Why Is My Boat Engine Still Overheating After a Water Pump Replacement?

If your outboard is still overheating after a water pump replacement, the issue is usually a blocked cooling passage, a stuck thermostat, or an improperly seated water tube seal. On a late-80s outboard, years of salt or silt build-up in the water jackets or a missing piece of the old impeller are the most common culprits.

Thermostats and Poppet Valves

Older outboards, such as a 1988 Mercury, Johnson or Evinrude, rely on a thermostat for low RPM cooling and a pressure relief valve, often called a poppet valve, for high RPM cooling. If the engine overheats at idle but cools down when you get up on the plane, the thermostat is likely stuck closed. If it stays cool at idle but overheats at wide-open throttle, your poppet valve is probably jammed with debris or has a ruptured diaphragm.

Remove the thermostat and test it in a pan of water on the stove with a thermometer. Most marine thermostats from this era should begin to open at around 60°C (140°F). If it stays shut, throw it away. A replacement usually costs between £20 and £35. Always clean the housing mating surfaces and use a new gasket when refitting.

Hunting Old Impeller Fragments

When an old water pump impeller shreds, the broken rubber vanes do not simply vanish. The pump forces them straight up the water tube where they lodge in the powerhead's narrow cooling passages. Even with a brand-new pump pushing maximum volume, the water cannot circulate past these rubber dams.

To clear them, you need to back-flush the system:

• Remove the thermostat and leave the cover off.
• Drop the lower unit, or gearcase, again so the water tube is exposed.
• Force water from a garden hose down through the thermostat housing.
• If water pressure does not dislodge the fragments, use compressed air at around 30 to 40 psi, blowing backwards through the cooling circuit.

Water Tube and Seal Alignment

When refitting the lower unit after a water pump job, it is easy to miss the water tube receptacle on top of the pump housing. If you miss the alignment by even a few millimetres, the pump will work perfectly but dump all its water straight into the exhaust midsection instead of pushing it up to the engine block.

Drop the gearcase and inspect the rubber grommet on top of the water pump. On older Yamaha and Mercury outboards, these guide tubes degrade, fold over, and pinch off the water flow when compressed. If the grommet looks squashed or torn, replace it. When sliding the gearcase back up, shine a torch up the midsection to confirm the copper water tube slides smoothly into the pump housing seal. Before bolting it tight, ensure the gearcase splines are greased with marine spline grease, and torque the lower unit bolts to the manufacturer spec, which is typically around 35 to 40 Nm for a V6 outboard.