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Posted: Sun Apr 14, 2013 4:52pm

Post Subject: overheating engine

1.5vmc overheating after half an hour run. what could be the causes? and how do i prevent it? it is not losing water, oils fine and i cant find any air locks. is it likely to be the head gasket? it did break peller blades in the cold weather and we didn't retrieve them all. is it likely they are causing a blockage somewhere? it is running a auxiliary water pump and 2 side tanks for cooling. thanks


Posted: Sun Apr 14, 2013 6:26pm

Post Subject: overheating engine

Do you have a dry exhaust or does water spurt out of it? I think it is probably a dry exhaust because you mention skin tanks. This is not the usual way of cooling a skin tank cooled boat and seems to be a bit of a hybrid. I think that your auxiliary (jabsco type) pump pumps water from the skin tanks, through a bundle of small bore pipes sat between the two rubber caps in the exhaust manifold and back to the tank. This system MUST have its own filler and is totally separate from the normal engine cooling water. When you changed the impeller you would have lost water from this circuit - you did top it up again? The engine water pump (the one driven by the alternator belt)pumps water from the manifold that sits around the bundle of pipes, into the engine and back into the manifold. The "tank water" flowing through the small tubes then cools the engine water ready for it to be used again. My first thought is that an impeller blade may be lodged across an elbow or there is a build up of rust flakes and old impeller blades across the entrance to those small tubes. Both of these may cause overheating. Take the rubber end caps off the manifold to check for tube blockages and check every elbow in the smaller pipe/hosework. Fill this circuit, including the skin tanks, and make sure all the air is vented from the tanks. I have no photos of your tanks so can not advise where to fill or where the bleed points are. There may be a plugs or screws on the top of the tanks or at the top on the face. This system normally expels any airlocks into the skin tanks so run the engine while topping this system up as required with the tank bleeds open. This system is NOT topped up via the filler on the manifold. The boat builder should have provided another filler. If it has overheated the thermostat might now be damaged so for less than £10 it should be worth changing it. Refill the engine. make sure the alternator belt is tight enough because the alternator can make it slip so the pump is not driven. Run the engine with the filler cap off and watch the coolant inside the manifold. If air bubbles out and it drops top it up. If you find a steady stream of air bubbles and the engine still overheats I fear you have a failed head gasket - especially if you topped the overheated and still hot engine up with cold water. If the overheating is cured then drain both systems down and refill with between 30% and 50% antifreeze mixture. Thsi is probably a very old engine so the degree of wear in the jabsco pump must also be questioned. Tony Brooks

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