Isuzu 4BG1 Instruction Manual page 82

2. COOLANT
Where the atmospheric temperature falls below freezing point, the
cooling system should be drained after engine operation, but to
eliminate the need for repeated draining and refilling, the use of
anti-freeze solution is highly recommended.
A 50/50 Ethylene glycol base antifreeze/water mix.
(which provides protection to -37OC (-34°F) is recommended for use
in these ISUZU diesel engines).
Concentrations over 65% adversely affect freeze protection, heat
transfer rates, and silicate stability which may cause water pump
leakage.
Never exceed a 60/40 antifreeze/water mix.
(which provides protection to about -5OOC (-58°F).
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