Why One Riser Runs Hotter Than the Other And What It Means

When both risers are working correctly, temperatures should be close — within 10 to 20 degrees Fahrenheit of each other. But if one side runs significantly hotter, something’s wrong. This isn’t a “wait and see” problem. Heat imbalance usually means restricted raw water flow, corrosion, or a failing riser gasket — and if left unchecked, it can lead to overheating or water intrusion into the engine.

Normal vs. Abnormal Temperature Difference

A small difference between risers is normal. The raw water flow path isn’t always perfectly symmetrical. But if you see a 30-degree or greater difference, that’s your red flag. One side is not getting the same cooling — and that puts stress on the exhaust and engine block.

Common Causes of Uneven Riser Temperatures

• Internal scaling or blockage: Salt, rust, or impeller debris can restrict flow through one riser while the other stays clear.
• Blown or leaking gasket: A riser gasket that’s failing can create heat buildup at the connection point or inside the exhaust passage.
• Collapsed or kinked raw water hose: A soft or poorly routed hose can limit flow to one riser.
• Air pocket in the cooling circuit: Air trapped in the raw water line can prevent full water flow to one side.
• Misrouted plumbing after repairs: If hoses were reinstalled incorrectly, water may be favoring one side.

How to Diagnose It

1. Run the engine up to operating temperature under load.
2. Use an infrared thermometer to scan the top, sides, and base of each riser.
3. Compare readings: 20 to 30 degrees hotter on one side means something is restricting water flow.
4. Check raw water hoses for softness, kinks, or improper routing.
5. Inspect the exhaust for steam, especially on the hotter side.

Also remove spark plugs after shutdown. If one plug is wet, corroded, or oddly clean, water may be making its way into the cylinder through a leaking gasket or cracked riser.

What to Fix

• Backflush the suspect riser: This can clear debris if the restriction is minor.
• Inspect or replace the gasket: Any sign of rust, seepage, or white deposits at the riser joint means the gasket is compromised.
• Replace the riser: If backflushing doesn’t help and the unit is more than 3 to 5 years old, internal scaling may be too advanced to clean.
• Check hose routing: Make sure all raw water lines are secure, unobstructed, and matched in length and diameter when possible.

What Happens If You Ignore It

• Riser overheats, warps, or cracks
• Gasket fails, allowing raw water into the exhaust or engine
• Exhaust temperatures rise, stressing cooling system and block
• Eventually, hydro-lock or full engine failure

Conclusion

If one riser is hotter than the other, don’t ignore it. It’s a clear sign that water flow is restricted or misdirected. The solution might be as simple as replacing a gasket — or it might require new risers. Either way, temperature imbalance is your early warning system. Catch it now, and avoid a much bigger (and more expensive) problem later.