Engine temp in a Nitro engine is controlled by 2 main factors.
1st: is Fuel Mixture
2nd: is Water\Cooling System.

The richer your fuel mixture the Cooler the engine will run.
The Leaner your fuel mixture the Hotter the engine will run.

If you have leaned the engine to the point it's affecting the Glow plugs, and the engine Still runs Cool, Then it's time to restrict the water flow.