In cold weather, a diesel engine at idle speed is generally not sufficient to maintain a normal operating temperature. Diesel engines require a certain amount of heat to operate efficiently, and at idle speed, they often do not reach the necessary levels of temperature, especially in cold conditions. When the ambient temperature is low, the engine may not produce enough heat through combustion to warm the coolant and engine components effectively.
Moreover, diesel engines operate with higher thermal efficiency when they are working under load rather than idling. When idling, the engine runs at lower RPMs, producing less heat and potentially leading to an inefficient combustion process. This can result in issues like fuel dilution in the oil or an increased risk of condensation forming, which can further impact the engine's performance and longevity.
In summary, a diesel engine needs to be worked at a higher RPM or under load to reach and maintain its normal operating temperature, especially in cold weather, making the statement false.