Effective glow plug fault finding and problem solving
As a diesel technology specialist and original equipment (OE) supplier to vehicle manufacturers across the globe, DENSO has abundant knowledge that it is happy to share with the independent aftermarket to help workshops undertake service and repair work efficiently and effectively.
To ensure the engine starts quickly and runs smoothly requires the glow plugs to be in good condition, but even the best of plugs, such as those manufactured by DENSO, will come to the end of their service life and need to be replaced.
Design
Both double coil and instant heating glow plugs use a combination of two types of metal alloy coils, one for quick heating and the other for regulating the current to protect it from overheating. This double coil assembly is firmly packed in an electrically insulated high heat conductive powder, encased in a body and manufactured in a variety of materials, in differing lengths, diameters and wire thickness. These variations allow for the production of glow plugs that operate to different parameters specific to their applications.
Working principle
A high electrical current flows from the battery via a glow plug control module, and through the glow plug terminal to the regulating and heating coils.
The electrical resistance of the materials allows the probe, which projects into the cylinder, to heat up quickly and glow red, to aid the initial combustion of the fuel at start up. To ensure the probe doesn’t overheat, as the electrical resistance increases the current is slightly reduced, which allows the glow plugs to deliver a prolonged service life.
Fault finding and problem solving
Should the probe found to be enlarged, the fault may lie with the alternator or regulator, so the charging system should be checked. Other causes could be from damp occurring during vehicle storage or excessive voltage, which could be down to a 12-volt glow plug fitted in a 24-volt system, for example.
In addition to problems with the alternator or regulator, or excessive voltage, causes can include the irregular/incorrect spray pattern from the injector or a timing fault with the injection pump, so respectively, the type of injector and its spray pattern must be checked, as well as the timing of the pump.
Other issues could be damage to the thread in the cylinder head or insufficient tightening torque on the plug, both of which should be checked.
This is generally caused by oil in the combustion chamber due to excessive engine wear and high oil consumption. This means that the piston ring clearance, turbocharger seals and crankcase ventilation system should all be checked.
Like the missing probe, carbon deposits can also be caused by a faulty/failing injector, irregular fuel spray pattern or injection pump timing issue. However, there could also be insufficient air in the combustion chamber, due to restrictions somewhere in the intake system.
This can again be caused by damage to the thread in the cylinder head, but whereas a broken tip could be the result of under tightening, the likely problem here is the plug being over torqued, or the use of the wrong tool during installation.
Like a probe touching the plug body, a bent or broken terminal can also be caused by damage to the thread in the cylinder head, over torquing, or the use of the wrong tool during installation.
In addition to the irregular/incorrect spray pattern from the injector or a timing fault with the injection pump, as well as damage to the thread in the cylinder head or insufficient tightening torque on the plug, the problem could be a failure of the power supply, so the ignition unit should be checked
Range and availability
The OE quality glow plugs in the DENSO Aftermarket range provide the independent service and repair sector with a comprehensive range of replacement options for the vast majority of the European vehicle parc and is widely available from its distribution partners across Europe.
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