GMC Repair: Overheat when AC is on, coolant system, radiator shop

Question
I have a 1995 GMC Sonoma truck with 1.8L shift stick. The engine overheats when air condition unit is on. The following are my diagnostics:
- Check engine light does not come on.
- It overheats only when air condition (AC) is on, and backs down when AC is off => means that thermostat is working properly.
- The coolant is full. When the engine is near overheating, I twisted half-way the radiator cap, the coolant is gushing out. This means that there is pressure in the coolant system, also means that the water pump is working (?).
- The belts, and hoses seems o.k.
- I did not notice significant leaks in the coolant.
- When the fan/water pump is turning, I did not notice any unusual noise => water pump bearings are o.k. (?).

Any suggestions what may be wrong and what to check ?
Thank you very much,
Rookie.


Answer
Hi Rookie. You have a problem with your radiator. When you turn on the AC, the condersor heats up to about 170 to 180 degrees, the ait passes thru the condensor before it goes to the radiator, so you are trying to cool the engine off with 170 degree heat. with the AC off, you are using outside air temp to cool it off. Look to see if the radiator is plugged either externaly or internally, a radiator shop will be able to check this for you. MANFRED