1996 Mercedes-Benz E300 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 the engine is cold, it will be hard, if not impossible, to start. However, the engine will start normally when it is warmed up.

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.

Glow plugs are normal maintenance items for most vehicles, and the service schedule is listed in the owner's manual. If the glow plugs are leaking combustion gases, weak, or fail before the specified mileage, they should be replaced early, and inspected for defects.

Diagnosing a glow plug is most easily done with a computer scanning tool. The OBD code set for the glow plugs indicates that at least one glow plug was not in specified range. The technician will use that information, and use the scan tool to cycle the glow plugs on, then off. Reading circuit voltage for all of the glow plugs will show which one is faulty.

If the glow plug relay, injector pump timing, and engine compression are not properly checked, then replacing the glow plugs may not address the original problem.

Locations for glow plugs are mostly simple to access, and replacement requires that the battery is disconnected, the glow plug is disconnected from an electrical connection, and unscrewed like a spark plug. The threads in the cylinder head are inspected, cleaned, and, if specified, coated with an anti-seize compound. The old glow plug will be inspected for signs of the engine running poorly. The new glow plug is screwed in, then its connector is plugged in. After the battery is reconnected, the glow plugs should be tested on a cold start, preferably after several hours of sitting.

For best results and to avoid future problems, replace the glow plugs as a complete set. Be sure to verify that the glow plug relay is operating properly.