1992 Mercedes-Benz 300SD 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.

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.

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.

We advise testing components before changing them, unless that component is being replaced in accordance with the factory scheduled maintenance interval. Failing to test parts very often causes good components to be thrown out, along with the money spent on replacement.

Glow plugs are the equivalent to spark plugs in a gasoline engine. They are not interchangeable, and powering a glow plug when not installed can lead to severe burns, fires, damage, or other injury to self and others. Lastly, on-board diagnostic (OBD) scanners may function in most vehicles, but not in select American trucks. The scanner should be universal, but some vehicles simply alert that there is no connection. For these situations, there may be an issue with the OBDII port or the scanner, or there may be a compatibility issue between the two.

Glow plugs can be changed by most people, and the job can even be a fun project for a weekend. Things can get tricky when trying to access the glow plugs, but generally, diesel engines are less cluttered than gasoline engines, and accessing hard to reach components is a little less of a chore. This will not be true for many diesel powered sedans, as the need for additional regulated components since the late 1990's has turned the already-too-small engine compartments into cramped spaces. For vehicles with access issues that are too difficult overcome, or if glow plug testing is needed, consult a certified technician to help you with this repair.