Why do glow plugs fail?

All about glow plugs - 3 causes of failure - workshop tips


to Part 1 Requirements for the glow plug | Structure and function of glow plugs |

Causes of failure of glow plugs

In warm and dry weather, the diesel starts, even if one glow plug is defective and only the remaining plugs are preheating. The start is then usually associated with increased pollutant emissions and possibly also with nails, but the driver is not necessarily aware of these signs or does not know how to interpret them correctly. The nasty surprise comes when it gets cold and clammy and the first night frost sets in: The “heat donation” of the diesel engine no longer works and at best it starts badly and smokes - more likely, however, nothing works.

Typical damage is shown below and its various causes are listed. In most cases, this diagnostic aid enables quick troubleshooting. (Source: Beru)

Diagnosis of glow plugs with an ohmmeter, clamp meter or glow plug tester

Does the vehicle have a Self-diagnosis, which also monitors the glow system, the error should be able to be localized quickly.

Otherwise it is advisable to always include thePower consumption to be measured with a current clamp (clamp-on ammeter) during preheating. For example, if there are 4 glow plugs in the engine and each one requires 5 amps, the total power consumption must be 20 amps. If the engine is still warm, you simply unplug the NTC from the cooling water so that the glow plug control unit also starts the glow plug. The current consumption will jump up to approx. 20A (depending on the type of candle) and then fall to a lower value of 15-8 A.

Glow plug test with clamp ammeter

Once the glow plug has been removed, it is best to check it with an ohmmeter. Is the displayed resistance at ∞ Ω, the glow plug is defective. If the resistance is below 5 Ω, the glow plug is functional (the resistance value of a functional ceramic glow plug is usually below 1 Ω).

Check the glow plug resistance

The method that is still often used in the workshop is to test the glow plug with a battery. Be careful. Today glow plugs are often designed either for voltages of 11V or for even lower voltages. This is why the method uses a glow plug that has been removed and clamped in a vice12V battery to energize, then often to a defect in the glow plug, if it has not already been.

Now there is also a new one from Beru Glow plug tester, with which you can now test glow plugs in vehicles with 12 volt on-board voltage easily, quickly and reliably. The glow plugs can be tested individually, without removing and without starting the engine. Kfztech.de has convinced itself of this. It is also not necessary to preset the glow plug type (steel or ceramic) and the voltage value of the glow plug of 3.3–15 volts is automatically recognized. The display is analogous. In this way, the heating up and the current reduction can be recognized, and it is thus possible to compare individual glow plugs with regard to current consumption and control behavior.

Diagnosis is very easy with the Beru glow plug tester

Assembly and disassembly of glow plugs

Important when replacing glow plugs: observe the torques!

The breaking torque must be observed when dismantling glow plugs. What to do when the breaking torque is reached? Then do not turn any further, otherwise the glow plug could break off. Instead, proceed according to the 3-point program: heating up - loosening - turning out:

1. Heating up: Warm up the engine or energize intact glow plugs with a separate cable for 4–5 minutes - this heats up the glow plug and burns it free.
2. Dissolving: Apply rust solvent or multifunctional oil generously to the thread attachment of the glow plug and let it work for approx. 5 minutes.
3. Unscrewing: Then start another unscrewing attempt and carefully remove the glow plug from the cylinder head with the appropriate tool. (Do not exceed the maximum loosening torque - see table below. It is essential to interrupt before reaching the breaking torque; if necessary, start another attempt by heating up.)

Glow plug thread Breaking torque
8 mm20 Nm
9 mm22 Nm
10 mm35 Nm
12 mm45 Nm

After unscrewing the old glow plugs, the thread, the cone sealing seat and the glow plug duct in the cylinder head should be cleaned with the appropriate tool (see below).

At the Screw in of the new glow plugs, the torque specified by the vehicle manufacturer must be observed.

Glow plug thread Tightening torque
M 810 Nm
M 912 Nm
M 1012-18 Nm
M 1222-25 Nm

In the case of glow plugs with a screw connection, the tightening torque of the connection nut must also be observed.

Connection nut thread Tightening torque
M 42 Nm
M 53 Nm

Note: Especially after coking (coking) between the glow rod and the cylinder head, the cylinder head bore often has combustion residues or dirt particles. In the case of cylinder heads with a 10 mm thread, this coking can be easily and safely removed with a special glow plug reamer.

Pre-clean the glow plug bore with a rag. Coat the reamer with grease in the cutting area and screw it into the cylinder head: The combustion residues stick to the grease and are removed when the tool is unscrewed. The new glow plug can then be fitted without any problems (please note the tightening torque again!). Before installing the glow plugs, grease them in the shaft and thread area with GK grease.

back to part 2 structure and function | back to part 1 requirements |

Articles on the ISS or AQGS quick-start glow system and the PSG pressure sensor glow plug are planned for a later date.

Source: Beru, NGK,

Author: Johannes Wiesinger

edited: 02/16/2021

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