Diesel motors

Hi Bruce, diesel motors are relatively simple motors to diagnose, some additional specifics would be good, so if you could describe in detail the circumstances when it loses power it would help. Also what year and model is the motor?

It's likely a fuel related problem, and it's not able to get enough fuel after running for a while. So that's a good place to start:

As a first step and the most obvious is to check the fuel filters on the motor, they may be clogged and need replacing. Usually, there's a primary main fuel filter, this is often a water-separating type. It has a paper cartridge filter inside a metal casing and a glass bowl on the bottom. And then there's a smaller secondary (finer grade) on the fuel pump itself. So the fuel path in order will be: tank, stop valve, hose, primary filter, hose, secondary filter, fuel pump, injector pump, metal tubes, fuel injectors. But all of that will fail after a while if your breather hose is blocked, if this isn't allowing air to displace the fuel that you're pulling out of the tanks it'll cause a vacuum.

Turn off your fuel valve at the tank, get a roll of paper toweling and a couple of those aluminium bbq containers to put under the filter to catch spillage and contain the parts as you disassemble (you can mold and bend them to get into awkward locations). Check the little glass bowl (water seperator) on the bottom for water and/or sediment before you disassemble and note what you find. It should be absolutely clear and clean and no water layer in there. Take lots of photos while you're doing it and post here.

So in sequence from the fuel tank to the engine cylinders, you have a number of places that the fuel delivery can be compromised;
1. tank - contaminated fuel (water and or diesel bug), also check where your breather hose is for the tank and ensure that's free to breath and not blocked somewhere, if it is, it'll cause your symptoms
2. stop valve blocked/faulty
3. lines to the fuel filter - old lines can break down internally causing contamination and blockage, diesel but residue can also block the lines, also lines can get pinched, so check that they're not compromised in that way
4. clogged primary filter (if so, investigate why, is it super old and hasn't been serviced for ages or is it being contaminated in some way?)
5. lines to the secondary filter - clogged or breaking down internally
6. secondary filter blocked? (why as above?)
7. fuel pump - may be failing
8. injector pump (specialist job)
9. fuel injectors (specialist job)