Overheating from Clogged Exhaust Manifolds: A Hidden Cause of Marine Engine Failure

Not all overheating is caused by a bad impeller or faulty thermostat. One of the most destructive — and commonly overlooked — causes of overheating in inboard marine engines is restricted exhaust water flow due to clogged manifolds. This issue is especially common in raw-water cooled systems without proper filtration at the seawater intake.

How Exhaust Manifold Cooling Works

In marine inboard engines, raw water is drawn in from outside the hull and routed through the engine block, heads, and exhaust system. A seawater pump (often belt-driven or crank-mounted) moves the water through passages inside the exhaust manifolds and risers, cooling the extremely hot exhaust gases before discharge. Unlike closed automotive systems, this setup is vulnerable to contamination — because it’s pulling in water directly from the environment, often at high volume and speed.

Why Exhaust Manifolds Get Clogged

When a boat operates in shallow water, especially near muddy or sandy bottoms, the seawater intake may pull in silt, sand, shells, or debris — especially during acceleration or sudden maneuvering. If the raw-water pump isn’t protected by a coarse strainer or filter, these particles are sent directly into the cooling system. The exhaust manifolds are the narrowest and most complex part of the system, with internal cooling passages that easily trap material. Over time — or sometimes within hours — these passages clog, restricting or stopping water flow entirely. The result? Superheated exhaust gases and immediate engine overheat.

Symptoms of Clogged Exhaust Manifolds

• Engine overheats within 1–3 minutes of startup, even with a new impeller
• No water discharge from exhaust ports at idle or throttle
• Steam, unusual smell, or metallic ticking from the exhaust elbow
• One side of the engine significantly hotter than the other (on V-config engines)

Damage Caused by Exhaust Overheat

If the exhaust water stops flowing, exhaust temperatures skyrocket — often exceeding 1,000°F. This can cause:

• Burned exhaust valves
• Warped cylinder heads
• Collapsed risers or melted exhaust hose
• Cracked manifolds or internal coolant leaks (on half-closed systems)

If the engine isn’t shut down within seconds of temperature spike, full mechanical failure is possible — including head gasket failure or piston seizure.

How We Diagnose and Prevent Manifold Clogging

• We remove the risers and inspect water passages for sediment buildup
• Backflush the manifolds with high-pressure water to check flow and resistance
• Use infrared thermography to spot hot zones across the manifold body
• Inspect seawater intake lines for lack of filtration or routing issues

In many cases, the raw-water pump is working perfectly — but the flow has nowhere to go. That’s why impeller replacement doesn’t solve the issue.

How to Avoid This Problem

• Install a proper seawater strainer with a clear inspection bowl
• Clean it regularly, especially after operating in shallow or dirty water
• Avoid revving in reverse near the bottom — this stirs up debris into the intake
• Flush the cooling system with fresh water after operation, if possible

Conclusion

If your inboard engine overheats even after replacing the impeller and thermostat, don’t overlook the exhaust manifolds. Clogged cooling passages from sediment are a silent killer — and they don’t fix themselves. Early diagnosis and filtration are the only way to prevent catastrophic engine failure caused by blocked exhaust cooling.