Volvo stopped recommedning the exhaust flapper is in most applications a few years ago. You may want to try running without them. Just be careful not to come off-plane rapidly or water can back up into the exhaust manifolds and then flow down into the cylinders that have open exhaust valves, when you shut it off. If you have no choice but to come off-plane rapidly, DO NOT forget to put the engine in neutral and rev it a few times to blow the water out, before you shut it off.
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