"The correct tool for the job is the telephone"

And a check book.

Things don't run forever without maintenance.

Why do you think 100C is too hot? What is the recommended operating temp? Some engines run a pressurized coolant system to allow higher water temperatures, what does the Cummins handbook recommend for this particular motor? What is your engine coolant, straight water or a water/anti-freeze mix?

100c (212F) might not be bad for the engine, newer engines can be and have been designed to operate at higher pressures and higher temperatures. Need to see what this particular engine manual recommends before looking to fix it.

The first thought I had was depending on what the operating temperature of the motor is, possibly the non return valve is not working correctly & restricting the flow more than it should.

As others have said find out the operating temp of the motor & compare that to the temps you get with & without the non return valve. If the temp is higher than specs then try replacing the valve. If that doesn't bring the temp to specs start looking for other causes

Anonymous--

You don't say if the NRV is on a public water supply feeding water to the DG set, or if it is on a recirculating sealed system similar to the cooling of an automobile engine. Big difference in the importance and purpose of the NRV.

--JMM

Dear Anonymous Poster #1,

I do agree with Post #1 and Post #5. If the NRV is part of the water cooling system of the DG, then please do not try experimenting with it. You may land up damaging the entire engine by overheating. By this I assume that the engine is now running at a higher temperature that before, which is an indication that the cooling system is faulty. Call in the engine servicemen quickly there is further damage to the unit.

If the NRV is external ( public water supply or own external water supply) then perhaps the NRV is defective and need replacement. If it had been just replaced prior to this problem, then you need to check upon the type of unit replaced. Normally for liquid supply, like water, one uses a swing type NRV ( or a disc type) rather than a lift type. Personally I prefer a swing type.

In addition, perhaps it may be of interest to you that many omit checking out the DG water cooling pump during the routine servicing and top overhaul at 5000 hours. I would normally replace the water pump ( whether defective or not) for every major overhaul at 10,000 hours.

Hope the above comments are helpful.

NRV (Non return valve) in water cooling system ? Never heard of this before.

Maybe you are looking at the by-pass thermostatic valve which helps to bring, quickly, the water temperature, up to the normal working temperature of the engine!

Removing this valve requires blocking the by-pass so that the water goes directly to the cooling radiator, straight from the start.

If this is what I am guessing at, removing it will increase the wear on the cylinders over time. The temperature difference as depicted by you, means that the "NRV" (your definition} is faulty and not shutting the by-pass when the engine reaches working temperature: A small leak is still by-passing the radiator, therefore, increasing the overall temperature of the water.

???

Would using the telephone to talk to the manufacturer help?