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2000 Yukon - Lowside Pressure 100+ PSI!

johnmc on Sun June 24, 2012 7:43 PM User is offline

Year: 2000
Make: GMC
Model: Yukon
Engine Size: 5.3
Refrigerant Type: R134
Ambient Temp: 94
Pressure Low: 120+
Pressure High: 125
Country of Origin: United States

Hi all,

Replaced AC Compressor on my 2000 Yukon 'bout a year ago - got a compressor kit right here from Added the correct amount of refrigerant and oil, vacuumed system, etc.....was working fine up til about a day or so ago. Yesterday I noticed the system was cooling down to about mid-50s, then this morning vent temp was about 80 - 85 deg. Before, nominal performance was typically 40-45 degrees at the vents.

I checked the system with my gauge set: Ambient temp is 94 deg & 41% humidity, static reading (engine not running) High Side is 125 PSI and Low Side is 120+ PSI (gauge only goes to 120, and needle is sweeps past 120 PSI).

With engine on and idling, I verified the compressor IS engaging; High side reading immediately jumps to about 275, then slowly creeps up to 320 and is more or less steady. Low side meanwhile is a constant 95 PSI. I have a large shop fan in front of the truck, so the condensor is getting some airflow.

Searching the internet, high lowside pressures typically mean overcharged or a failing compressor. But.....the compressor is building pressure to 320...I have noticed that it has been a bit noiser last few weeks (or maybe my paranoia is hearing things..ha). Could system be overcharged? It has been working fine since I replaced the compressor last summer. Even though the last week is really the first time it has gotten really hot out (95 - 100 here in Alabama) Just to check, I did vent a little bit of charge out, but no effect, i dont really want to vent the refrigerant if I dont need to.

Edited: Sun June 24, 2012 at 7:52 PM by johnmc

Cussboy on Sun June 24, 2012 8:00 PM User is offline

If you had added the correct amount of refrigerant then, and it did work fine, then you are not overcharged (refrigerant can't magically increase).
Wait for a "real pro" - not me - to respond. My next step would be to hose off the condenser with the hose while that fan is on and see what the pressures are with the engine running. Static pressures don't provide a ton of information, and are those pressures read at about 2000 rpm?

Edited: Sun June 24, 2012 at 8:01 PM by Cussboy

TRB on Sun June 24, 2012 8:17 PM User is offlineView users profile

Proper evacuation and change level is a must.


When considering your next auto A/C purchase, please consider the site that supports you:

845rabbit on Wed June 27, 2012 9:57 PM User is offline

Own a 2003 Yukon. Failed compressor took out everything including rear expansion valve. Check it if you have rear a/c. You'll get weird pressure readings on split systems that half work.

johnmc on Sat June 30, 2012 6:35 PM User is offline

johnmc on Sat June 30, 2012 6:58 PM User is offline

Thanks for the suggestions everyone. Yes, my Yukon has rear air, but I changed the compressor before it puked its guts out. The orfice tube was fairly clean, no obvious blockage. As such, I did not check the rear air expansion valve. I was going to replace the rear valve as a new valve came with the compressor kit, but I couldnt find the dang thing. If it is on the rear evaporator, then the whole rear air unit will have to come out of the vehicle to replace it. I really didnt see the need for replacing it anyway.

Also, I remembering charging the system with 42 oz of R134 (3.5 cans).......Seems to be conflicting info on the factory charge - my underhood sticker has faded to the point I cant discern what is printed on it. But from what I can tell, a short wheelbase (not the Yukon XL) with rear-air Yukon takes 42 oz.? Of course, I cant now find where I saw that.

Morning of 6/25, (driving to work), 78 deg ambient, vent temperatures will (eventually) drop down to 40 - 42 degrees, which I think is about as good as it can be.

So its 100+ here in Alabama, decided to tinker around with the AC again.....
TODAY - 6/30:
Ambient: 102
Initial startup lowside 62 PSI, highside 205-210.
After idling for a few minutes, system more-or-less stablized to lowside 47-50 PSI, highside 215 PSI. Vent temperature at 64 but seemed to be slowly dropping.
Raised engine RPM to 1500, highside increased to approx 250 PSI. Lowside no change. Hard to read the gauges from inside the truck tho.

Highside is in the range of 2.2 - 2.5 ambient, but why is the lowside still high?
Vent temps will drop to mid-fifties after 20 -30 minutes of this nominal performance in this kind of hot, 100+ temperatures?

Still dont knowwhat happened last week when the ac wouldnt cool. I havent seen those symptoms again.

Edited: Sat June 30, 2012 at 7:00 PM by johnmc

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