Hey, we recently replaced most of the hoses on our 87 4Runner except for the one to the heater core. Drove 4 miles today and it was swollen like a grapefruit once we stopped. Could it just be a bad hose or the heater control valve? It doesn't look like it's opening but the heat works. Thanks!

 

I'd think that when the valve is fully closed, no heat called for, the hose should be just fine, so it sounds like you have a weak hose. change it before it blows and leaves you stranded.

 

I think the heater control valve is just fine. Even if the heater control valve is bad, it would not cause a hose to balloon up.

You have a bad hose and as Melrose 4R said, you will want to replace it before it completely fails.

It is also possible that there is too much pressure in the cooling system too and the weakest hose is what expanded.