I have a 1986 toyota pick-up, 4 cyl, 4wd with a 22r engine. the truck had been overheating and I found a leak in the rad, I replaced the rad and everything was fine for the past few months. Recently it started overheating again, the first time it happened I topped up the fluid and it lasted 3 weeks without overheating again. Then it started to overheat again last night and when I checked the coolant levels it was still full. The other symptom is that there's some white smoke from the exhast when I start the truck that lasts until it's heated up. What could th problem be here? Water pump? Head gasket? Any suggestions?