I was loosing a half gallon or so a week of cooling fluid last winter. Now I'm not. I suspect that it is a heater core issue? Heat turned off for months. I have no air conditioning system on my truck.

I was loosing a half gallon or so a week of cooling fluid last winter. Now I'm not. I suspect that it is a heater core issue? Heat turned off for months. I have no air conditioning system on my truck.

I don't think you have a heater core issue, there is coolant in the heater core even when the heater isn't being used. It sounds like you have a coolant leak with cold temperatures. I would check for signs of coolant tracking at all your hoses and gaskets. The most common one I have seen is a radiator hose clamp leaking when temps get down pretty low.

how did you turn off the heater core? Did you disconnect the hoses? I'm not familiar with the control... does it block coolant flow?

As you can see there is no valve in the heater core just hoses back out to the block. The only thing that controls temperature is a butterfly valve that routes air to or away from the heater core.

I stand corrected. Just assumed there was a valve on the coolant going to the core. I'll look elsewhere. I've got a cooling system pressure tested. Any ideas on how high pressure I can go without causing more problems than I'm solving? Like 3-4# above cap pressure? I've done it before without success, or should I say without finding a leak.

I typically test cooling systems cold at about 16-18 PSI and look for wet spots to form. Like on Mom's 96 Dodge the intake manifold gasket was bad and was leaking but never dripped on the floor because the block temp would dry it up before leaving a trace. But pumped up cold and let it sit the leak should show itself within about a hour if its slow.