Northern Lights Generator shuts down after 5 min.

[La Vie Dansante]

I have a Northern Lights M773L W3.3 3-cylinder, 4-stroke diesel generator with 2100 hours. It is fresh and raw water cooled.
It had been running just fine from the time we purchased the boat a few weeks ago until we arrived at her new home in Key West, including some time while we were under sail. I have a scoop raw water intake. Now the generator starts up just fine, but only runs for 5 minutes then shuts down. The intake, seacock and strainer are clear, but the pump gets hot and I'm not seeing a stream from the raw water outlet. What are some items I need to troubleshoot? Impeller? Something in the raw water system?
Any advice appreciated.

[Eder]

Happened to my NL 6kw gen....impeller went from pristine to only 2 tines left in 1 day. Easy fix though.

[Lepke]

If the impeller has gone bad, make sure missing pieces aren't plugging the heat exchanger. When you put in a new impeller, grease the inside of the housing and end plate with waterproof grease. Then when you start for the first time, the grease lubes the impeller before water arrives and because it makes a tighter seal between the impeller and the housing, it draws water quicker.