Don’t trust the jug, jug being the radiator reservoir jug. I’ve seen this problem a lot. Your car or truck seems to be running hot, but you have plenty of coolant in your radiator reservoir jug, so you think it’s fine. But it’s not and your engine gets ruined.
This is especially serious on your Isuzu npr, Isuzu nqr, Isuzu ftr and frrs, the UD1200, UD1300, UD1400, UD1800, UD2600 trucks because they are much more expensive than cars.
What happens is, in a vacuum reservoir type system, the engine gets a leak or the hose gets a leak . This system counts on a vacuum to draw water out, which is fine until you get a hole in the cooling system somewhere. Then it can’t draw water out — can’t pull a vacuum; therefore, your truck runs hot. This ruins a very expensive motor and causes thousands of dollars in damage within just a few minutes.
The best way to avoid this problem, is to physically lift the cab before you crank in the morning when the engine has been cool for at least 2-3 hours and physically check the radiator level.
DO NOT OPEN THE RADIATOR UNTIL THE ENGINE HAS AT LEAST SEVERAL HOURS COOLING TIME. Open the radiator, on a cold engine, very slowly. If you open the radiator on a warm or hot engine, you will risk getting seriously burned.
Just because the reservoir jug may be full, does NOT mean you have coolant in the radiator. Please check the coolant in the radiator while the truck is cold.