Drop in PSI when "revving" the engine?

Tiny
DAVID VITTITOW
  • MEMBER
  • 2011 DODGE NITRO
  • 4.0L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 90,000 MILES
The A/C doesn't seem to be working as well. About a year ago there was a leak in the system that was repaired and recharged. We haven't had any problems since.

I got a recharge kit and upon checking the pressure on the low side it was right in the middle of the green at 37psi (between the recommended 25 and 50). I asked my son to give it some gas and when this happened, the psi dropped to about 25-27.

Is this drop normal? He is going back to school in Miami, FL and if there is a problem, I would prefer it to be fixed before he goes.

Thoughts? Ideas
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Sunday, July 30th, 2023 AT 8:37 AM

3 Replies

Tiny
BRENDON S
  • MECHANIC
  • 653 POSTS
Hello DAVID VITTITOW,

Unfortunately, the can hooked to the low side is not an accurate way to measure system pressure or to refill an Automotive A/C system.

You need either a machine or a set of gauges and a tank of refrigerant. Unfortunately, the refill cans you buy are a gimmick and that's all.

The reason for this is the refrigerant and oil need to be added at a very specific amount or it will damage the system.

Now there could be something going on like a clog possibly but without the proper tool to measure it we don't even know if any of the readings are correct, especially because we can only measure one side.

These are sealed systems and refrigerant should not be leaking from them unless a component is faulty.

Can you tell me what symptoms you were having that caused you to refill or top off the system? You stated it wasn't working well, could you elaborate for me?

Thank you,
Brendon
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Monday, July 31st, 2023 AT 10:28 AM
Tiny
DAVID VITTITOW
  • MEMBER
  • 2 POSTS
A cooling system leak was fixed about a year ago by a shop and everything has been good. My son is just complaining of the air not being as cold but it's still cold just maybe not as cold as before. I wonder if it is simply a byproduct of the summer heat here in GA.

When I checked the pressure using a refill kit by connecting to the low-pressure port, the digital pressure gauge was within range. I did not add any refrigerant. As far as I know I do not see any refrigerant that might indicate a leak.

My only concern was that when he increased the RPMs while I had the kit still connected to the low-side port, the pressure gauge dropped from about 37psi to about 25psi. 25 is just outside of the low range. You said that is not a good mechanism to test with and not a good point within the system to test with.

He leaves for college in a week and honestly, he will have to keep an eye on it and if it gets worse take it into a shop.

Thank you very much for your response.
Take care!
Dave
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Wednesday, August 2nd, 2023 AT 8:23 AM
Tiny
BRENDON S
  • MECHANIC
  • 653 POSTS
Hello DAVID VITTITOW,

You are very welcome. I am here to help.

Pressure will drop on the low side as RPM increases and high side will go up in pressure. It also depends on the ambient temperature.

If pressure is too low the system's low pressure switch will turn the compressor off.

In Georgia especially if he is in traffic, the A/C won't work as well because there isn't air moving over the heat exchanger/condenser. The cooling fans should help but they can only do so much.

If it gets better driving down the road as opposed to sitting in gear in traffic then this is normal to an extent.

If it is too much of a temperature difference, then getting an A/C machine on it and having it checked and refilled might be a good first step. They will pull a vacuum on it for about 20 minutes and see if there are any leaks.

You can also get a gauge set and check the pressures yourself. I will add an image of them for you.

I am also adding a temperature chart for you. These ranges are approximate values. Keep in mind if you do use gauges to check it, some refrigerant will be lost.

Personally, I would run it for a while and have him keep an eye on it and see if it gets worse.

A good accurate way to measure the temp coming from the vents is with a food thermometer or something similar. He could use this to keep an eye on the temperature. They have ones that clip onto the vent as well.

Hope this helps.

Thank you,
Brendon
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Wednesday, August 2nd, 2023 AT 10:09 AM

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