How to test spark plugs: complete step-by-step guide

how to test spark plugs (1)

Concise guide outlines how to test spark plugs, including checking external condition, resistance and ignition spark.

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How to test spark plugs: Regular checking is advised
External condition: visual inspection of the spark plugs
Technical condition of the spark plug: check resistance
Ignition Problems? Check ignition spark
Have spark plugs checked in the workshop

When the yellow engine control light flashes or the engine suddenly starts running unevenly, you should go to the workshop as quickly as possible . If you ignore these signals, you run the risk of costly engine damage . Sometimes these are “only” defective spark plugs. And in most cases, changing the spark plugs is quick and doesn’t leave a big hole in your wallet. But be careful: In addition to the signals described above, there are other signs of defective spark plugs.

If a spark plug has problems generating sparks, the engine will no longer run optimally. This is often noticeable in a loss of performance and increased fuel consumption. If no spark is formed at all, it will not ignite. This is particularly noticeable when the engine is not running smoothly or when the engine is jerking while driving – sometimes the engine simply cuts out. 

A defective spark plug can sometimes also only be found during an emissions test, which will reveal if the exhaust gas values ​​are incorrect or an error message is stored in the control unit. 

In many cars, if the engine control light comes on while driving, this can indicate a defective spark plug. However, all of these symptoms can also occur if other components are defective. 

How to test spark plugs

If spark plugs malfunction, reduced engine performance is just one of the possible consequences. If the spark plugs of an engine are not in top condition, the combustion in the cylinder does not run optimally. Other components of the car, such as the catalytic converter, can then also be damaged and thus also the environment. Therefore, it is useful to know how to test a spark plug regularly.

How to test spark plugs: Regular checking of the spark plugs is best

A car’s spark plugs are no longer as easily accessible as they were in previous generations of vehicles. Before you look at the mechanical aspects of how to test a spark plug, you must remove the cladding and covers. Checking and testing is therefore a little more time-consuming. 

Regular testing and regular replacement of the spark plugs generally takes place as part of the inspection and maintenance appointments, but it is something you can do yourself if you have a little mechanical skill. 

Signs of possible defects on the spark plugs are above all misfires, for example when accelerating and delayed starting of the engine when starting. 

Always check the appearance of the removed spark plugs first. The condition, discoloration and damage of the spark plug say a lot about whether the car’s engine is working optimally or not.

External condition: visual inspection of the spark plugs

The testing of spark plugs essentially consists of two sections. After the spark plugs have been removed from the engine, the inspection begins with a comprehensive visual inspection. With experience, you can immediately see the condition of the spark plug, and the motor from the appearance of the spark plug. If the electrode looks fawn and is dry, everything is usually fine. 

The ceramic insulator should be light brown in color at the tip. Whitish deposits indicate overheating due to a too lean mixture (e.g. carburetor setting, secondary air), black soot indicates poor combustion or a mixture that is too rich with too much gasoline or too little air. If the candle is charred, too much oil could get into the combustion chamber. At first it is enough to clean them with a brass brush. 

But a conscientious checker also looks for breaks and cracks in the insulation when examining the candle, checks for wear on the electrode, looks for damage to the sealing ring and traces of burns on the insulator. Discoloration can already indicate malfunctions.

All of these features show a professional very precisely whether the combustion in the respective cylinder is running exactly as it should. The damage pattern of the spark plug can differ from cylinder to cylinder or be identical for all plugs. If the spark plugs are worn out, they need to be replaced. If a malfunction occurs, troubleshooting continues. Sometimes the wrong spark plugs are simply installed. Even then, the spark plugs provided by the vehicle manufacturer must be installed.

Technical condition of the spark plug: check resistance

The resistance test of all spark plugs is only carried out in a further step. Each spark plug type has an electrical resistance that is specified in ohms. This resistance is measured using a multimeter that shows the current resistance of the spark plugs. If the measured result is not within the manufacturer’s specifications, the need to install new spark plugs.

Short to ground and high internal resistance

A quick test of the spark plug can be carried out without unscrewing it. To do this, the ignition plug is removed from the plug and the resistance between the upper plug connection and the vehicle ground is measured with a multimeter. 

The smaller it is, the more “short to ground” there is and the spark plug will not generate any high voltage. Ideally, the resistance is very high. You get the right value if you screw in a new spark plug and measure its resistance for comparison.

But some spark plugs are already defective when you buy them, mostly because of poor contact transitions in their inner workings. Since spark plugs consist of screwed individual parts, you can often unscrew the upper contact bolt (threaded piece) from the insulator. If you scratch the transition to the exposed inner electrode clean, treat it with contact spray and screw it back together, it usually works again. The spark plug should then no longer be used for long, but it is usually sufficient as an emergency repair.

Ignition Problems? Check ignition spark

Despite regular maintenance of the spark plugs, it can also happen that the car’s engine refuses to work completely and does not start. In such cases, the expert will need to run diagnostics to find the fault. If the ignition system is suspected of not working, its voltage supply is checked first. If this is OK, you next check the ignition cables. To do this, the resistance of the cable is simply measured.

The spark plugs are only checked when it has been ensured that the ignition cables are in perfect condition. To do this, the spark plugs are unscrewed one after the other. Then the professional re-attaches the plug and places the connected candle on a metal part of the car. Permanent sparks should be seen on the electrode when attempting to start the engine. 

If there is an ignition spark, the spark plug can continue to be used, provided that the assessment, testing and measuring as well as the maintenance booklet say nothing else. This test should only be used in extreme cases on vehicles with a regulated catalytic converter in order to avoid damage to the catalytic converter.

Creeping currents

These are formed by a conductive coating on the insulator, so that the spark creeps over the insulator instead of jumping over. Such a coating is, for example, damp soot or abrasion from a cleaning action with a brass brush. So just scrub with feeling and rinse with brake cleaner. Leakage currents can also occur in other components, especially in damp weather and in winter. To prevent this, the outside of the candle, all plugs, cables and the ignition coil should be sprayed with oil (e.g. WD-40) and wiped off thoroughly.

Electrode distance

You should use a feeler gauge to check the distance between the center electrode (in the insulator) and the curved ground electrode. Check what it should be for your engine. If it is too big or too small, it can be adjusted by carefully bending the ground electrode.
If you have tuned the engine and increased the compression, the distance should be a little smaller than the standard, otherwise the ignition voltage is not sufficient for a flashover. However, it is better if you install a more powerful ignition coil and choose a larger electrode gap. This always gives the best spark under high loads.

Caution: The earth electrode should not be bent too often or too far to adjust the distance. If you overdo it, there is a possibility that it will break off in the engine and cause damage. Some manufacturers even advise against readjusting in their operating instructions. Assembling new candles is better in this case and does not cost the earth.

Spark patterns

Now you put the plug in the spark plug cap and place it with its thread on the cylinder head. Hold them by the insulation of the spark plug connector or the ignition cable. When the engine is started, a relatively wide bluish spark jumps over between the electrodes. If not, or if the spark is very small and yellowish, the candle must be exchanged for an equivalent that is approved. Avoid any experiments here, it will only damage the engine.

Tip: You can also check the plug individually. Anyone who has a so-called “electric shocker” for personal protection can also abuse it for the candle test. Simply hold it to the upper spark plug connector and body and you can tell whether a clean spark is jumping between the contacts.

Did you get a “wipe” during the procedure? Good, because now you know that the spark plug connector or the ignition cable are defective and allow side currents. So get away with it! Otherwise, a careful visual inspection for cracks or holes is sufficient for the check.

You also check the spark plug connector right away, without the spark plug. You place it with its insulated side surface close to the motor housing and start the motor. If there is a spark on the plug, it is defective and must be replaced.

The emergency repair

If the spark plug does not provide a sufficient spark, is very sooty and covered with deposits, the wire brush can no longer do anything. If no new part is available, there is only one thing: a blowtorch or welding torch flame.

The plug (s) burn freely for a few seconds or up to a minute, ie let it glow bright red. Now let it cool down slowly!

When the plug is cold again, you can remove the tinder with the wire brush and check the distance between the electrodes. Then put it in the engine and the plug will usually spark like on the first day.

Have spark plugs checked in the workshop

As already mentioned, the checking of the spark plugs and spark can also be done in a specialist workshop. This is especially true if the engine no longer wants to start. In the event of a defect in a spark plug, the cause must be found in order to ensure an even and best possible combustion again. If the test reveals that a spark plug is defective, workshops will always replace all of the engine’s plugs if they have been used for a long time. This prevents similar problems in the near future.