What to Check If Your Boat Engine Idles Fine But Overheats at High RPM

An engine that overheats at high speed but not at idle presents a different set of problems than one that runs hot all the time. The cooling system appears to work at low RPM — raw water is flowing, the thermostat opens, and temps are stable. But once you apply throttle, the temperature rises quickly. This usually points to restricted flow, collapsing hoses, or components that can’t keep up with demand at high volume.

Common Causes of Overheating at High RPM

• Partially clogged heat exchanger: At idle, the cooling system can handle low flow. But at speed, debris or internal scale prevents enough heat transfer to keep up.
• Worn or damaged impeller: A few missing vanes may still circulate water at idle but can’t keep up at higher pump speeds.
• Collapsing raw water intake hose: A soft or delaminated hose may allow flow at idle, but collapse under suction at high RPM, starving the pump.
• Intake restriction: Sea strainer partially clogged with weeds, plastic, or sand can limit volume — especially under high demand.
• Circulation pump failure (on closed systems): The freshwater pump may be spinning, but a damaged impeller or worn bearing reduces flow under load.
• Thermostat stuck partially closed: The engine may run cool at idle, but not enough coolant moves at high power.

What to Inspect First

1. Raw water pump: Pull the impeller and check for missing blades, rubber swelling, or hardened vanes.
2. Strainer and intake: Clean the basket thoroughly and backflush the thru-hull to check for obstructions.
3. All intake hoses: Squeeze by hand and check for soft spots. If the hose flattens easily, replace it.
4. Heat exchanger: Remove the end caps and inspect for scale, corrosion, or stuck debris.
5. Thermostat: Remove and test in hot water. Replace if sticky, slow to open, or corroded.

Other Checks Worth Doing

• Use an infrared thermometer to compare riser temps at idle and after throttle-up
• Watch for steam or water flow changes at the exhaust — weak flow under load is a clear warning
• Check the belt driving the circulation pump — slippage under load can reduce flow

Why It Gets Missed

Engines that idle cool but overheat at speed often trick owners into thinking the cooling system is fine. But the real problem is volume. These engines run hot only when flow rate or heat transfer can’t scale with demand. That means the issue won’t show up in a bucket test or idle diagnostics — you have to simulate real conditions to expose it.

Conclusion

If your boat overheats only after throttling up, don’t assume your impeller is fine just because it works at idle. High RPM overheating is usually about restriction — not total failure. Look for components that are good enough at idle but inadequate under load: clogged heat exchangers, soft hoses, weak pumps. Fix the volume issue, and your engine will stay cool no matter how hard it’s working.