If the engine stalls and stutters, the first thing to check is the spark plugs. This common wear part, which can be easily found in the engine, causes such symptoms at the end of its life span. Replacement is not always necessary. Cleaning might sometimes be enough. How this works, and how to identify “burnt up” spark plugs, is shown in this manual.
|1. The task of car spark plugs|
|2. Cleaning and adjusting spark plugs|
|3. Which spark plug is the right one?|
|4. Step by step: replacing spark plugs|
|5. Common mistakes and troubleshooting|
The task of spark plugs in the car
Unlike the diesel engine, temperature in petrol engines is not sufficient for ignition of the air-fuel mix. In this case we speak of electric ignition. For this type of ignition, the spark plug is responsible. Per cylinder, one and sometimes two spark plugs are available. They give off a small ignition spark at the right moment, causing an explosion, ultimately driving the engine. Modern spark plugs generate several thousand sparks per minute.
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A worn out spark plugs causes typical symptoms like a faltering ignition, leading to a restless running of the engine and a complete failure of the cylinder, causing heavy shocks. Manufacturers recommend a certain maximum mileage for their spark plugs. However, there is no stipulated replacement interval. To determine whether a spark plug has reached the end of its life, it should be removed first. If the electrode has black residue or the gap between the electrodes is too small, the spark plug is up for replacement. With a special feeler gauge the distance between electrodes can be determined very precisely up to tenths of millimetres. No replacement is necessary if the discolouring is light to dark brown. In this case the spark plug can be cleaned and adjusted.
Cleaning and adjusting spark plugs
Soot-coloured, brown deposits and a too large gap between the electrodes can lead to symptoms indicating a spark plug defect.
In this case, no replacement is necessary. Instead, the spark plugs must be thoroughly cleaned and adjusted. This is done as follows:
|Step 1: Remove all spark plugs and mark the corresponding cylinder.|
|Step 2: Use a brass steel brush to clean excess soot and dirt from the spark plug. You can wipe dirt residue from the housing with a damp cloth.|
|Step 3: Check the electrode gap with a feeler gauge by inserting its flat side. If the gap is wider than the usual 0.6 mm (check the manufacturer’s data!) adjust the metal mass electrode by slightly knocking the spark plug on the engine block. If it is adjusted, re-install the spark plug after cleaning the screw thread with the steel brush. Read how to do it below.|
|Step 4: At a temperature of 400˚C or higher, the spark plug is able to burn the residue by itself. On short distances this temperature is not reached and therefore it is useful to drive at least 30 minutes on the motorway after cleaning and adjusting the plugs. Doing this, you can meanwhile identify possible installation errors.|
Which spark plug is the right one?
All spark plug components should match. These are its material, the screw thread angle, its length, width and properties such as its heat resistance or ignition speed. The car manual often has a list of recommended spark plugs. Alternatively, dealers have a table with spark plugs and corresponding data. In the internet the matching products can be found with the car or its model number.
|– Iron-nickel chromium alloys are the best, but the least durable. If you are looking for long-life spark plugs, choose silver, platinum or even iridium. With regard to price this is especially worthwhile for people doing a lot of travelling.|
|–Spark plugs with several mass electrodes are good because the current can divert if one electrode is blocked by deposits. The spark plug will not fail quickly and has a long life as well as, of course, a corresponding price tag.|
|– If your engine runs on gas (LPG, CNG) you need corresponding spark plugs. Check the car manufacturer data or consult the internet on the AA, MOT or DEKRA websites.|
Step by step: replacing the spark plugs
You need the following tools for replacement:
|– a car manual
– a fitting box / socket spanner
– or a spark plug spanner
– or a torque spanner with a spark plug socket
– a screwdriver if needed
– a cloth
Please note: Spark plugs must be replaced or inspected in a cold engine to avoid the risk of burns.
|Step 0: Open the bonnet and find the spark plugs in the engine. The position can be found in the manual. If they are protected by a cap or lid, remove this and put it aside. Remove the spark plug terminal and make a note of the corresponding cylinder.|
|Step 1: With a spark-plug socket, spark-plug spanner or a box or socket spanner (generally key width 20.8) loosen old spark plugs and then unscrew them by hand. Old spark plugs do not belong in household garbage, but must be disposed of at the car scrapyard or the local recycling facility. Wipe the worst dirt from the screw holes.|
|Step 2: Screw every new plug in the corresponding screw hole by hand. Just insert and screw loosely.|
|Step 3: Put the torque spanner with the spark plug socket on the torque stipulated by the manufacturer and tighten the bolt just as long until it “slips through”, or alternatively work by the following thumb rule:
– In case of a spark plug with a gasket, after manually screwing it tight, tighten it another 90˚.
– In case of a conical spark plug without gasket, tighten the bolt another 15˚ after screwing it tight manually.
|Step 4: Insert the spark plug socket.|
|Step 5: The lid or cap can now be placed back.|
|Step 6: Leave the bonnet open, sit in the car and start the engine. Apply the gas pedal a couple of times and observe the reaction. Is the engine running smoothly without stalling and stuttering, the replacement is successful.|
Common mistakes and troubleshooting
With the proper tools, the replacement of spark plugs generally poses no problem for DIY’ers. If any problems occur, please observe the thumb rules but remember that this is not an optimal solution. Possible problems will occur during the first drive, and therefore a test drive after replacement is essential. The following problems might occur:
|– Rattling sound in the engine: This is an indication of something not being correctly mounted. This could be the spark plug caps. Check the mounting of these parts and if necessary mount them again.
– Error message – Engine: Modern cars are constantly supervised by the board computer and a spark plug replacement can cause an error message. In normal cases it will reset as soon as the computer notices everything to be in order. In rare cases, the error message has to be removed by the garage.
|– Stuttering, cylinder failure: These symptoms indicate the spark plug not being properly installed or the spark plug terminal not being sufficiently tight. The screw thread sometimes has a slack which might cause the terminal to move a couple of millimetres. As soon as the engine has cooled off, check if the spark plugs are installed properly and insert the cable again. If the spark plugs are loose, a faulty screw thread can be the cause. In this case you need to file a complaint with the manufacturer.|
|– Bad performance: In order to run optimally, the timing of the ignition spark must be exactly tuned to the engine. After a spark plug replacement, modern cars are often confused and have to “learn” the timing anew. This lasts a while. If this problem doesn’t disappear after a few kilometres, the computer must be reset in the garage.|
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