When a diesel car refuses to start - So, you change the glow plugs!
Auto repair

When a diesel car refuses to start - So, you change the glow plugs!

Diesel engines are so-called self-igniting. They do not have standard spark plugs that ignite the fuel-air mixture with an external spark. In diesel engines, the rapid compression of the fuel is sufficient to cause a fire. To do this, the engine must reach a certain operating temperature.

The reason for this lies in the fact that the compression in diesel engines is very high. If the engine is too cold, there is too much clearance between the piston and the cylinder wall. Too much compression is lost and the engine cannot start. Only when the engine is warm enough do the metals expand, allowing the combustion process to take place. Therefore, the diesel engine needs help to start. This is where glow plugs come to the rescue.

Glow plug function

When a diesel car refuses to start - So, you change the glow plugs!

The diesel engine glow plug is made of hard carbon steel; electrical voltage causes it to glow. When the injection system sprays the diesel-air mixture into the combustion chamber, it ignites even at low engine temperatures. The warm-up process takes 5 – 30 seconds .

Once the engine is running, the entire engine block heats up quickly. The engine goes into self-ignition mode and no longer requires ignition assistance. The glow plug goes out and no longer works while driving. This explains why diesel cars cannot be started with conventional jump ropes or by pushing. While the engine is cold, it will not start without the help of a glow plug.

Service life of the glow plug

When a diesel car refuses to start - So, you change the glow plugs!

Glow plugs are not used most of the time and therefore last significantly longer than spark plugs. It is difficult to make assumptions about average life expectancy. The more often a car is started during the day, the shorter its service life. If the vehicle is only used for long distance travel, a set of glow plugs can last more than 100 km . Thus, the glow plug is only replaced if it reports an imminent failure. If the engine is difficult to start, repair is required.

Now it's important to act now . As long as the engine is still igniting, changing glow plugs is much easier.

The deterioration of the glow plug leads to additional wear of the exhaust gas cleaning system. Diesel particulate filters clog more easily, as does the EGR system. Only clean combustion during the warm-up phase can reliably prevent damage. Therefore, if there is a possibility of damage to the glow plug, a more accurate diagnosis is necessary. Fortunately, this is very easy.

Resistance test

When a diesel car refuses to start - So, you change the glow plugs!

Glow plugs can be easily checked with using a multimeter by checking their resistance and thereby providing diagnostics.

The procedure is as follows:

– Switch off the engine.
– Disconnect the plug from the glow plug.
– Set the multimeter to the lowest resistance level.
– Connect the negative pole to earth, for example directly to the engine block (clamp connection is ideal for this).
– Hold the positive pole against the top tip of the glow plug.

If "continuity" is indicated, meaning there is no or very little resistance, the glow plug is good. If it shows "1", the glow plug is defective and must be replaced. The corresponding multimeter costs approx. 15 euros.

Glow plug replacement problem

When a diesel car refuses to start - So, you change the glow plugs!

The glow plug in a diesel car performs the same task as the spark plug. However, both parts have a different design. The spark plug for a gasoline car is short, with a rounded wide threaded base. The glow plug, on the other hand, is quite long with a small diameter due to the fact that it has to withstand the high pressure in the combustion chamber while driving.

When removing it, there is always a considerable risk of breaking it. . Due to constant temperature changes and years of use, the glow plug can become overgrown in the threads of the cylinder block. You should always take into account the fact that it is tightly glued and can easily come off.

To safely remove the glow plug, you need four things:

– Time and patience
- Oil
– Suitable tools
– Heating

There is absolutely no benefit in acting impatiently and giving in to time pressure. Let's boldly say: a broken glow plug is a big deal . It has to be drilled out, which is often only possible by completely disassembling the engine, turning the replacement parts for 15 pounds for repairs costing several hundred pounds .

When a diesel car refuses to start - So, you change the glow plugs!

The best tool is an adjustable torque wrench. These wrenches provide resistance up to a certain torque. Exceeding this value causes them to slip, preventing excessive force from being applied to the glow plug.

If that doesn't work, it will take a lot of patience. The location of the plug allows it to be lubricated with oil.
Oil, ideally a highly effective rust remover such as, for example,  WD-40  , liberally sprayed onto the threads of the spark plug.
Subsequently, the car drives 3-6 days and constantly pour oil into the threads. Oil penetrates gradually, stimulating engine heat and temperature changes along the threads.

When a diesel car refuses to start - So, you change the glow plugs!

A lubricated glow plug should be removed when the engine is warm. Although it must be warm enough, it must be turned off! Engine cooling stimulates the glow plug to loosen. A hot engine is a burn hazard. Therefore, handle it with care and always wear protective clothing!

Installing a new glow plug

When a diesel car refuses to start - So, you change the glow plugs!

A new glow plug should not be installed too soon. The carbon in the steel of the old spark plug and especially the soot from the engine may have eaten into the shaft. The consequences may be:
– deterioration in performance
– sticking
– breaking off . The shaft must therefore be thoroughly cleaned before installing a new glow plug. . Retailers offer suitable reamers. By carefully inserting the reamer, the thread is securely cleaned. Direct introduction of the reamer is important. An oblique insert will certainly damage the thread. Silicone-free lubricant is applied to the tip of the reamer. By inserting it into the thread, the lubricated tip will reliably clean the shaft. AT 25 – 35 euros reaming is not exactly cheap. In any case, it will always be cheaper than repairing a broken glow plug.

Before installation, it is recommended to check the glow plug with a multimeter . Connect the negative pole to the thread and press the positive pole to the end. It must indicate "continuity", otherwise it is faulty.

A new diesel engine spark plug is installed with the specified tightening torque on the packaging. The click of a wrench is enough. " Don't push too hard " and " take it easy both are adequately applicable here.

Glow plugs wear out at the same time . Therefore, they are always replaced as a set. One stands from 5 to 15 euros . As with spark plugs, the components must match the vehicle or model. A glow plug that is too long can damage the engine when screwed in.

If the diesel refuses to start

When a diesel car refuses to start - So, you change the glow plugs!

Before the last glow plug has expired, the pre-glow relay often fails. . It is important that the old glow plugs are loosened for a few days and that the engine is warm. Therefore, checking and, if necessary, replacing the glow plug relay is a quick and cheap way to leave the car on the road for a few more days. However, this period should be used to eliminate worn glow plugs.

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