Regularly changing the spark plugs is crucial for the perfect function of the gasoline engine. AUTO BILD explains how you can recognize defective spark plugs, how expensive a repair is and how you can change spark plugs yourself.

how to clean a lawn mower spark plug

How do I recognize a defective spark plug?

Defective spark plugs are usually noticeable quickly. Particularly noticeable when there is a misfiring, causing the car to jerk or a cylinder completely misfires. Both lead to a significantly different engine run. By then at the latest, the cause should be investigated as quickly as possible and the spark plugs replaced if necessary. Because misfiring can cause unburned fuel (watch out for the smell of petrol) to get into the catalytic converter and destroy it. A further indication is increased consumption, from incomplete fuel combustion results. 

Additionally, the basic rule for gasoline engines is that they should reach temperatures of around 400 degrees as quickly as possible in the combustion chamber (not to be confused with cooling water or oil temperature).

The spark plugs can only burn freely from this temperature level. As a result, the engines of a large walk behind mower suffer when short journeys are predominantly used. In most cases, a defective spark plug can be identified by encrustations and deposits on the spark plug electrodes. If the electrodes have a uniform, fawn-brown color and are free of white or black deposits, the spark plug is fully functional.

Repair, cost and duration

The cost of changing spark plugs in the workshop depends on the number of cylinders in the engine. From which the number of spark plugs results and ranges between 80 and 300 euros. In the case of engines in small cars with only three cylinders, the change is of course cheaper than in engines with six, eight, ten or twelve cylinders. 

Moreover, it is also expensive for engines that are subject to high thermal loads and require the use of high-performance spark plugs. It is worth comparing prices, even if conventional spark plugs cost between around two and 20 euros. There can be a considerable price difference even with four-cylinder engines. The duration of the change also depends on the vehicle. In most cases, the change is done in 30 minutes.

how to clean a lawn mower spark plug

When do spark plugs have to be changed?

As a rule, spark plugs should be replaced every 30,000 to 60,000 kilometers. But now there are also spark plugs that last up to 100,000 kilometers. Many spark plug manufacturers recommend that you check the spark plugs for functionality once a year. Furthermore, the prescribed change interval for the car can be found in the vehicle manual. 

In most cases, one spark plug per cylinder is always used in gasoline engines. But there are also engines – mostly with very large individual cubic capacities – with what is known as double ignition. Here two spark plugs ignite the mixture in order to achieve the most homogeneous combustion possible. A four-cylinder engine with double ignition requires eight new spark plugs when changing spark plugs.

Structure of a spark plug

The spark plug consists of a center electrode that is located in an insulator made of ceramic. At the lower end of the spark plug, the ceramic is surrounded by a metal housing, the thread that is screwed into the cylinder head. The ignition cable is connected to the spark plug by means of a spark plug connector via the top connection of the spark plug. 

At this point, the electrical impulse previously applied to high voltage by the ignition coil reaches the spark plug. After the ignition voltage has passed the spark plug via the center electrode, a spark jumps over the center electrode head to the ground electrode. The spark ensures that the fuel mixture in the combustion chamber ignites. However, this principle only applies to gasoline engines. 

Glow plugs are used in diesel engines, which are only required for a cold start of the engine. The temperature required to ignite the diesel mixture is created by the significantly higher compression of the engine, which causes the mixture of diesel and oxygen to heat up so much that it ignites without sparks.

how to clean a lawn mower spark plug

How do spark plugs differ?

Outwardly, two types of spark plugs can be distinguished. Those with a sealing ring and conical ones without a sealing ring. However, depending on the requirements of the engine, spark plugs have other specific characteristics. This includes the pitch of the spark plug thread, the thread diameter. The length of the spark plug, the number of electrodes, the electrode material and the heat value. 

The heat value quantifies the maximum temperature resistance of a spark plug. The different heat values result from the ability of a spark plug to dissipate heat to the cylinder head. In general, turbo engines run hotter than naturally aspirated engines. Accordingly require particularly heat-resistant spark plugs, i.e. those with a high calorific value. However, if the heat value of a spark plug is too high, the temperature required for burning free may not be reached, which means that the spark plug collects deposits.

Another distinguishing feature of the spark plug is the electrode material used. In addition to the conventional iron-nickel-chromium alloys. Precious metals such as iridium, platinum or silver are also used. The material has an influence on the temperature stability. The durability of the spark plug and thus on the frequency of the change intervals. 

Furthermore, in contrast to the standard spark plug, which has only one ground electrode, there are spark plugs with several ground electrodes. Candles with two, three and four ground electrodes are among the common designs. The advantage of having several ground electrodes is that the ignition spark always looks for the cheapest route. If one electrode wears out, the current takes its way via another, still functioning earth electrode.

Find the right spark plug

Unfortunately, the manufacturers of spark plugs use different nomenclatures when describing the properties of their spark plug types. Anyone who wants to buy spark plugs is therefore dependent on so-called spark plug tables, which list the right spark plugs for the most common vehicle types. 

In addition, manufacturers such as Bosch, NGK and Champion. The right spark plug can be found here by specifying the vehicle manufacturer number. Or with the help of vehicle information on the model, year of construction and engine version. With the product number shown at the end, you can then order the spark plug from specialist retailers or online.

Changing spark plugs yourself - Instructions

Changing the spark plugs is quick and easy, especially in older vehicles. With many models, after opening the bonnet. You can already see the spark plug caps that are attached to the spark plugs. But with many younger cars the first thing to do is to remove the panels. 

There are also engine types that have an ignition coil per cylinder, which is mounted directly above the spark plug connector. The necessary dismantling of the fairing and the removal of the ignition coils increase the workload accordingly. But even in such cases you can change the candle yourself with patience and a little skill.

Required tools

The following tools are required: Spark plug socket with rubber insert (holds the plug in the socket after it has been unscrewed)

• Ratchet / ratchet with extension

• Torque wrench for torques from ten to 50 Newton meters

• Compressed air (alternatively large brush)

• Cleaned rag

Other requirements:

The spark plug change should only be carried out when the engine has cooled down, as the spark plug and the cylinder head expand when heated. A new, cold spark plug that is screwed into a hot. Therefore, expanded cylinder head can get stuck in the cylinder head. If the cylinder head is cold, the spark plug cannot be screwed in that far. 

Very important: The ignition of the vehicle must be switched off and the earth pole of the battery disconnected. Even more important: people with a previously damaged heart or people. With pacemakers are generally not allowed to work on ignition systems. Touching parts of the ignition system when the ignition is switched on can also be life-threatening for healthy people.

Requirement 2:

Once access to the spark plugs has been established, the spark plug connector is removed first. The individual spark plug caps are each assigned to a cylinder and must be attached to the same cylinder. Again after the spark plug has been changed. If there are no numbering and the assignment does not result from the length of the plug connector cable, the connectors should be numbered with masking tape. 

After removing the connector, the upper part of the spark plug can be seen, consisting of the connection nut, the ceramic casing and the hexagon nut. To ensure that the periphery around the spark plug is free of dirt, the spark plug shaft should be blown out with compressed air before it is unscrewed. Alternatively, cleaning can also be done with a hard brush. So it is assured!

Requirement 3:

Now the spark plug wrench is put on the tightening nut and the spark plug is unscrewed. Work with caution when loosening the spark plugs so as not to damage the thread in the cylinder head. If there is any contamination in the spark plug thread of the cylinder head, remove it with the wiper cloth. Then screw in the new spark plug by hand. It is essential to ensure that the thread is screwed incorrectly. 

A little trick helps: Apply light pressure to turn the spark plug of the best lawn mowers take counterclockwise about one turn, the threads of the plug and cylinder head adapt properly. Then turn in the right direction. If the candle is not screwed in by hand, there is no feeling of having hit the correct thread. There is a risk of costly damage to the cylinder head thread when tightening. 

Requirement 4:​

A piece of hose can help with deep-seated spark plugs that cannot be screwed in by hand. If the spark plug sits securely in the thread, it is tightened with the torque wrench and the correct tightening torque. Then put the spark plug connector back onto the spark plug connection.

The use of the torque wrench is so important because if the tightening torque is too low, the spark plug can be destroyed by vibrations. The spark plug is also responsible for sealing the combustion chamber, which can therefore result in compression losses. 

Requirement 5:

Also improper: tightening the spark plug too tightly. This can destroy the thread. In the worst case, the entire cylinder head has to be replaced – expensive and unnecessary fun. The correct torque depends not only on the size of the thread, the shape of the spark plug seat (conical or sealing ring). But also on the material of the cylinder head. 

However, it is best to use the exact tightening torques provided by the manufacturer. The information provided by the spark plug manufacturer – to be found on the spark plug packaging. You should correspond to the information provided by the vehicle manufacturer in the logbook.

how to clean a lawn mower spark plug

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