2013 Sienna - Front AC is weaker when rear AC is turned off - huh?

[Craig in Canada]

My SO has a 2013 Sienna LE. It has a slow freon leak of some kind which will need to be addressed professionally. What she usually notices is that in hotter conditions the driver's outlet air will warm up compared to passenger. A little top up (like every year or two) and it's back to running well.

In taking a look at it yesterday I observed something puzzling. My "common sense" would tell me that if you shut down the rear HVAC it should improve performance of the front AC (by which I mean vent temperatures, not cooling the air volume inside the car). Instead it has the opposite effect. It's quite hot/humid here right now. She had the rear system off, forgot, and a rear passenger asked if she could turn it on. The driver's side was struggling a little, which is something she was used to. She hit the button to turn on the rear which went full blast and almost immediately the driver's vent cooled off dramatically, despite all the extra load from the rear.

With a pressure gauge hooked up, I observed that when the rear is shut off, low side pressure dropped dramatically along with corresponding warming of the driver vents. What's going on? Lots of liquid refrigerant loading up in the rear evaporator due to no load, accentuating the low charge condition? Some kind of reduction in compressor output due to switching off 2/3 of the vehicle demand?

It was about 100F and humid outside. High fan setting, low pressure readings:
All on, normal idle, 45-50psi.
All on, 1500rpm, 35-40 psi
Rear off, 1500rpm, 20psi

My rough estimate gauge says low side should be 50-55psi in these temperatures, but I know it can be more complicated than that. I'm also not clear if the "guideline" low side pressure targets are at idle or raised from idle.

At any rate - I'm curious if anyone can explain why shedding heat load reduces performance in this undercharged system.

 

[michaelluscher]

Rear A/C lines like to corrode out near the right rear wheel
SMA did a video on him repairing a Sienna
This performance screams low charge to me
Have you checked condenser air flow and cabin filter?

 

[Bailes1992]

Most certainly low refrigerant level.

You get more flow over the evaporator with the rear vents open which results in colder A/C.

 

[Craig in Canada]

Rear A/C lines like to corrode out near the right rear wheel
SMA did a video on him repairing a Sienna
This performance screams low charge to me
Have you checked condenser air flow and cabin filter?

It is low charge - well let's say 99.9% low charge. I am curious why removing load reduces the performance of the remaining parts of the system, unless it's an counterintuitive low charge effect. e.g. liquid refrigerant accumulates in the rear lines since the rear expansion valve is closed, and really starves the rest of the system for refrigerant. When it's all running there's still enough to work "OK". ???

I'll search for the rear lines video. This is the salt belt.

Condensor air flow is OK, but in test conditions the vehicle was stationary so performance was even more reduced. Cabin filter is good, inside air flow is good.

 

[Craig in Canada]

You get more flow over the evaporator with the rear vents open which results in colder A/C.

That doesn't make sense to me. As far as I know, there is one compressor and two evaporators - front and rear. With the rear system off, presumably the rear expansion valve closes. Then low side pressure falls and front cooling performance drops on the driver's side first (the imbalance being a typical sign of low charge).

Turn on the rear full blast - low side pressure increases and everything gets colder.

 

[JMJNet]

I suspect the leak is in the front system.
May be behind the dashboard?

 

[Craig in Canada]

I suspect the leak is in the front system.
May be behind the dashboard?

Could be, but I hope not for the sake of repair cost.

For clarity, every 1-2 YEARS it benefits from a little top up (like 5-10 seconds of trigger squeeze from an auto-store can, held upright, charging w/ gas not liquid). I don't think the nature of the leak is involved in the system behaviour I've described. No argument that it needs fixing, but it is not a severe leak IMO. The system has never completely lost pressure, failed to run, or been left open to the environment.

 

[KrisZ]

Interesting stuff. My 15 Grand caravan has a leak around the rear expansion valve, already identified with a dye, and when the charge gets low, the rear AC starts to get warm first. But I never tried shutting off the rear to see if it has any effect on the front.

But as others have noted, it’s low on charge and probably when the rear evaporator gets separated from the rest of the system, it traps a significant amount of refrigerant. This behavior might be specific to this particular system.