I have issue with engine coolant rapid level drop in my Toyota Avensis 1.8 vvt-i petrol engine. I did some inspection and noticed these things:
- When engine coolant temperature is still low, some bubbles rising from bottom of coolant expansion tank.
- When thermostat opens and fan starts, about 1/4 of volume of coolant disappears from the tank.
- When thermostat shuts off, coolant level stays the same. So after ~4 cycles of thermostat opening, all coolant in tank disappears.
- All hoses seems to be filled and not flat
- No leakages visible from any of components - hoses, water pump, thermostat, radiator, radiator cap valve, coolant expansion tank.
- Engine never overheats, temperature is within limits.
I am a bit confused, because what I believed is that first the level of coolant in expansion tank should rise when thermostat opens and pressure in radiator increases, hence opening radiator cap relieve valve and pushing coolant back into expansion tank. When thermostat opens (I assume it opens when fan starts), after ~5-10 seconds coolant level starts dropping in expansion tank, which means that pressure in radiator/cooling system is below the pressure in coolant tank (below atmospheric). That normally should happen when pressure in radiator drops due to liquid being cooled, but in normal circumstances coolant should be pushed into expansion tank first due to high pressure through radiator valve and only then same amount of coolant should be sucked back. My only guess is that somehow when thermostat opens, cooling circle always has lower pressure than atmospheric, hence sucking coolant from tank. Is it possible that there is just not enough coolant in the system or maybe there is some air there? I already filled about 4-5 liters of coolant and it seems it's never enough. I can't find any leaks and I would doubt it has something to do with engine gasket as coolant level drop won't be so rapid and there is not residue in oil system and no white fume from exhaust. Any thoughts on this?