I’m pretty sure the heater core underneath the dash is leaking onto the passenger side floor of my truck. How do I best disconnect the heater core from the engine block? Do I need to loop the hose?

I would just loop a section of new hose between the outlet and inlet on the engine.

Technically you should be able to plug both of them and not affect coolant circulation. I just play it safe and loop the hose.

That was my thought but glad to have confirmation. 👍😇
Thanks!

The proper way to taken the heater out of the system is to plug both hose ends at the engine but a field repair is to remove 1 hose from the heater and the other hose from the engine then connect the 1st hose free end to the engine fitting for the other hose.
Tony

Be aware that the source of the water on the floor may not be from the heater core but from water condensed by the air conditioner if the drain is plugged. If your truck has a functioning AC, I would check the drain by probing with a screwdriver or similar tool before removing and/or rerouting the heater hoses.

I forgot about the AC drain. Except I did not think there was enough humidity in Tucson this time of year to create any condensation!

Should be pretty easy to tell which if the engine has anti freeze in the coolant. Stick your finger in the puddle and smell it.

Not good to taste it!

If fluid is slippery it has antifreeze in the water. If just non slippery water it is condensate.

If a drop onto concrete stays as a drop it is condensate from AC system. If it spreads out a little then has antifreeze or some anti rust agent in it.

If it is ac condensate it wont leave a trail but coolant will leave a trail as the water based ingredients evaporate, this trail maybe white or a paler version of the coolant color. The AC drain will most likely be a hose that leaves the ac enclosure and drops through the floor. It is not uncommon for this hose to block up.
Tony

What year truck is it? Some of the trucks could start to rust around the windshield and water would get in there. Or if the windshield is not properly glued. I had this happed to me on an 04 Chevy and thought it was the ac only to find a thin line of rust allowing the rain in.

It's an '84. Even if there is rust around the windshield I doubt if that is the source of the water. Peter lives in Tucson, AZ. When I was there in early April it was already hitting 100 during the day and really dry.