Does incipient TXV failure explain my Subaru A/C symptoms, or what other problems cause this?

Question

I’m researching what is likely wrong with my 2014 subaru 2.5L outback A/C. It’s charged and operates normally most of the time. After everything gets up to normal temperature on the engine there are occasions when the air out of the vents is is warmer than it had been and might be a little humid. The interior fan speed is slow and the car is coasting or idling at a stoplight. Things improve when I speed up. It doesn’t always happen. Although this link is about variable displacement AC compressors How is a variable displacement A/C compressor controlled has interesting footnote on the TXV, which reads “...the TXV is that systems featuring this kind of compressor need a tailor-made thermal expansion valve that will never hunt for superheat nor close fully, or they will experience unstable vent temperatures at low heat loads and possibly starve the compressor of lubricant” My AC symptoms seem consistent with this, but I am uncertain that I am in a low heat load or if there are alternative things to look for. How does one test the TXV to be sure it is not malfunctioning or to find out if debris might be limiting the proper operation of the valve at the low idle condition? Anything else I can evaluate that relates to my symptoms?

Answer 1

It’s not the best idea to diagnose based off of web articles. The very basics of automotive ac system diagnostic is the following:

1. verify blower motor operation across all speeds

2. verify blend door actuation occurs

3. verify power to compressor

Some a/c compressors have a clutch, others are duty cycled. Sounds like Subaru uses a duty cycle style compressor. This means a certain amount of power will be sent depending on command from its control module.

If you don’t have power going to the compressor this can be for various reasons. The ECM or whatever controls HVAC will look for proper pressure at the high and low side pressure switches, it will look for ambient air temp, cabin air temp, and evaporator temp. If it lacks data from any of those of the data is faulty or incorrect the system will generally not power on the compressor.

Best diagnostics after the above is to evacuate and recharge the a/c system, introduce uv dye and run the system for 3 hours (drive it around with a/c system on) and then look for leaks using a uv light at the pressure lines, compressor, condenser, etc… if no leaks are found then you need to make sure you have power to the compressor since the system is properly charged. If not then look for codes. If no codes are present in any HVAC related module then you need to watch pressures on your ac machine and make sure they are fluctuating properly.