Question: Recently I was running along at cruising speed and suddenly the overheat alarm on my boat sounded. Fortunately I was not too far from my slip at the marina and we were able to nurse the boat back into its slip OK. After some inspection we discovered that the impeller in my water pump was shot (see photo). I’ve replaced the impeller and have run the engine and things seem to be back to normal. Am I missing anything?


This rubber pump impeller clearly has some of its blades missing; tracking down the missing rubber is a top priority after overheating.

Answer: You may be quite lucky here, or it is entirely possible that more trouble is lurking around the next bend on the waterway.

Several things related to your mishap come to mind. First, in the photo you show a rubber pump impeller that clearly has some of its blades missing. This can be a problem because unless you know exactly where those blade parts are, you could be at risk. They can and will circulate through your engine’s cooling system and could end up blocking a critical cooling passage inside the engine. To avoid further problems, you need to find these pieces and remove them from your cooling system.

You don’t say whether your engine is an inboard our outboard, but here is the way things typically play out. On inboard engines the unfound impeller parts usually end up in the end of your engine’s heat exchanger. Most heat exchangers will have a removable cap. Remove the cap and look for the impeller parts.

With outboard engines, there are three common points where the blade parts end up.

  • A side cover on the actual engine block itself.

  • The engine’s water jacket, behind either the water-cooled charging system voltage regulator or on-board CPU if the engine is fuel-injected.

The back side of these components is sometimes facing directly into the engine’s cooling water jacket to maintain a constant temperature. Simply remove the fasteners that hold the device in place and pull it away from the engine. If you do this carefully you shouldn’t damage the sealing gasket. If the gasket tears, get a new one. Look for the impeller parts inside this section of the cooling system.

  • At the top of your engine there will be a housing that holds the engine’s thermostat.

On outboards this can also be a common point for the impeller parts to get stuck. In freshwater cooled inboards, the impeller is isolated from that part of the cooling system.

With either the outboard or inboard installation, after a severe overheat, the thermostat should be replaced as they are quite frequently damaged during an overheat event.

Once you get all the missing pieces out of the cooling system and replace the thermostat, you should be OK.