Northern Lights 12kW Generator shutting down

Let it cool down. Fill with water. Pressure test if you can. Check impeller.

I have witnessed strange cooling problems with the Later NL heat exchangers. Usually after somebody worked on them.
If not positioned correctly in the exhaust manifold, coolant and sea water combine and will eventually overheat.

Uh Oh !!

Is your model 12 with the heat exchanger in the exhaust manifold or over the front of the engine?

Ah, another Shibaura block. Between the models I was thinking of.
That is usually a good block.
If you replace it; May we deal over this old one? I'll wash your car..

There is not much to those head gaskets.
You can re-torque the head and pressure test the system to around 10PSI. If the gasket is not scored, put some water in there and run it a bit.
Still runs good, drain, re-charge the coolant and go fishing.

Water can also get pushed by the piston rings into the sump.
Until you get it running again, you need to keep turning that engine over daily before rust forms on the liners and valve seats .

I hate hearing this given I have two NL gens with the Luggers. Hopefully it was a fluke as you hear more stories of them running amazingly long hours vs the early failure you experienced.

Can’t you set up some slide boards to get the engine out and a new one in? They will slide pretty easily if you use a piece of UMHW or Starboard type material between the mounts and wood. Usually a basic come along moves them pretty easily. And getting creative with small bottle jacks or even better, a porta-power with jack heads can usually get the job done.

Run it a lil more under a load and send an oil sample out to a lab.

If you don't have a coolant recovery bottle, use a 2l coke bottle full of water and put your coolant vent hose in it. Under load, watch for bubbles.

If it’s just a head gasket then repair is a no-brainer IMHO. Even with a higher rated pressure cap, you have to ask where is the pressure coming from? Certainly could be head gasket. If you repair, I would take the head to a good shop and have it thoroughly checked out for any cracks or issues.

With.
You need pressures to build up to normal.
Your looking for exhaust gasses leaking past the fire ring in the head gasket.

When was the last time you checked coolant level cold in that gen-set before this issue?
History of ever low?

Also; You may consider testing your over-temp shut down senders and relays.

Frak
Look for a OEM sticker on the engine.
Under the alternator in the casting, over the oil pan seam, is a number if you can not find labels.
With some data, we can shop for a new head if needed.

It was cheaper for me to buy a new , complete head, with whole engine gasket set for my Kubota than to send my original head to the shop for cleaning and inspection, before parts.
Just for two dirty and stuck valves.

I'm together and running stronger than ever.