Coolant that disappears with no puddle on the ground is almost always a small external leak you missed, a bad radiator cap, or a head gasket starting to fail. Catch it before overheating destroys the engine.
A weeping hose clamp or water pump shaft seal drops coolant slowly. Pressure test the system - leaks show up at 15 psi that hide at idle.
A weak cap spring loses pressure and lets coolant boil off as vapor. Cheapest fix on this list - a $15 part.
Coolant gets into the cylinders or oil. Look for white sweet-smelling exhaust, milky oil on dipstick, or bubbles in the reservoir at idle.
A hairline crack in the plastic tank or a loose clamp weeps under pressure. Often missed because it dries fast.
A leaking heater core drips inside the cabin (wet passenger floor) or evaporates through the defroster. Sweet smell inside is the giveaway.
Temperature gauge climbs above normal, you see steam, the coolant reservoir bubbles at idle, or you smell coolant inside the cabin. Severe head gasket failure can hydrolock the engine.
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Modern cars should lose essentially zero between flushes. Anything more than 1 inch drop in the reservoir per year is a leak to find.
Yes to get home, but flush and refill with proper 50/50 within a day. Plain water lowers boiling point and accelerates corrosion.
White exhaust, milky oil, bubbles in coolant at idle, or unexplained coolant loss. A $30 combustion gas test kit confirms in 5 minutes.
Sometimes for a small seep, never for a real failure. Use only as a tow-it-home patch, not a real repair.
Heater core leaks evaporate through the defroster vents. Look for wet passenger floor or sweet smell inside the car.