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"98" 252 Sunesta Hydroloc


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Had a hose from the raw water pump to the thermostat housing burst, made quite a noise. We stopped the boat quickly and shut it down. made a quick temporary repair with gorilla tape and tried to start the motor and it was locked up. After being towed back to our dock we discovered that we had water in the cylinders after removal of the plugs. The engine cranked over freely withe the plugs out, trying to clear the cylinders of water. We then sprayed WD-40 into each cylinders and cranked some more. Replaced the plugs and started the engine after some cranking. We found a blockage in the thermostat housing, so having just changed the manifolds and risers a to-do will be to check the housing as well. All is back together and completed a hour and a half of continuous running all seams to be well. 

My question is; who has had the same problem with abruptly stopping and experiencing a hydroloc?  What have you done to prevent it from happening. I have had many merc-cruisers and not experienced it with them, this is my first Volvo Penta stern drive.

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I`ve never heard of such a sequence of events. I`ve experienced T-stat blockage before, with the by-pass channels, but that only resulted in the engine running cooler than normal. Don't see the Hydrolock being related unless you built up cooling water pressure and blew a head gasket.

 

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43 minutes ago, 1948boat said:

Had a hose from the raw water pump to the thermostat housing burst, made quite a noise. We stopped the boat quickly and shut it down. made a quick temporary repair with gorilla tape and tried to start the motor and it was locked up. After being towed back to our dock we discovered that we had water in the cylinders after removal of the plugs. The engine cranked over freely withe the plugs out, trying to clear the cylinders of water. We then sprayed WD-40 into each cylinders and cranked some more. Replaced the plugs and started the engine after some cranking. We found a blockage in the thermostat housing, so having just changed the manifolds and risers a to-do will be to check the housing as well. All is back together and completed a hour and a half of continuous running all seams to be well. 

My question is; who has had the same problem with abruptly stopping and experiencing a hydroloc?  What have you done to prevent it from happening. I have had many merc-cruisers and not experienced it with them, this is my first Volvo Penta stern drive.

Sure sounds like a restriction prevented the raw water from exiting the engine. The raw water pump is positive displacement and can build quite a bit of pressure, but it does not under normal conditions as the discharge side restrictions are minimal. Blowing off a hose could be an indication that you sucked up some sand and packed off an internal passage causing the water to back up through the risers and into the cylinders. Start pulling hoses and look for debris. W

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Thanks for the reply, I thought I was clear about the hose bursting and that we fount the cause and repaired it. My questions is the hydro lock, from stopping abruptly and water coming back in through the exhaust. There is/are flappers in the exhaust yet Volvo Penta has a service bulletin suggesting the removal of them, BTW our boat (1998) didn't have any.

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17 hours ago, 1948boat said:

Thanks for the reply, I thought I was clear about the hose bursting and that we fount the cause and repaired it. My questions is the hydro lock, from stopping abruptly and water coming back in through the exhaust. There is/are flappers in the exhaust yet Volvo Penta has a service bulletin suggesting the removal of them, BTW our boat (1998) didn't have any.

All engines experience reversion during periods of heavy deceleration, whereas the exhaust pulse reverses direction causing a momentary vacuum in the manifolds. It's worse in a marine engine as the cooling water mixes with exhaust gas just down stream of the riser and this quench exacerbates the momentary vacuum. Merc uses flappers, and they help some, but need to be maintained as that are simple metal plates jacketed in rubber, and they live in a pretty harsh environment. Volvo deleted them on newer models, and had some issues until they redesigned their risers. Still happens on a very hot engine that gets snatched from high speed back to idle. Best case is to learn to back her down more slowly, and allow her to idle for several seconds before shutdown. Easier on the engine too as it avoids a severe post shutdown heat soak, but it's sometime hard to pull off if an owner has just went aground as the tendency is to get her back to neutral in a big hurry. In your case I'd check to see what risers you actually have, and if they are first generation, you may want to look to alternatives when its time for them to be changed out. Merc is using dry joint cast stainless units on their newer engines which work fairly well, but I am not familiar with the current Volvo offerings. W

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