I just bought a 1990 4 runner with 22re engin and it ran fine and smooth at normal temperature as i drove it home for 300km at highway,as soon I arrived in my city the temperature went to red ,so I stopped and saw that the water was boiling out,and i heard a loud hiss sound from the thermostat housing.I waited till engine cooled down put some water in the radiator and went home-the temperature was in the middle all the time ?!?
The next morning I went to the mechanics and everything was fine-temperature in the middle, fan working properly,until afternoon when I saw that temperature is going up again,so I stopped and saw steam coming out of the thermostat housing again and the water from the radiator went to the plastic container right next to it.I towed car home, and when I started it the idle was going up and down and steam out of thermostat housing,and the exhaust was puffing white smoke,water went from radiator to overflow tank.I must tell that the mechanics tried to take thermostat out, but the screws were stuck and they were afraid to broke them,because the new thermostat will arrive in 8 days.
Could it be that the only problem is thermostat,radiator or the worst case - my headgasket is blown-how could I tell?

 

bumpity bump

 

if white smoke is coming out the exhuast its most likely the head or the head gasket... i am currently going through replacing hte head gasket and decking/pressuretesting/possible valve job on the head..... there is a chance the block is cracked, however seeing as the head is aluminum and hte block is cast iron, the chance of it being the block is very slim....

 

White smoke is coming out only when the engine is cold or when the water starts to boil.I can see some rust inside the radiator - is it possible that the radiator is clogged-and how to explain that the truck went fine and the temperature in the middle at highway speed-130km/h and when i stopped it started to rise slowly and i could hear hissing sound-when I get back to highway temperature was back to normal until I got to the city and stopped-it started to boil water out of the overflow tank?!

 

When you're driving at highway speeds there is enough air flowing over the radiator to provide sufficient cooling. Is your radiator fan coming on when you stop? If not, that could be the culprit.

 

Yes the fan clutch is working.

 

Hmmm...well I guess just start out with the t-stat since that's easy and cheap...might also try replacing the radiator cap (might not be pressurizing the system)...if that doesn't fix it, might be time for a trip to the radiator shop...

 

Has the timming chain been checked/changed lately? The guides might have become toast and you have worn thru the cover into the water pump. Mine had those symptoms when my chain wore thru.

 

Its not the cap because the system is under pressure,I didn't have time to check the chain because I just got this truck-but it is not loud or anything(the chain ).