When the diesel car refuses to start – So you change glow plugs!

Diesel engines are so-called self-igniters. They don’t have regular spark plugs igniting the fuel-air mix with an external spark. In diesel engines the quick compression of the fuel is sufficient to cause combustion. To this end the engine must reach a specific operational temperature.

The reason for this lies in the fact that compression in diesel engines is very high. If the engine is too cold, there is too much play between piston and cylinder wall. Too much compression is lost and the engine cannot start. Only when the engine is sufficiently warm, metals expand enabling the combustion process. A diesel engine therefore needs starting assistance. And this is where glow plugs come in.

Function of a glow plug

The diesel engine’s glow plug is made of hard, carbon steel; electric voltage causes it to glow. When the injection system sprays the diesel-air mix into the combustion chamber, it ignites even at low engine temperature. The warming-up process lasts 5 – 30 seconds.

As soon as the engine is running, the entire engine block quickly heats up. The engine enters self-ignition mode and no longer requires ignition assistance. The glow plug extinguishes and is no longer operational during driving. This explains why diesel cars cannot be started with help of regular jump leads or by pushing. As long as the engine is cold, it will not start without assistance of the glow plug.

Life span of a glow plug

Glow plugs remain out of use for most of the time and therefore they last considerably longer than spark plugs. It is difficult to make assumptions about an average life span. The more often a car is started in a day, the shorter the life span. If the car is only used for long distances, a set of glow plugs can last for more than 100.000 km. A glow plug is therefore only replaced if it announces its imminent failure. If starting the engine becomes more difficult, repair is due.

Now it is important to act immediately. As long as the engine still ignites, replacement of the glow plugs is considerably easier.

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A deteriorating performance of the glow plug leads to additional wear in the exhaust cleaning system. Diesel particle filters clog more easily, as does the EGR. Only a clean combustion during the warming-up phase can reliably prevent damage. Therefore if damage of the glow plug is likely, a more accurate diagnose is necessary. Fortunately this is very easy.

Testing resistance

Glow plugs can easily be checked with a multimeter, testing their resistance and thus enabling diagnosis.

The procedure is as follows:

If “continuity” is indicated, i.e. no or very little resistance, the glow plug is ok. If it shows “1”, the glow plug is defective and must definitely be replaced. An adequate multimeter costs ca. 15 EUR (± £13).

A glow plug replacement challenge

A glow plug in a diesel car has the same task as a spark plug. Nevertheless, both parts have a different construction. A spark plug for a petrol car is short with a rounded, broad threaded base. A glow plug on the other hand is rather long with a small diameter due to the fact that it must be able to resist high pressure in the combustion chamber during driving.

When removing it, there is always a considerable risk of breaking it off. Due to the continuous temperature changes and its use of years, the glow plug can be encrusted in the thread of the engine block. You should always consider the fact that it is tightly stuck and can easily break off.

To safely remove a glow plug, you need four things:
– Time and patience
– Oil
– Suitable tools
– Heat

There is absolutely nothing to gain by acting impatiently and giving in to time pressure. To state it boldly: a broken glow plug is no small matter. It has to be drilled out, which is often only possible by complete disassembly of the engine, turning the replacement of a £15 spare part into a repair of several hundreds of pounds.

The best tool is a torque spanner with an adjustable release torque. These spanners offer resistance up to a certain torque. Exceeding it causes them to slip, preventing excessive force being applied to the glow plug.

If this does not work, plenty of patience is required. The plug’s location allows it to be lubricated with oil.
The oil, ideally high-performance rust solvent like e.g. WD-40 is liberally sprayed into the plug thread.
Subsequently, the car is driven for 3 – 6 days and oil is continually poured into the thread. The oil gradually penetrates, stimulated by the engine heat and the temperature changes along the thread.

The lubricated glow plug must be removed when the engine is warm. Although it needs to be sufficiently warmed up, it must be switched off! A cooling engine stimulates the loosening of the glow plug. A warm engine constitutes a scorching risk. Therefore handle with care and always wear protective clothing!

Installing a new glow plug

The new glow plug should not be installed too soon. The carbon in the steel of the old plug and especially the soot from the engine has possibly encrusted in the shaft. The consequences can be:
– deterioration of performance
– caking
– breaking off
Therefore the shaft must be thoroughly cleaned prior to installation of a new glow plug. The retail offers suitable reamers. By carefully inserting the reamer, the thread is reliably cleaned. Straight insertion of the reamer is essential. Obliquely inserting it will certainly damage the thread. Silicon-free grease is applied to the tip of the reamer. Inserting it in the thread, the greased tip will clean the shaft reliably. At 25 – 35 EUR (± £22 – £31) a reamer is not exactly cheap. At any rate it will always be cheaper than the repair of a glow plug that broke off.

It is recommended to check the glow plug with the multimeter prior to installation. Connect the negative pole to the thread and hold the positive pole against the tip. It should indicate “continuity”, otherwise it is defective.

The new diesel engine plug is installed with the indicated torque which is mentioned on the packing. A click of the spanner is sufficient. “Don’t push too hard” and “easy does it” both adequately apply here.

Glow plugs wear simultaneously. Therefore they are always replaced as a set. A single one costs between 5 and 15 EUR (± £4 – £13). As with spark plugs, the components should match the car or the model. A glow plug which is too long will damage the engine when being screwed in.

If the diesel refuses to start

Before the last glow plug is giving up the ghost, the pre-glow relay is often defective. It is important that the old glow plugs are loosened over several days and that the engine is warm. Therefore a check and if necessary a replacement of the pre-glow relay is a quick and cheap way to keep the car on the road for a few more days. This period should however be used to tackle the worn-out glow plugs.

Foto: marekusz, yauhenka, vchal, Ikonoklast Fotografie, pryzmat / shutterstock.com