A.C. very weak. Low side 30 psi, high side 150

Hi, all.
I have a 2005 passat tdi with around 95k miles on it. The ac has been pretty ineffective ever since I bought it a year ago.

I got a set of ac gauges from the autozone tool rental program and checked the pressures. With ambient temperature around 70 degrees the low side pressure is about 30 psi, and the high side pressure is around 150. The air at the vent is barely cool. If you put your hand on the high side line it is neither warm or cool. The low side line is cool (not cold) to the touch.

A.C. clutch is on, Electric fan is spinning. Does this set of symptoms make sense? The low side pressure is around where it should be, I guess. This argues to me that there is enough r134a in the system. The low high side pressure suggests to me that the compressor isnt pumping enough, but then, why is the low side cool to the touch? It seems like it should be around ambient, if all the r134a boils off in the evaporator.

If the expansion valve was stuck closed, the high side pressure should be too high. If it was stuck open, the low side pressure should be higher.

Is there an internal flap before the evaporator that could be stuck and prevent the cool air from making it out the vents?

Maybe I'm just over thinking this and should just put in a can of r134a and see if it works any better. It doesn't seem like I risk overcharging it with the high side pressure so low.

Thanks for any insight you might have..

Max Behensky

It sounds to me like you are a little low on freon. R134 will blow almost ambient temp if it is down as little as half a pound on some systems. Of course do the basic checking for fans running, condenser air restriction, etc. all first.



I charge by temperature more than pressure like the HVAC guys, though you do use pressure as a reference for major blockage or something like that. There are way too many variable to use pressure for anything more than that.


If you are using refill cans, try to find one with no sealant or stop leak in it. Dye is ok. Introduce it slowly in the low side while running. Often on a top up I don't hook up the high side at all to minimize intrusion and possible leakage from disturbance. Add freon till the liquid line (low side) hose starts getting cold. Then Put in a little freon and let it circulate a minute or two to normalize while monitoring vent temps. Keep charging slowly till the vent temps get as low as they can go. (usually in the 40's on a moderate day) If and when the vent temps start to reverse you are getting into overcharge conditions, do not add any more it will just get worse, best to stop when the temps get into the low 40's to avoid that.


The liquid line will be cold and sometimes sweating condensation out of the air. Unhook your refill can and go for a cool test drive.

If the system had a slow leak and have had refill cans used, they usually have extra oil...the gas leaks out, the oil may or may not...so many DIY fixed cars slowly get more and more oil... result is like overall capacity for cooling, especially in warmer weather...but psi appear normal ish..
If you get the SYSTEM vacuumed, it will then get filled with the proper amount of oil and r134a....
If you read the side of most refil cans, they say to shake then fill or hold it one way to add only gas, or another to add lube...
Some more pro style cans are gas or lube only...but you have know way if knowing if it was "topped off" a few times before you got it.

^^^^^
True, post above. Many systems end up over oiled, or overcharged due to procedural errors alright. I was assuming it was untouched, good point to note if it was not.


Evacuating system will not remove the oil. It might cough up some excess oil if it had been overoiled, but not a guarantee in any case. Hell, it has been proven that evacuation will not even remove all the freon even after repeated attempts, so many systems get overcharged due to that.


Often though a simple top up is all that is needed, and that was what my recommend was based on.

Are the outlet of the accumulator and the suction line at the compressor cold?

The variable displacement compressor will work to keep the low side at 30 psi, so you can't really go off that pressure. The high side does seem a little low.

Since the Passat is an OT / accumulator system, it's not as sensitive to charge levels. To prevent damage all you need is there to be liquid getting to the accumulator.

It might be worth taking the blower fan out and verifying the AC evaporator is clean.

-J

Thanks for all the quick replies. The single previous owner of the car was a former San Francisco fireman who didn't drive it very much (85k miles in 12 years). He didn't strike me as a DIY guy, and the car has been in SF for most of its life. It hardly ever gets hot there, so I think the AC probably has been ignored. It is possible that the system has been refilled and oiled lots of times, but that seems unlikely to me.


The idea that the evaporator is plugged up or very dirty is possible. I would think that the low side line out of the cabin would be very cold in that case (not just cool). I'm getting plenty of airflow out the vents, as well.



I think I'm going to add a little r134a (without oil) and see what happens.


I'll report back the results.


Max Behensky

When you add gas, be sure to purge the lines.

Well, I tried it this morning, when it was pretty cool out, and the low side pressure was in the low 20s, and the high side pressure was under 100. I think I mis-remembered, and the high side pressure yesterday was more like 100.


The vent temperature was in the high 60s with the fan on position 2. I slowly added r134a to the low side. The high side pressure started climbing, and the vent temperature started dropping. After putting in 8 oz of r134 (measured with a digital kitchen scale), the vent temperature was around 48 or so. With the fan in the lowest position the vent temperature got as low as 40.


Since the system was basically working, and if I added too much I was going to have to get some professionally extracted, I stopped. With the fan on High the high side pressure got a little over 250, the low side pressure was in the low 30s, and the vent temperature was in the low 60s.


I'm going to go drive it around and see how it works. We're supposed to have a heat wave in CA in the next few days, so I'll be able to give it a good test.


The system obviously has a slow leak. 6 ounces of r134a in 13 years doesn't seem too bad. Just for reference, r134a has 1400 or so times the global warming potential of carbon dioxide. Thus, one pound of r134a is equivalent to 1400 lbs of CO2. Since each gallon of diesel burned releases 22.4 pounds of CO2, this is equivalent to 62.5 gallons of diesel. At 38 miles per gallon, this is the amount of diesel burned in 2375 miles of driving. If I've only leaked 8 ounces this works out to around 1200 miles of driving. Over 13 years this is under 100 miles of driving equivalent per year. Not great, but not exactly an environmental disaster either.


Of course, it may be leaking faster now than it has on average, and this may not be the first time it has been filled up. I'll see how it does over the next year, and if I have to add more I may have to try and find the leak.


Thanks for all the help,


Max

You need to continue adding gas until the outlet of the accumulator is cold and the suction line at the compressor is cold. If the charge is low and there's only gas coming out of the evaporator (which means the vents will still be cold!) you'll be starving the compressor of oil.

-J

The pressures don't seem that bad. Wonder if there's a blend door issue?