Why After getting no heat or engine not warming up you replaced the thermostat flushed rad etc twice with the same result Overheating what else could it be on an 87 suburban Chevy 350?

Answer 1

If I understand this right, there were 2 original problems. 1-no heat, 2- engine not warming up. Now the new problem is overheating, correct? Overheating is usually a function of poor circulation or poor heat transfer, assuming the coolant level is at proper point. 1) Sticking thermostat - if open, no heat and poor engine warmup. if closed, no flow and overheating. 2) Radiator fins degrading and beginning to rust/corrode or come apart. This will compromise the rads ability to do heat transfer and the result will be overheating. Copper rads are famous for this, aluminum less so. 3) If the impeller on the water pump has become worn down, it will be less and less efficient at moving the coolant around in the motor. This will usually give overheating. 4) Last item is big problem - if there is a leak at a head gasket allowing combustion pressure to leak into the cooling system - this will cause overheating problems. Note: No heat if all else is good usually means a plugged heater core. Also, watch out for a slight drop in the coolant level in the rad tank as the pick off point for heater hoses can end up below this level and suddenly you have no flow thru the heater core.