I have a 1999 Cherokee limited with a 4.0 engine

Been having an issue where the heat is inconsistent. The engine warms up and reaches operating temp reading 210 on the needle with no issues. The heat will get hot when first turned on then gradually, sometimes quickly, cool off and become either cold or luke warm. I back flushed the heater core today and no change. Flushed heater core a second time from both directions and no change. I checked the blend door actuator and it appears to be working fine. The engine is reaching temperature and the coolant seems full. It will get warm to hot if I drive it hard, but that is short lived. I thought it could be a vacuum issue as there was an issue this last summer where the AC would work great, but after driving for a while, or driving under a load like in the mountains, there would be no air from any of the vents at all despite the fan being maxed out. If I'm understanding what I read here is that if there is a vacuum leak the vents should default to the defrost which it did not. I was never able to find what the AC issue was, but am wondering if it relates to the heating issue.

When the engine is at temp after driving the city I checked both heater core hoses. The top hose is nice and hot while the bottom is warm. There does not appear to be a heater control valve as the hoses go straight from the thermostat housing to the firewall. The cruise control also only works some of the time, which is another reason I thought it could be vacuum related for all these problems. Right now I'm mainly concerned with the heat.

Any help would be appreciated

 

Then I’d say you have a blockage in the heater core.

 

And that's a new heater core right? I thought about running some coolant system cleaner through it and letting it sit for a while before flushing it out again.

 

A new heater core is the absolute last step. I would try to add cleaner per instructions and flush it with water. Do not try to use compressed air. You’ll only damage the heater core and then you’ll have to dive down a deep rabbit hole to replace, if you want heat.

what does the coolant look like? Great time to replace the coolant with some pretty green fluid after flush.

 

I didn't check the coolant from the radiator, but what I was able to see in the reservoir it looked time for a flush. I will try some cleaner and back flushing it again then flush the entire cooling system. I was also reading that if the system was low on coolant it could cause the problem, but the reservoir was full to the mark. Thanks for the assist. I'll post back here when I have results.