Glow Plugs

Glow plugs heat the combustion chamber to help diesel engines start and run cleanly when cold. Even in warm climates they matter: modern common-rail engines use preheating and afterglow to reduce white smoke, noise and emissions, and failed plugs can trigger warning lights and rough idle.

Types

• Metal-sheath glow plugs – a heating coil inside a metal tube. Designs include single-coil plugs for older systems and dual-coil self-regulating plugs that heat quickly and then limit current.
• Low-voltage quick-heating plugs – designed to run below system voltage with a controller or resistor on some older indirect-injection engines. Fitting a plug with the wrong voltage rating leads to rapid failure.
• Ceramic glow plugs – reach higher temperatures faster and are used on some newer engines with controller-managed afterglow.
• Pressure-sensor glow plugs – integrate a combustion pressure sensor, used on a small number of engines.
• Intake air heaters – many truck engines use a grid heater in the intake instead of, or in addition to, glow plugs.

Specifications

Rated voltage, thread size (such as M10×1.25 or M8×1.0), overall length, sheath length and diameter, terminal type (threaded stud or plug connector), and heating time characteristics. Direct-injection engines typically use longer, slimmer plugs than older indirect-injection engines.

Applications and failure drivers

Diesel engines such as Toyota's 1KD and 1GD, Isuzu's 4JJ1 and Mitsubishi's 4D56 and 4N15 use glow plugs specific to their generation and control system. Failures come from over-voltage, failed controllers, carbon buildup and poor injector spray, and seized plugs can break during removal, which requires careful workshop practice. Pickups such as the Isuzu D-Max often have their plugs replaced as a set. Related: sensors.