I have purchased 2 boats with diesels and have not had a dedicated mechanical inspection on either. In both cases, the surveyor gave the engine a look and spent approx 2 hour on our latest boat. While I would OT discourage an inspection, I do know some things to look at based on personal experience and what I know of the mechanical inspections. For diesels, a list would include but would not be limited to:
1. Cold inspection. Any wet liquid on or under oil pan or transmission? If so, what is it and why is it there.
2. Green corrosion on heat exchanger? If so, may be indicative of a leak under pressure.
3. State of zincs. If they are not even there or corroded away, could be indicative of general maintenance.
4. Belt state. Any obvious rubs or cracks?
5. Oil state. Should be black and not milky. If so, run away.
6. Any grinding or hesitation when starting? Starter
could be on the verge of failing.
7. Trans fluid should be pink and the reservoir should be full and not overfilled. If overfilled, could be indicative of a leak that is being neglected.
8. Water streaks around fresh or raw water pumps
could be indicative of a leak.
9. New paint
10. Supply of spares. Could be indicative of general service. No spares could mean that the PO wasn't all that mechanically interested.
11. Run at cruising speed (~2000 RPM). After warmup, should not have a plume of white smoke (if ever).
12. Run at MAX RPM (~3000 RPM). Boat should hit hull speed at about 75% or 80% of MAX. Does excessive black smoke develop? Not necessarily bad (we get black smoke at MAX RPM but I rarely run that hard).
13. Run in reverse. Should not be a lot of grinding noises.
14. Smooth transition into and out of gear while engine is running?
15. Smooth engine sounds? With less than 4 cylinders, may be hard to tell.
16. While running and the engine is fully warmed up, verify that the oil pressure and water temp are per the manufacturer's specs.
17. While running and the engine is fully warmed up, verify that the heat exchanger does not leak. Verify that the exchanger end caps are not leaking. If so, micro cracks could have formed from over tightening (easy to fix) or due to excessive backpressure.
18. Verify that a generous amount of water spits out the back of the boat (verifies that pump
works well and that there are no severe blockages.
19. While running and warmed up, verify that there are no pin holes or leaks at the exhaust
to raw water discharge Y fitting.
20. Verify that ALL hoses are correct type for the application and in good condition with no rot, cracks, rubs. For this age engine, none should still have the original manufacturer's paint
on them (should have been replaced long ago).
21. Double hose clamps on raw water intake lines
and exhaust lines
22. If possible, remove the heat exchanger upstream endcap and verify that there is no debris or impeller vanes blocking water flow. DO NOT do this without owner permission or without a new gasket immediately available.
This is what I can think of for now. I'm sure that there is a mechanic out there who will provide an even more detailed list. I checked Nigel Calder's Diesel book and there is nothing about inspections or survey. Too bad.