One Side of the Engine Runs Hot: Diagnosing Asymmetrical Overheating in Marine Engines

If one side of your inboard engine is noticeably hotter than the other — especially on V6 or V8 configurations like Mercruiser, Crusader, or Volvo Penta — that’s not “normal.” Asymmetrical overheating usually points to a cooling system imbalance, blockage, or internal restriction. If left unresolved, it can lead to warping, head gasket failure, or even full engine shutdown.

Why One Bank Might Overheat

In most inboard marine engines, raw water is distributed to both sides of the engine through a system of T-fittings, manifolds, and risers. These components are designed to split flow evenly — but it doesn’t take much to disrupt that balance.

Common Causes of One-Sided Engine Overheating

Clogged exhaust manifold or riser on one side
If one manifold is partially blocked with rust, scale, or sediment, water can’t flow through it properly. That bank overheats while the other stays within spec.
Collapsed or kinked water hose
A single crimped line can starve one side of cooling water, especially after improper reassembly or hose routing during service.
Unbalanced water flow from a failing seawater pump
If the impeller is damaged unevenly or partially blocked, pressure may favor one side. This is common when pieces of an old impeller have lodged in the water distribution system.
Failed or blocked thermostat (closed cooling systems)
On systems using coolant and heat exchangers, an air pocket or blocked bypass can keep hot coolant trapped on one bank.
Salt corrosion inside cooling jackets
Over time, especially without freshwater flushing, internal corrosion can clog water jackets unevenly — causing hot spots in one cylinder bank.

How We Diagnose the Problem

We never assume anything based on feel or “guess where the blockage is.” Instead, we test:

Measure surface temperature on each riser and manifold with infrared tools
Flow-test each water discharge independently
Inspect hoses and fittings for kinks, internal collapse, or reversed routing
Remove risers and inspect for corrosion or sediment buildup
Flush both sides of the cooling system independently under pressure

In half-closed cooling systems, we also test coolant flow rate and check for trapped air or vacuum-lock situations.

What Happens If It’s Ignored?

When one side runs hot, thermal stress builds unevenly. You might get away with it once or twice, but continued operation risks:

Warped cylinder head or manifold
Cracked riser or exhaust elbow
Premature head gasket failure on the hot side
Increased cylinder wear due to localized detonation

In worst-case scenarios, one side of the engine can suffer significant mechanical damage while the other appears “fine” — misleading less experienced techs.

Conclusion

If one side of your marine engine is running hotter than the other, don’t dismiss it. It’s not a minor imbalance — it’s a mechanical warning. Asymmetrical overheating is nearly always tied to restricted cooling flow, whether from corrosion, blockage, or routing issues. With proper testing and a system-level approach, we isolate the problem, correct the flow, and protect your engine from long-term damage.