[jdmccright]

Your 125-deg temp is not at all a normal engine coolant reading (from the instrument panel)...it is very low. Normal coolant temp range is 185- to 210-deg. I am wondering where you are getting your temp readings from. Are you reading this from your instrument panel or are you measuring from an air duct? That said, if your 'Check Gauges' light is coming on and you're coolant temp needle is in the red zone, then yes you have an engine overheating problem. Either add coolant or check the thermostat and fan clutch as mentioned before.

Alternatively, if you mean that you're cabin air is too hot and you cannot adjust it then you will need to have the temperature select switch and mixing door servo motor checked.

Finally, hearing sizzling sounds from the right side of the engine is a definite indication of a water-based leak. Does it smell like engine coolant inside the engine bay? Any other pungent smells? Do you notice orange-colored drips underneath the truck's engine bay or anywhere else? If the aluminum skid plates are under the truck, they can "catch" the drips and make them fall elsewhere or mask a small leak by acting as a drip catcher. Unbolt them and see if you notice any signs of coolant collecting within the ridges. Then look under the engine for any orange-colored drips and follow them back up to the source. My guess would be a leaking heater core hose that is dripping onto the hot exhaust header, which is routed through the right side of the firewall. It may be a hose clamp that has loosened or just an old hose that sprung a leak after all those kms. You may be able to borrow a coolant system pressure test kit to pressurize the system while the engine is off to pinpoint the leak.