I have a 99 Cherokee XJ 4.0. I was getting a coolant smell coming from my heater vents, I put in 1/2 a bottle of barrs leak and the coolant smell went away.

However, the other day I was not getting very good heat(only about half as hot as it usually gets). So I replaced the thermo and did a coolant flush(with water only). The heat came back and has been working great until this morning. I let my truck run for 15 mins before I got in and there was NO HEAT. Im thinking clogged heater core? I have tried doing searches on this subject and cant really come up with anything...

Any suggestions???

A quick heater core swap is in order here.. Before swapping do several coolant flushes. Get that damn barrsleak out of the system. Stopping up the heater core with it should tell you what it has done to the radiator itself.
The coolant smell was just what you thought it was... a leak in the heater core. Then the barrsleak stopped up the leaky area and prolly much more.
Big JIm

Thanks for the peace of mind Big Jim. I will do a few flushes then install the new heater core. Im printing off an all data on how to R & I the core. Anything inpreticular I need to know?

Yeah keep your patience in check.. The first thing on the assembly line when building a Jeep is the damn radiator core... The rest of the jeep is built around it..
Big JIm

Are the waterpump veins made out of plastic or metal? Can they go bad? Can my jeep not pump coolant? The thermo is wide open but not pumping coolant through upper rad hose. SO the upper rad hose is cold, heater core hoses are warm, but not HOT.

I never saw any Jeep pumps with plastic veins. Fords are notorious for the veins rusting away.. But I think it is the caliber of owner Fords have... Using the 1/2 by 1/2 water and coolant mixture should cause the pump veins to last the life of the bearings in the pump.
What is the temperature outside when you are testing the temp of the top hose? If it is really cold and the engine isn't toiling, I would suspect the top hose to be cool at worst..
Big JIm

Ok, its done......For a minute I thought it may be the water pump, which by the way the pulley is squeaking?

Turns out it was the heater core, with some help I got it swapped out in about 4 hours, and lets hope i never have to do that again...

The system was having a problem holding a vacume, not anymore though Still kinda strange why I had NO heat this morning....I guess tomorrow morning will be the real test, its suppost to get REALLY cold tonite.

Make sure you bleed the air out of the cylinder head by removing the heater hose from the thermostat housing while you fill the radiator until coolant shows or drips out of the thermostat housing. An air pocket will form in the head if you don't!