Glow plug malfunctions.

So that the candle disturbs the fuel turbulence as little as possible, increasingly thinner glow tubes are used. Currently, the diameter of the tubes in candles is 5-6 mm, and in the latest solutions it was even reduced in the zone of the strongest glow to 4 mm. This tube shape, it is used in spark plugs that glow after starting the engine.
Modern glow plugs can withstand the least 15 000 starting cycles and 100 000 km mileage. In the case of car operation with a predominance of long routes, these plugs have a service life of up to 200000 km mileage. Companies are among the leading manufacturers of modern glow plugs: I take, Bosch, Champion and Slovenian Iskra.
Modern glow plugs are characterized by a high degree of reliability. However, during the operation of the car, the heating material becomes thermally fatigued with time and they begin to fail. A common cause of premature spark plug failure is injector failure, incorrect angle and injection pressure too low. This manifests itself in problems with starting the engine, because the cylinder with the damaged candle starts to work only after the combustion chamber walls have warmed up. During the period of no ignition in the cylinder, which may take tens of seconds, the engine runs very unevenly, fumes and does not reach the expected power. If two or more spark plugs are damaged in the engine, it generally fails to run, even by towing.
Checking the operation of the candle is not a difficult activity and a multimeter is enough for measurements. Without any instrumentation, you can also find out about the efficiency of the glow plug. To do this, simply remove the glow plugs from the cylinder head. Then check it out, whether they are mechanically damaged. Burned or worn tips indicate a bad shape of the injected fuel stream (damaged injector).
If the glow plugs are in good mechanical condition, you can check their operation, connecting to voltage 12 V (in the case of spark plugs powered by this voltage in the engine!), which allows us to conclude, whether they heat up evenly and in a timely manner. It should be remembered here, that the glow plugs will become red. Therefore, you need to hold them in a pair of pliers or a vice, and then let it cool down after checking. The test should be carried out as short as possible. The tip of a functioning glow plug will glow red after approx 5 seconds from power on. Every candle, which takes longer to heat up or does not glow along the entire length of the heating rod, it is damaged and needs to be replaced.
Before reinstalling the glow plugs, the thread of the housing must be lubricated with a special anti-seize lubricant. Then tighten to the required torque (generally within the range 15-18 Nm). Do not over tighten, because there is a risk of pinching the air gap between the heating rod and the body and, as a result, damage to the spark plug due to overheating of the rod's shell.
A common cause of difficulties in starting the engine is improper selection of the glow plugs. For example, installing cheaper single-spiral candles in place of the more expensive two-spiral candles will burn them out for approx 10 startups, if the engine is equipped with an auxiliary heater. In turn, the use of candles without a regulating winding, in place of candles with a two-piece spiral, will overload the battery during each start. In winter, the engine will hardly "start", if it can be run at all, because the candles will not reach their heating temperature in the required time.
I do not recommend buying cheap products with trademarks of reputable companies, as they are usually mere fakes, quality significantly different from the originals.
Design and manufacturing defects of such candles cause not only their reduced service life, but they can be the cause of expensive engine repair. For example, if the candle was filled with powdered magnesia in the production phase, which was not properly dried, after screwing it into the head and starting the engine, the tube at the end of the candle will expand. Such a candle can no longer be unscrewed from the engine without removing the head.