Air not coming out of vents, but only when in motion.

[mpower36]

2005 Legacy 2.5i.  When I start the car, the blower and all the vents work fine.  The vent mode changes as it should.  After I go down the road about 5 miles, the velocity of the air decreases to almost nothing.   It doesn't matter what vent mode is selected.  When I park it in the garage without turning off the engine, after about 5 minutes, the air velocity returns to normal. 

I've checked the blower motor and it's running as it should, all speeds work fine, but the air is just not coming out of the vents when the car is in motion.  I pulled out the glove box to check the mode actuator and it's working fine.  I had my son drive while I watched the mode actuator when the air velocity was nothing.  It's working, but you can't hear the air changing routes as the doors change position.  I think this is because there is not airflow.  This is repeatable. 

I will also mention that in the summer, the AC is cold for about 15 minutes, but then it fades out.  I checked the AC pressures and after 15 minutes, the high side goes really high, like it's overcharged.  

It seems like the air is being blocked somewhere between the blower and the vents.  I pulled out the blower case assembly.  Everything is clean.  No blockages to prevent airflow from the fan from entering the heater core and evaporator case (HVAC case).  I looked into the HVAC case with a mirror and saw a bunch of debris from trees laying against the AC evaporator.  This is not surprising as my daughter used to park this car under a tree until I showed her all the debris on the cabin filter.  I blew all the debris out with an air hose and vacuum cleaner and put it back together.  The problem remains.

Any ideas? 

[mpower36]

A little new info.  I suspect that the evaporator is icing up and blocking the airflow.  At idle, I don't think the compressor is turning fast enough to cool down the evaporator to make ice, but when driving it is.  When I then park it and let it idle after driving, the compressor is turning slowly and maybe the ice is melting allowing air to flow through the evaporator again.

I've also noticed that the longer the car idles, the harder the compressor pulls on the engine.  The car has a tough time recovering idle when the AC compressor clutch engages, which makes me think the compressor is heavily loaded.

Today, I unplugged the AC compressor relay and drove it.  The air velocity did not diminish while driving.  I'm going to try putting the AC relay back in and increasing the rpm to 1500-1800 rpm while it's parked in the garage and see if the air velocity decreases.  That will at least tell me if it's rpm related.  I'm also going to check the evaporator temperature sensor and see what kind of readings I get while idling and at elevated rpm.

If you've experienced something like this, please let me know. 

[KZJonny]

You already figued out the only thing that at least I could think of to describe those symptoms, namely the evaporator icing up.... I was wondering if this time of year, if you had the defrost w/AC on, that might be doing it as well while you were driving.

So possibly a failing compressor or expansion valve?

[mpower36]

I pulled the evaporator out expecting it to be plugged with debris, but it had very little in it.  I cleaned everything and reassembled.  I charged the AC system per the FSM, but the high side pressure goes way high to 450 psi, and I didn't even get the full charge in.  

Took it for a test drive and it takes a little longer, but still the air velocity diminishes to almost nothing.

[KZJonny]

I’m not an AC tech, but I would be considering a failed expansion valve if the pressures are that far out of range.

A Denso unit is like $25 on Rockauto…

[silverton]

You're describing two separate issues.  airflow and temperature.  are you perceiving reduced airflow because of a change in expected temperature? If it is the temperature issue and it goes from ice colds to luke warm then it's either the charge amount, the relay, or the compressor itself.  If it is just airflow, which reduced airflow can make it seem like the temperature is incorrect, I'd suspect the blower motor or the blower motor resistor.

If you charged the AC yourself with cans from an automotive store, I highly recommend you take the car to an AC specialist for a recharge, as those cans create more problems than they solve.  IMHO those cans should be illegal to sell.

[KZJonny]

30 minutes ago, silverton said:

You're describing two separate issues.  airflow and temperature.  are you perceiving reduced airflow because of a change in expected temperature? If it is the temperature issue and it goes from ice colds to luke warm then it's either the charge amount, the relay, or the compressor itself.  If it is just airflow, which reduced airflow can make it seem like the temperature is incorrect, I'd suspect the blower motor or the blower motor resistor.

If you charged the AC yourself with cans from an automotive store, I highly recommend you take the car to an AC specialist for a recharge, as those cans create more problems than they solve.  IMHO those cans should be illegal to sell.

You think they are bad in the US? In Canada they don't even have R-134a in them.

It's a blend of propane and isobutane, and sold as a 1:1 replacement for 134a systems. Banned in the US, but allowed in Canada?

I cross the border and stock up on your 'good stuff' cans to do AC repairs when I have to.

[mpower36]

Silverton,  the temperature remains as commanded.  It's just the air velocity from the vents that's affected.  The blower motor and resistor check okay.

This only happens when the car is in motion for about 7 minutes.  I tried to duplicate it in the garage by using the rpms to 200p for 10 minutes, but that doesn't produce the failure.  

Also, pulling the AC relay solves the problem, so it seems it's somehow AC related to me.

[silverton]

The only part of the AC system that may reduce air flow is the evaporator.  You did mention you saw a bunch of debris against it, that on top of an inefficient AC system from an incorrect charge amount will make it ice up excessively and would effectively block air flow.

[mpower36]

Thanks for the input everyone.  Since pulling the AC relay seems to fix the issue, I'm going to chase the AC system at this point.  The FSM says high side pressure too high could be a plugged condenser.  AC service websites and KZJonny recommend the expansion valve.  I think I'll start there as it's the next easiest (and mostly likely) possibility.  Since I have a work-around, I may wait until spring.  It's a bit difficult to charge the AC when the ambient temp is 35°F.  Thanks for the info on the quality of the R134a cans.  I have 6 left from the case I bought.  I've successfully charged some of the other cars with these, so I think the ones I have are probably okay.