2014 BMW 535d xDrive Glow Plug Replacement Cost

Glow plugs heat up when the ignition switch it turned to the run position and the engine is cold. The glow plugs receive electricity, and turn that electricity into thermal energy. You can think of glow plugs like BBQ coals. If lighter fluid (fuel) is thrown on cold coals, there may not be a fire. But if the coals are started and are hot, the fuel will ignite. Much like toaster coils, when the glow plugs are powered, the tips turn red hot, igniting the atomized diesel fuel as it flows into the combustion chamber. The ignition of the fuel and air creates more heat until, eventually, the engine is hot enough to run without a glow plug assist.

When glow plugs become weak, the engine will take longer than 3-5 seconds to start, and may not start at all after they degrade. Diesel engines with external block heaters have a better chance of starting the diesel engine than those without. The glow plugs can also cause the engine to run rough until it reaches near-operating temperature, because one cylinder will misfire. Smoke will billow from the tailpipe in this case, as raw fuel is dumped into, and burned in the exhaust. The check engine light will illuminate, and OBD codes will suggest the fuel delivery system, emissions and exhaust system, and possibly glow plugs. When the engine is warm, and restarted, the check engine light may disappear.

Driving a vehicle with a bad glow plug should be fine. In fact, when only one glow plug fails, the engine should run normally once near-operating temperatures are reached, given the engine can start. Diesels with block heaters may be easier to start, but the vehicle will exhaust smoke as the engine warms up. If the vehicle will not start, the glow plug must be replaced before driving. The engine may have extreme drivability issues and misfires until it has been completely warmed for several minutes. If there is a faulty glow plug causing a misfire at idle, the vehicle should not be driven until the engine has warmed thoroughly. Continued use in this condition will cause emissions component failure and carbon build-up in one or more cylinders, which can lead to hot-spotting, and detonation knocking after the engine is turned off.