 psi at idle.


Your pressures actually are pretty good. I would certainly DO NOTHING MORE at this point. Congratulations.

Re: Ac not as cold when car warms up

Posted: Fri Dec 06, 2019 4:17 pm
by Miggy
Cusser wrote:
Miggy wrote:Ive run out of funds to buy a vacuum pump, but thankfully i stumbled upon this: https://www.instructables.com/id/Making ... cuum-Pump/


Autozone in warmer areas of USA has loaner vacuum pumps, I once borrowed one when 100 miles from my own. But I don't know your country or location.


Miggy wrote:I bought a 1kg r134a tank, then i started charging the ac. I dont have a weighing scale so i charged with pressures. Its my first time charging an ac system so i dont really know if i did it right. Here are the new pressures at 2000 rpm:
Low side: 26 psi
High side: 250 psi
Ambient temp: 31 degrees celsius
Vent temp: forgot to take, again.

I could tell that the ac is MUCH MUCH colder now, even at idle. But i have no idea if the system is undercharged or overcharged based on these pressures. I just added refrigerant until the low side read about 30 psi at idle.


Your pressures actually are pretty good. I would certainly DO NOTHING MORE at this point. Congratulations.


Unfortunately, im not from the US.

Looks like i got lucky charging the car for the first time. What i did was charge till the low pressure side read 30 psi at idle. After the low side reached 30 psi, the high side kept rising and the low side kept steady at 30, so thats when i stopped.

Is there any reference or chart i could look at when charging the ac with pressures? A baseline pressure to look for when charging? Thanks.

Re: Ac not as cold when car warms up

Posted: Sat Dec 07, 2019 6:39 am
by Cusser
Miggy wrote:Is there any reference or chart i could look at when charging the ac with pressures? A baseline pressure to look for when charging? Thanks.


I'm not personally aware of such chart; others may have information.

Understand that I am NOT an AC professional. Personally, I charge by weight from refrigerant cans (No SEALER !!) and hope to get 10-11 oz. in from each can as they never completely empty. My guideline is more focused on the high side pressure, seems to me to be full about 250 to 270 psi, and good if low side falls between 25 and 35 psi. So you're there, good work.

Re: Ac not as cold when car warms up

Posted: Sat Dec 07, 2019 8:40 am
by Tim
Cusser wrote:
Miggy wrote:Is there any reference or chart i could look at when charging the ac with pressures? A baseline pressure to look for when charging? Thanks.


I'm not personally aware of such chart; others may have information.

Understand that I am NOT an AC professional. Personally, I charge by weight from refrigerant cans (No SEALER !!) and hope to get 10-11 oz. in from each can as they never completely empty. My guideline is more focused on the high side pressure, seems to me to be full about 250 to 270 psi, and good if low side falls between 25 and 35 psi. So you're there, good work.


Charging my weight is really the best option. Doing an R12 retrofit, charge by pressures. But its always a guess on retrofits.