Do I need a heater bypass valve?

[burma]

Hi,

This summer I have noticed my '96 Previa's AC doesn't really cool well. The curious thing is, in the first 5 minutes of operation when the van is starting from cold, it works pretty well, with vent temps in the 45F range. However, as the engine warms up, it gradually creeps up to 65-70F.

One day I did notice that if I leave the AC compressor off, it will blow slightly warm air even when set to full cool. Poking around a little further, I also noticed that the heater core pipes under the dash (and under that small plastic cover with 3 screws) were hot.

I did verify that the control cable tied to the slider was moving the heater control valve (on the left behind the coolant expansion tank) and that it seemed to be moving to fully closed. I even popped the cable off and pushed it manually but it didn't move, and the heater core still seemed to be getting coolant flowing through it as it still warms up.

My question is - is this normal operation and maybe I have some other issue? Or is the valve blown and letting too much coolant pass through when shut?

Thank you!

[John Kaufmann]

Those were the exact symptoms when my leaky heater valve needed to be replaced.

[burma]

Ah that's helpful to know, just sucks that the valve is $100+ from Toyota. I might pull it off and inspect, and maybe rig up some sort of inline shutoff valve that is manually operated. In my area the need for a heater core is somewhat limited

[burma]

So the heater valve was definitely cooked, just barely touched the hose and one of the tubes snapped right off. All the plastic was turning into that chocolately brown that cooling system parts do when they're getting really fragile.

For the time being I just replaced it with manual spigot-type valve from the auto parts store. Happy to report that the AC is working MUCH better than previously.

One question though - does anyone know if these valves need to be slightly open for everything to function properly? I ask because I have it completely shut, and I'm noticing that the rad fan is running much more frequently while the temp gauge is fluctuating a little more. Not getting hotter, but actually showing a bit cooler most of the time. I know the fans might just be because its very warm outside and it now doesn't have the heater core acting as another radiator, but I just want to make sure I'm not overly impeding flow through the system. With the head gasket issues these vans are known for I'm pretty sensitive to issues with the cooling system.

Thanks!

[man_btc]

The heater valve cuts off flow to the heater core completely when closed- your temp fluctuations might be because of what you surmised about the heater acting as an auxiliary radiator, or perhaps you introduced some air into the system. While you're Waiting For The Problem To Go Away (always a good first step), here are some pics from a long-ago failure I had.

1 -Something inside Previa heater valve.jpg

2 -Gasket extracted from heater valve.jpg

3 - Exploded diagram of heater valve.jpg

[burma]

Thank you! That's super helpful. And yes my valve looked about like yours when I took it off.

After a couple days and another burp for good measure things seem to be back to normal with the fans and the gauge. WFTPTGA seemed to work!