I just currently bought a 2000 jeep cherokee sport 4.0L and since ive had it the heat dont work it blows cold and maybe sometime lukewarm. I check the coolant level and it was fin but noticed the common brown sludge pasty type substance on the cap and in the radiator.......
HEEELLLPPP!!!! PLZ
Thanks

 

You will need to flush your cooling system, as well as the heater core. There are a few threads here on doing this to help you through it.

 

X2

 

X3^... This will most likely need to be done multiple times, probably best to replace thermostat (195°F), water pump, upper and lower radiator hoses, and heater hoses while you're at it, this will allow you to flush the radiator, engine, and heater core all separately.

 

I havent read all the threads on this subject but wanted to offer some insight on what we used to do to clean sludge out of the coolant tank and the rest of the coolant system. When I worked at the ford dealership we would use Cascade dishwasher dish detergent. Run that through just like you would a Prestone coolant flush. It breaks down the sludge pretty well. Then flush the system out with some clean water a few times. Then refill to spec with new coolant/water mixture.