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If I have to drop the RPMs every half hour or so then, as far as I'm concerned, the motor has serious problems..
Agreed... as as mentioned it's not something we've ever had to do.

Our first engine (actual Pathfinder conversion) eventually needed a rebuild after 15 years or so.. was very inexpensive compared to marine parts. The one we have now is a Brazilian conversion with some differences in layout and parts used, but in both cases the only time we've had issues has been with external, non VW parts like the exchanger, RW pump, fuel filters etc.

I'm not saying Yanmar, Volvo et al are bad engines.. all I'm saying is that the Pathfinders and their like are also not bad engines... and don't deserve the negative rep they appear to have.
 

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You know; with all engines there are a certain percentage that have chronic problems. I was not trying to say that the Pathfinder engines are of poor quality; just that the original design spec for the VW engine was not for continuous high loading. If the engine was not at all capable of it, it would be safe to assume that many, many more pathfinders would have failures.

In your case Faster, a 42 HP engine is probably not going to be worked at a very high load when pushing an 11,500 lb hull. That's pretty light displacement for 42HP. It's possible that more problems with head gaskets occur when the boat displacement is closer to 18-20k lb. More water to push out of the way and more wetted surface will increase the engine loading quite a bit.

I was only pointing out a potential reason for the reported reliability problems; but it could also be due application mis-match, over-propping, engine quality control, etc.
 

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I would do A LOT of research about that engine. Older VW engines have a lot of parts made out of magnesium.
VW diesels do not have any magnesium parts. The magnesium engine blocks were on the much older aircooled VW engines.

I think the difference is that the VW diesel was not specifically designed to be put under constant load (like an engine designed to do heavy work). I have heard that there are issues with the cylinder heads warping and blowing head gaskets. This is probably associated with over-loading the engine or running at constant high load for long durations.
VW diesels are widely used and marketed as heavy duty industrial engines. The six cylinder version of this engine is used in military vehicles, and vans all over Europe.

The only issue is that they have aluminum heads- and do not survive overheating as well as an all cast iron diesel. This is simply a matter of properly engineering and sizing the cooling system to allow 100% constant load without overheating.

I think the VW diesels would make great marine engines, but even better would be the old Mercedes diesels with cast iron heads- these engines would have less issues with galvanic corrosion, and can survive mild overheating without damage.

Another issue with VW diesels is that they require thousands of dollars worth of specialized tools to perform major work on them, whereas most marine diesels can be worked on with standard tools.
 
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