Start with the complaint, not the parts bin

"Overheat alarm" calls resolve fastest when you establish when it overheats: at idle, at cruise, immediately on start, or only after long runs. Each pattern points somewhere different.

The flow

1. Verify it's real. Infrared-thermometer the heads and thermostat housing with the engine running on muffs or at the dock. A failed temp sender or a corroded alarm wire causes plenty of false alarms — confirm actual temperature before touching the cooling system.
2. Check the telltale. Weak or no stream:

• Blocked telltale fitting (mud daubers love them — clear with weed-whacker line, gently)
• Impeller worn or vanes missing → see the impeller article; replace the full pump kit
• Blocked intake screens (plastic bags, grass, or paint from a careless bottom job)

1. Overheats only at idle, fine at speed: classic tired impeller — not enough pump output at low rpm.
2. Overheats only at cruise: suspect restrictions downstream — partially blocked passages, a failing poppet/pressure relief valve, or salt buildup in the heads on older salt-water engines.
3. Overheats immediately from cold: stuck-closed thermostat or a completely failed pump. Pull the thermostat and test in hot water; check the service manual for the opening temperature.
4. After the fix, verify: temps stable across the operating range on a water test, alarm self-test works, telltale strong.

Salt-water engines

Old salt engines can be scaled internally. If a new pump, thermostat, and clear passages still run hot, quote a descaling flush or explain the block passages may be beyond economical cleaning — be honest before the customer spends more.

Common mistakes

• Replacing the impeller without checking why the old one failed (running dry, sand ingestion)
• Ignoring missing impeller vane fragments upstream — they lodge in passages and cause the comeback
• Removing the thermostat as a "fix" — the engine then runs too cold, carbons up, and on some models actually overheats at speed due to altered flow
• Trusting a single overheat alarm as sensor fault without measuring temperature

When to walk away

An engine that has been run hot repeatedly may already have scuffed pistons — do a compression test before investing the customer's money in cooling parts, and document pre-existing damage before you touch it.

Safety: never run without water supply even briefly; hot heads and steam can burn — use the IR gun, not your hand.