Bukh DV24 doing strange things

[Nolfese]

It all started when we bought our boat last year, a 40 year old somewhat neglected sloop. The engine, a Bukh DV24, sounded like the hammers of that hot place. Anyway, much improvement has been made and it has brought us down from Wales, across to the Solent, across the channel, down to St. Malo and through the canal to Arzal. The engine over heated in the canal a few times due to vegetation in the strainer.

Anyway, all was well (noisy as ever but well) until two days ago. An hour after leaving Pornichet I notice the revs dipping - something that never happens.

Ripping the cover off the engine seemed incredibly hot. Talked to Bukh uk, they explained how to remove the thermostat and force all of the water from the pump through the block. Did that, brilliant. Set off again today and I could leave my hand on the block, it was so cool.

However, after an hour, same problem as before. Time to explain one point. The previous owner disconnected the oil pressure switch. Now then, when first started the oil pressure warning light goes out. But it has always been the case that once thoroughly warm the light (and buzzer) would come on again at tick-over. Hence the disconnection.

This situation persists but now the light comes on within 5 minutes and stays on. What I would give for an oil pressure gauge - the pressure should be 0.8 bar minimum.

The symptoms are that after an hour at 1800 revs the engine revs dip and there is an overall loss of power. The cooling system is now such that the engine is warm (not red hot) to the touch. She won't rev to 2100 as usual but taking her out of gear she will. We limped back to port at 1300 revs today and there were many moments when we thought she was going to stop altogether.

The bizarre thing is that now that the engine has cooled she'll start instantly and operate perfectly (noisily but perfectly) again for another hour under load.

Am I not seeing the wood for the trees? What is going on here? Are the big-ends stuffed? Would they not just knock like Billy? It feels as though due to lack of oil pressure something is tightening up.

Any ideas anyone?