I recently did the dreaded heater core swap on my 1997 Expedition. It appears to be succesful as i get heat up front and all controls work as expected.

The Problem: Rear heat was working fine before I started this journey.

Now, it seems to only blow cold air through the rear system. The front heater works great, all controls work properly.... Just can't get hot/warm air through the rear vents.

What did I screw up? And what steps can I take to fix? I searched around for some help. I see others have had similar prob but no complete answer.

Thanks in advance

 

The coolant flow will take the path of least resistance. Since the front core is new, most of the coolant can be flowing through it while the older rear one being slightly restricted gets less flow.

It's that or the rear is simply air bound. Get a couple of small flat pieces of wood and a c-clamp. Use the wood pieces as cushions on each side of one heater hose (after the Y-adapter) going to the front core, and clamp them tight to try and prevent flow to the front core. Rev the engine a few times, this will force coolant to the rear core and push any trapped air out of there.