2000 XJ 4.0 with 185k miles

I don't have any experience with this jeep as I bought it in need of a rebuild. Rebuild is finished now, but I don't have much heat at all. The engine gets up to operating temp just fine, no overheating or anything. Thermostat, water pump, radiator, radiator hoses, and heat hoses are brand new.

I haven't tried flushing the heater core yet, but with the engine fully at operating temp and the heat on, the top heater hose gets plenty hot but the bottom is only lukewarm. Is that a dead giveaway that it's a clogged heater core? Or should I be looking for a bad blend door? When switching from hot to cold I can see the little electric motor/servo rotate that would be moving the door, but haven't opened up the dash to physically see the door move.

 

If blower is working and pushing air - flush it. Drain, fill with fluid and cleaning solution and run engine with heater on high. If the core is really clogged run some hot water and vinegar in the top hose of the core and let it sit for 20 minutes. The flush it out. Repeat until water cycling through everything is clear

 

My things are 10 years older. Since you have no valve for the coolant to the heater core and one hose is hot, and the other cool I'd guess coolant isn't circulating. The fins on the water pump can get smaller and smaller till it's flat as a dime. Might be somewhere between it could still cool an engine but not move much water through the core. More likely it's clogged though. Mine, I disconnected hoses, then just holding a garden hose and the hose end in my hand, flushed back and forth. Got a little gunk, but it's still marginal, OK here though. At least I didn't explode something exposing the core to 50, 60# of pressure when it's use to less than 20!

This here DJB provided us >>>>

Coolant system flush, THANKS DJB!
"Remove the top radiator hose and top heater hose from the t-stat cover. Remove the bottom radiator hose from the water pump (very easy if the plastic air filter box is removed 1st). Stick a garden hose in the heater hose and flush. Stick the garden hose on the heater hose nipple on the thermostat cover and flush. Stick garden hose in the top radiator hose and flush. All areas are now flushed without crud moving from one place and lodging in another."

 

do not use high pressure to flush the heater core. I replaced mine once i found the passenger floor boards soaked. oddly i replaced all of my cooling system prior to this and once the heater core was replaced my heat was never as hot as it once was.

however flush your heater core backwards with some CLR or vinegar as stated aboved(any good safe and non corrosive cleaner) and let it sit a while to loosen up any crud. then gently rinse with garden hose or such.