I actually have a G5200 with factory sender. There are 3 lines on it.
- Below lower line is "COLD"
- Above upper line is "HOT"
- Everything in between is "NORMAL"
Perhaps this helps you understand what is 'normal' range.
Beware, the D600 engine is a little large for the radiator. Everything needs to be working at top efficiency to keep the engine cool.
- Antifreeze (Proper mixture to be able to move heat)
- engine cooling passageways (clean-- no rust)
- waterpump (antifreeze flow)
- radiator (clean-- no scale)
- fan (airflow through radiator)
Any of the above can weaken the entire chain.
My G5200 used to climb above the upper line after mowing over an hour. (90F ambient air)
I did the following:
- cleaned the INSIDE of the cooling system
- changed leaking waterpump
- changed hoses
- removed radiator and used garden-hose to rinse inside in BOTH directions
- Fill/flush cooling system several times - distilled water until clear
- blow out with leaf-blower between flushes
Now, my temperature gauge goes above the upper line ONLY if the airflow thru the radiator is impeded. After shutting down and blowing the chaff from the airflow... the gauge is back to normal whilst running.