Methods and systems are providing for improving engine coolant level estimation to reduce engine overheating. The level of fluid in a coolant overflow reservoir is inferred based on the fluid level in a hollow vertical standpipe fluidically coupled to the reservoir at top and bottom locations. An ultrasonic sensor positioned in a recess at the bottom of the vertical standpipe transmits signals intermittently, receives their echoes upon reflection from the coolant surface, and estimates the fluid level in the standpipe based on echo times.