Problem with my old truck ?

[kjphilippona]

I have a old '88 Isuzu pickup that is giving me trouble and i cant figure what is wrong. Last week i started her up and could hear coolant rushing thru the heater core, never did this before. I opened the hood just in time to see coolant squirting out of the over flow bottle, the engine was still cold so i thought the thermostat was stuck, replaced it and flushed the system. It still does it, replaced the radiator cap, no good, i also have bubbles in my over flow bottle. I have no white smoke from the tail pipe, no oil in my coolant and no coolant in my oil. Any ideas ?

[Unkle]

Stuck heater valve ? It's open when water / coolant not hot enough ? 2 cents worth .