Mechanics

PROPER LOW AND HIGH SIDE PRESSURES FOR A RETRO FIT

1990 Nissan 300ZX • 6 cylinder 2WD Automatic • 180,000 miles

I had my 1990 300ZX retro fitted for R134a when the freon was low. Although I bought this Z new in 1990, it is no longer my daily driver. I am told by the AC tech that I have a very small leak in the compressor and discharge hose. He used a Freon sniff tester (I lost 1 lb of R12 in 2 years). According to the invoice, they put in 26 oz of R134a plus PAG oil.
I happen to have an AC manifold gauge and took a reading when I got home and got 34psi low side and 250psi high side. The ambient temperature was around 92 degrees. I had the doors closed, windows open and a box fan on high at the nose of the car. Center vent temp are anywhere from 48 to 58 degrees depending on whether I am using a regular temp gauge or an infrared non contact gauge (infrared gauge read lower).
Since this is a retro fit are these readings reasonable?
Avatar
BMail
July 6, 2011.




They are dead on not bad for retro especialy without drier replacement, flush and TXV replacement. Vent temp good should drop a bit at hyway speed.

Docfixit
Jul 6, 2011.
Thanks for the quick response.
You are correct, when moving the temps do come down.
I've been getting many varied answers from other forums from those that said my vent temps are too high and should be in the mid 40's to your high side is way off unless it was over 105 degrees and will cause the excess heated refrigerant to flow back to the low side preventing proper heat absorption.
Hard to know what or how is right considering this is retro fit.
By the way. How did you know that the drier wasn't replaced?

Tiny
BMail
Jul 6, 2011.
You did not say it was replaced. High side readings depend on ambiant temp and where reading is taken the closer to COMP the higher the reading. Inside car temp better guage of cooling common rule is a drop of 20 to 25 degrees from outside temp is good.

Docfixit
Jul 7, 2011.
Docfixit,
Thanks again for the great info.
To be sure I understand, for future readings, are you saying that I "should" take the ambient temps reasonably close to the compressor to calculate if the high side psi reading on the gauge is good or not? Wouldn't the temps close to the engine skew the surrounding ambient temperature reading higher than they really are?

As for the 20 to 25 degree drop from outside temps at the air vents. That also makes sense to me except that I keep seeing other sites that say otherwise. Two other sites said if the vent temps are not in the mid to high 40's you have a problem? My temps at 90 degrees were in the mid to high 50's which calculates to a 30 to 35 degree drop! Given your suggestion this seems good.

Lastly, have you had any positive or negative results from using Artic Freeze Ultra Synthetic with advanced leak sealer? Since I have such a small leak (took two years to loose 1 lb of refrigerant)and I only drive the car 400 to 500 miles per year now a days, I thought I might top off my system the next time it runs a little low with this product rather then spending in excess of $1,200 to repair it. Interested in what you or any of the others have to say about this product.

Thanks again for your help.

Tiny
BMail
Jul 7, 2011.
To clarify not air temp next to COMP the high side reading if schrader is close to COMP reading will be higher. No clear cut on vent temp if your comfortable in car then AC is doing its job. Not about the Artic stuff but have had good results on similar sealers. They work well on low side leaks.

Docfixit
Jul 9, 2011.
Thanks that really helped.

On another related question.
Just purchased a AC manifold gauge to test low and high side pressures and for adding freon if needed.
For those that have used them, I'd like to see if you have run into this issue.
After hooking up the low and high side couplers to the AC fittings and turning the hose valve couplers clockwise to open (the manifold blue and red ****s are left in the closed position), I take the readings, then turn the car off, turn the coupler's hose valves off by turning them counter clockwise and remove the low side first then the high side connector. The low side comes off easily with barely a whisper of pressure, but when I remove the high side coupler, it spews, forcefully for approx. 2 to 3 seconds, freon from the hose coupler, that was built up in the hose!
Is that normal?
Seems like such a waste of freon. Glad I don't have longer pressure hoses.
By the way, the high side coupler is quite hard to attach to the fitting and requires a fair amount of pressure to push down and make a connection. Don't know if that has anything to do with the above issue.

Tiny
BMail
Jul 11, 2011.
Don't know why there were **** after the word red in my last post, but they should be replaced with the word "****s"

Tiny
BMail
Jul 11, 2011.
What tha. Let's try it this way
k n o b s

Tiny
BMail
Jul 11, 2011.
Normal High side has lots of pressure so wait before removing until gague reading falls
AD

Docfixit
Jul 13, 2011.
Thanks for your help. That's a great idea, although I guess it will take quite a while for the pressures to come down. I realize the gauges will not completely Zero out so some freon will be lost. I just hate to discharge freon into the air if I can avoid it.

Tiny
BMail
Jul 13, 2011.

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