Update:
After talking to some rad shops, it was too expensive to fix.
So, I ended buying a whole new heater system from heater craft. (3 vent, via my dealer)
Good news, this new system came with the Y adapter to fix the low heat at idle.
I never had the Y adapter, (just two hoses from block to the heater core and for years have always had cold air on idle)
So, I'm excited to fix my heater, and fix the low heat at idle.
Attached is the pic of my Y adapter. From what I've read online, is that it's best to put the Y adapter in "front" of the impeller. But, apparently Malibu puts the Y adapter "after" the impeller. Now looking at this particular Y adapter, I'm pretty sure it's not the correct size to fit on the "front" of the impeller or raw water intake hose. (Perhaps my older boat has a larger raw water intake hose?) It looks way too small.
I don't have a recent photo of my raw water intake hose (the heavy stiff one) but I believe its too big to fit this Y adapter. And, I do kinda recall that the impeller connection is smaller "after" the impeller that feeds into the transmission cooler. And this Y adapter looks like it would fit on that section of hose.
So, for those of you that have modern Malibu boats or other boats with a Y adapter "after" the impeller, do you have heat a low idle?
Thanks in advance for advice.