A plugged heater core is the most likely cause of low flow, especially on vehicles where it sits higher than the radiator. Pull the two heater hoses off the engine and run water through it both ways from a garden hose. Avoid pulling the hoses off the heater core itself because it's common for a nipple to break off. Also be aware that sediment buildup can block a leak in the heater core. Flushing out the blockage can result in a leak and the need to replace the heater core.
GM also has a lot of trouble with corrosion due to the normal acid buildup in the coolant. Corrosion inhibitors in the antifreeze wear out in about two years. If you still have the red Dex-Cool, (Dex-Mud) antifreeze, GM advertised that as "lifetime" coolant to make their cost of maintenance appear to be lower than their competitors, but then they put on a sticker that says to replace it every three years. Even the Dex-Cool company says to replace it every two years to get the acids out and replenish the additives.
To check the acidity of the coolant, use a digital voltmeter with one probe on the negative battery post and one dipped into the antifreeze but not touching anything metal. A reading of more than about 2.0 volts indicates high acid content and you will want to drain and refill the system.
Wednesday, February 12th, 2014 AT 1:15 PM