Automotive Service Technician (310S) Engines Practice Exam

The heat range of a spark plug is about how quickly the plug tip sheds heat into the engine. This determines the tip temperature during normal operation. The goal is to keep the tip within a sweet spot: hot enough to burn off small deposits and prevent fouling, but not so hot that it stays near ignition temperatures and promotes pre-ignition or overheating.

If the plug runs too hot for the engine conditions, the tip can stay unusually hot, increasing the risk of pre-ignition, detonation, and damage to the plug and nearby components, especially under heavy load or high temperature. If the plug runs too cold, it tends to accumulate carbon and other deposits, which can cause fouling, misfires, rough idle, and reduced performance and fuel economy.

That’s why the heat range is chosen to match the engine’s combustion chamber temperature, fuel mixture, timing, and operating conditions. It isn’t about the number of electrodes or the thread size, and it isn’t limited to cold-start duration.