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What is a Block Heater?

By Lori Kilchermann
Updated May 23, 2024
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A block heater is a device used in vehicles to protect the engine from the cold. Typically mounted in the engine block coolant passages, it functions much like an electric hot water heater in a home. When plugged into a household electrical outlet, the block heater warms the coolant inside of the engine block—a warm engine is more easily started than a cold engine. Both gasoline and diesel engines can benefit from the installation of this device.

In cold temperatures, the spark plugs in a gasoline engine can actually become coated with frost. This is due to moisture from a hot-running engine sitting to cool off in the cold temperatures. Residual moisture from the combustion process is left to freeze inside of the cylinder walls, creating a difficult starting condition. By placing a heater inside the coolant passage of the engine, the entire engine remains warm, resulting in easier cold weather starts.

Another benefit of a block heater is that the engine's oil remains fluid and pumps easily throughout the engine upon start-up. In a cold engine, the oil may remain very thick and unable to pump through the lubrication system for several seconds to several minutes. This creates lubrication problems that greatly reduce the life of critical engine components such as the oil pump, camshaft, lifters and main bearings.

In a diesel engine, the benefits are basically the same as in a gasoline-powered engine; however, the difficult-to-crank diesel is made all the more difficult in cold weather. The diesel engine is started by creating heat in the combustion chamber of the engine. In a cold engine, this task is made more difficult, requiring a block heater to assist in the cranking ability of the engine.

The most common mounting procedure for a block heater is to remove one of the engine's freeze plugs and replace it with a heating module. The design of the block heater allows it to be a direct replacement for a freeze plug. The heating element or coil of the block heater is placed into the coolant passage of the engine block, and the mounting plug fits snugly into the freeze plug orifice. Once in place, the coolant can once again be topped off, making the heating element completely submerged in the coolant. An electrical cord, complete with plug, is attached to the heating coil and run to the grill area of the vehicle, where it can be easily accessed and plugged into a power source.

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