I believe the D4 engine relies heavily on ECU’s. And these are particularly expensive if they go wrong. There are many reported issues with the D4, but tbh I think a lot of engines will have their own issues if not looked after properly. And let’s be honest, not all boaters are as careful with their motors as they should be. Just get a good engineer to check it over for you. If you need the number of a good guy down this way, PM me and I’ll supply you with one!