There is a potential 'flaw' in the *steam riser pipe* exhaust systems used in the P30. Internal to the vertical pipe section is a small 'diverter cap' that prevents water injection from flowing INTO the dry exhaust vertical riser-pipe which if not there (eroded away) will lead to water backflow into the engine. On old installations especially the copper assembly should be cut apart, the 'witches hat' be visually inspected or replaced ... an easy job for a 'red-metals' shop/welder. Can easily be 'assayed' with a flexible optical 'bore-scope' ... find a shop that has one to do the assay.
If the system was adequately protected by the use of 'pencil zincs' (at the 'drain' on the bottom of the riser ... replaced yearly, etc.) the copper usually will be found to be intact.
How to check the integrity of riser-pipe system: Take the whole assembly, both the vertical section AND the radiused horizontal jacketed section that leads to the vertical riser pipe to a 'boiler shop' and have them "pressure test" (quite high pressure water) and simply check for leaks emanating into the 'dry side'. Copper is easy to 'work', the components are 'common' and a boiler or heat exchanger shop will be able to either rebuild or repair a 'faulty' heat riser system and at 'economical' prices.
Also a good time to pressure check the (A4) exhaust manifold for pin holes between the dry side and the wet side ... pin holes can be quite common with old engines ... can easily be repaired by 'brazing' or simply get a 'new' manifold from sources like Moyer Marine Atomic 4 Engine Rebuilding and Parts
Pin holes and 'slab rust' (which can intermittently BLOCK the cooling water flow in the exh. manifold and the riser system) will be quite common on engines that were 'dried-out' (no antifreeze) for long term or winter 'storage'... this is the reason that large ships NEVER shut down their engines/exhaust systems as such keeps the formation of 'protective' ferrous (black) rust intact on the cast iron.
hope this helps.