Originally Posted by nissanh

Based on the area you live and the quality of water, you may have calciumcarbonate (CaCO3) buildup inside the heater core. I bought a LandCruiser with the same symptoms. Little to no heat. I treated its heater core with 9% vinegar (most stores carry 5% but look for 9%) and problem solved.

Vinegar is a mild acid and reacts with CaCO3 to give off water and CO2 (carbon dioxide).

I attached two transparent hoses to both heater core lines (NOTE: if the factory hoses are hard to remove from the aluminum tubing, use a pick to break loose the rubber hose from Aluminum tubes: NEVER use a pier to twist the hose and filled with 9% vinegar and allow it to sit over night. Next day I emptied the heater core with come low pressurized compressed air. I got white loosened CaCO3 out of the heater core!! Did that few times.

CLR is OK but I wanted to go soft on the heater core: My cousin ran 100% CLR on his 2000 pathfinder and obtained good heating, but I would not recommend CLR as the first option.