TOYOTA 2010 Sienna - Manual (Page 467)

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4-3. Do-it-yourself maintenance
Engine coolant
The coolant level is satisfactory if it is between the “FULL” and “LOW”
lines on the reservoir when the engine is cold.
Reservoir cap
“FULL”
“LOW”
If the level is on or below the
“LOW” line, add coolant up to the
“FULL” line.
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If the coolant level drops within a short time after replenishing
Visually check the radiator, hoses, engine coolant reservoir cap, radiator
cap, drain cock and water pump.
If you cannot find a leak, have your Toyota dealer test the cap and check for
leaks in the cooling system.