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And if ... glow plugs are faulty

  1. Purpose and construction The fuel-air mixture in the cylinders of a diesel engine ignites at a temperature...
  2. Causes of problems
  3. Diagnostics

Purpose and construction

Purpose and construction

The fuel-air mixture in the cylinders of a diesel engine ignites at a temperature of 800-850 degrees. But in a cold diesel engine (less than 50–60 degrees), it is impossible to reach such a temperature only by compressing air with pistons. That is why the glow plugs, the working part of which is located in the combustion chamber for heating the air, have been introduced into the design of this type of engines.

The main task of the glow plug is to turn the ignition key to the first position as quickly as possible to reach the working temperature in order to heat the air in the combustion chamber. Candles heat up to 850–1300 degrees for 2–30 seconds and on average consume a current of 8–16 amps (ie, 96–192 W) each. The duration of the current supply to the candle and the voltage value is controlled by a relay or a special electronic unit. Having “redeemed” a light bulb on the torpedo, which prohibits the driver to turn on the starter, the relay is still for a while - he continues to heat the candles for 3 minutes while the engine is running. However, some modern candles are disconnected from the network immediately after turning off the starter, since they remain hot for the time required to warm up the motor. In the most advanced designs, the electronics monitor the temperature of the engine, and if it recognizes it warm enough, it does not include candles at all.

The glow plug is a heating element housed in a housing and enclosed by a casing. In the simplest case, it is a spiral of refractory metal. More complex candles have two spirals of different materials. By changing their resistance depending on the degree of heating, they provide automatic temperature maintenance and avoid overheating without the help of an external relay.

Symptoms of the problem

In a modern diesel engine, a serviceable candle with a normally operating control unit warms up to operating temperature in a few seconds.

The lack of heating of the internal volume of the cylinders is the most likely cause of a difficult start of the diesel. A serviceable engine should start up immediately after starting the starter, from the first crankshaft revolutions. In warm weather, the failure of one candle is not always noticeable at the start, but the first minutes after starting some motors (depending on the model and state) will work unevenly, since the combustion process in a cold cylinder will be disturbed. Two problem candles are already seriously complicate the launch. Failure of three or four candles at once is unlikely, therefore if the engine does not start, the reason may lie in the control relay or wiring. There are cars that have a malfunction of the candle signal lamp on the dashboard.

With faulty candles, a warm (from 60 degrees and above) diesel starts without problems.

Causes of problems

The nozzle should not "pour" diesel in one stream directly onto the candle.

The candle fails for two reasons: the helix resource is exhausted (approximately after 50–75 thousand km of run), or the fuel equipment is faulty. If on the old car one candle refuses, you need to change the rest - they will still soon be out of order.

A faulty fuel equipment shortens the life of a candle, for example, a polluted or worn nozzle that directs a powerful stream of fuel directly to the working element of the heater. From this in the shell of the heater is formed a hole, and the spiral is destroyed. Finally, the candle is easy to break with an unqualified installation. The long plugs of many modern engines break when dismantled, as they stick to the head of the block.

A faulty control relay can cause candles to overheat - their shell is deformed, melted, the coil burns out. The relay itself can also “burn out”, and the wires coming from it can break or fray due to improper installation after repair.



The fact that the candle is heated and you can turn on the starter, will tell the extinct signal lamp.

The simplest diagnosis of glow plugs is to check their electrical conductivity. The spiral should conduct current, its resistance in the cold state is within 0.6–4.0 Ohm. If you have access to candles, you can “ring out” them yourself: not every household tester can measure such low resistance, but any device will show the presence of a heater break (resistance equals infinity).

More accurate diagnostics with tracking characteristics of each of the spirals are already produced by specialists. In the presence of a contactless (induction) ammeter, you can do without dismantling the spark plug from the engine. But it is often necessary to check the site on a special stand and visual inspection of the working part, which may show signs of overheating - melting, deformation of the tip until its destruction.

In some cases, including the refusal of all the candles at once, it may be necessary to check the vehicle’s electrical equipment. Namely - the relay control candles and its chains.

Igor Shirokun
Photos Bosch, Beru

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