I have a 98 Cherokee 4wd inline six 215k. I just replaced both heater core hoses about 2 months ago when I flushed the heater core for a second with a little more heat coming now than the first time. I noticed there’s air coming out of the area where the hoses go in when i was running the defrost on high!!!! I’m clueless and I also noticed the floorboard on the passenger side is damp also guess cause the heater core is leaking!!! I flushed the core the first time with some prestone cleaner but didn’t drive it around for a day or 2 like I should have. Then I did the 2nd with just water and back flushed it with no really difference from the first, I guess it’s time for a new core 🤦‍♂️

 

It is possible the hoses are not tight enough, or too tight, and leaking into the cabin. It is also possible the core tubes were damaged during the install. Getting the tubes caught up on the firewall slidng the box back in, for example. Also, ive seen some cores that have moveable tubes. Never bought one yet, but i can only assume the tubes are sealed with an o-ring? If you have one of those cores, maybe an o-ring has failed? Could be a bad core out of the box, too. For the air coming out of the firewall into the engine bay, id say the case isnt sealed completely. But those are only guesses.