1991 era 6.5 Kohler genset. Cranks easily, runs very smoothly for about 40 minutes under load, then dies. Good waterflow. Will crank immediately after it dies and runs smoothly for about 10-12 minutes, then dies. Does this regardless of load. I would think that a temp issue would prevent it from re-cranking right away as it would still be hot. oil level fine. ideas? thanks in advance.
Check the water temp with an infra red. Does it have an oil pressure gauge and water temp gauge? If water temp and oil pressure are ok then my guess is you've got a bad shutdown sender (ie oil pressure, exhaust temp, coolant temp, etc). you're going to have to ohm each sender and see which one is bad.
I had a similar problem and it was the fuel pump. Mechanic asked me how old the fuel pump was and I said original 10yrs. He said change it and that will fix your issue. It did. Mine would run for different times as much as 3 hrs and then shutdown. Then I would restart, it would run for awhile and shutdown again.