Tip & How-To about Toyota Corolla

Engine Diagnosis Old Spark Plug Condition

Old spark plugs removed from your engine can tell a lot about the condition of the engine. Experienced mechanics can use this information to help diagnose several conditions simply by examining the state of the old plugs.

This tip is written as a useful guide to help diagnose engine problems and give a general indicator of the state of your engine. If you have ever wondered what you can tell from the condition of your old spark plugs please see the illustrations below.

As an example note that conditions such as over advanced ignition timing, lean fuel mixture or intake manifold leaks can often be determined from a plug that shows absence of deposits and a burnt or eroded electrode. However, sometimes this condition is caused by incorrect spark plugs having been fitted at some stage and in this case can be easily rectified by replacing the spark plugs with plugs that are in the correct heat range.

More serious conditions can be determined from oily deposits which indicate that oil is leaking past the valves guides or piston rings. In this case the mechanical fault will need to be rectified which will involve an engine strip down. These symptoms when seen on old spark plugs are a very significant diagnostic pointer as to how the engine has been performing up to this point and can help identify faults in the engine itself

The first picture in the illustration is of a healthy spark plug. Please follow through each of the images and you can compare your old spark plug against each of the images for a description of the engine problems that are associated with it.

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Normal: Here we see a normal used plug which should be a brown to greyish color with only slight electrode wear. This plug has been operating correctly and is the in correct heat range for the vehicle.




Worn: On this plug you can see a rounded electrode. The normal colour deposits are seen but this plug has simply been in the vehicle too long.




Plug Run Too Hot. The symptoms here are a blistered white insulator with an eroded electrode and an absence of deposits.
Causes: Incorrect heat range plug used, over advanced ignition timing, lean fuel mixture, intake manifold vaccuum leak, sticking valves or insufficient engine cooling.




Carbon Deposits: Dry sooty deposits like this indicate either a rich fuel mixture or a weak ignition. This is a good indicator of a clogged air filter or a problem in the fuel or engine management system. Also check for ignition problems.




Preignition: Here we see melted electrodes. You will note the insulator is white. This is an indication of severe misfiring and overheating. Can be an indicator of severe engine damage. Causes: Check for over advanced ignition timing, lean fuel mixture, insufficient engine cooling and lack of lubrication in the engine. Also check correct heat range plug has been used.




Ash Deposits: Encrusting of light brown deposits observed on the electrodes. Causes: Oil or fuel additives - try changing the gas brand that you are using. Severe cases often indicate engine valve seal problems whereby oil is seeping into the combustion chambers.




High Speed Glazing: Insulator looks glazed and yellow. Condition is associated with sudden temperature rises from hard accelleration of the engine. Plugs in this condition can cause misfires at high speeds. These plugs will need to be changed and consider replacement with a colder temperature range plug.





Oil Deposits: Indicative of oil leaking past the valve guides or piston rings and fouling the plug. Vehicle will run badly. Engine will need stripdown and repair.





Detonation: Insulator appears cracked or chipped. May have resulted from poor gapping technique which has damaged insulator. Can lead to piston damage. Replace plugs and ensure fuel anti knock values are correct. Ensure plugs are carefully and correctly gapped.





Gap Bridging: In this case deposits can clearly be seen lodged between the electrodes. The plug will not fire and this results in a dead cylinder. Clean or replace the plug.

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2 Answers

misfire in cylinder #2


Hi Roger:
I'd start by removing the spark plug and having a good look at the tip. It should tell you a lot about what is going on. Is it oil fouled, burnt, eroded?
Then put in a fresh plug and run it for a while. Has the problem gone away? What is the condition of the spark plug lead wire? Try looking at the wires with the engine running in the dark. Can you see sparks? If so, it may be time to replace the spark plug leads.
Depending on the miles on the engine, the problem could be as simple as, it's time to change the spark plugs, to the first indication of a bad head gasket or worn piston rings.
One step at a time. Deal with the symptom to identify the problem.

Jun 20, 2015 | Ford F-150 Cars & Trucks

1 Answer

87 maxima sometimes cuts off, sometimes wont start can wait a few minutes and will start, what can it be


this could be a lot of things ... problem with the fuel system (dirty fuel filter, dirty injectors, etc.), it could be due to an Extremely dirty air filter (but that is unlikely), it could be an issue with your car's computer (but this is also unlikely), your ignition system might be going (spark plugs or wiring could be bad), or a lot of other issues.
If there is any other information about exactly what noises the car makes, whether or not the car shakes when it is not working right, or if the car just turns itself off like you turned the key, this information could help diagnose the problem. It would probably be easier to take i to a mechanic and have them look at it. This would give them all of the information that they need to correct the problem. If you are too broke to take it somewhere (like I usually am) then you can try buying a Haynes manual for your car. These manuals have a lot of good input as to why problems happen to cars (specific to make and model of the car) and might be able to help you find the issue for yourself. Once you know exactly what is wrong, you can still take it to a mechanic, but you can save yourself a lot of fees by telling the mechanic what you need fixed. If your mechanic tells you that he thinks you have the wrong cause, there is a strong possibility that they are right -- mechanics have usually dealt with similar issues many times, so if you say that you think it is one thing based on looking through the Haynes manual, but he says he has solved this same problem in a different way before, he probably knows what he is talking about -- and if the solution is complex enough to require a mechanic ... he's probably right about the fix. Haynes manuals are designed to help the home-mechanic work on his own car .. so if you find that it is something relatively simple like spark plugs give it a shot. If it turns out to be something that you are not sure you can handle (because it is too complex, or because you are not sure you have all of the tools you would need) , take it to a mechanic before you pull the engine apart -- you'll save yourself a lot of time and hassle by getting it fixed right the first time.

May 28, 2011 | 1987 Nissan Maxima

2 Answers

I recently bought a 2nd hand 2003 galant 2.0, after my mechanic replaced the timing belt, the engine won't start. There was no spark on the cables. What's wrong? Please help.


It is imperative to have a very experienced mechanic performs a timing belt service!! If not done correctly on this vehicle, severe engine damage may occur. I would recommend checking the cam position sensor, this controls spark!! If damaged or disconnected, you will experience a "NO START" condition!!

May 24, 2011 | 2002 Mitsubishi Galant

1 Answer

engine shakes, misses when idling


It sounds like you might have an ignition problem. Did you change your spark plugs recently? If so, what kind did you out in? If not, how long have the old ones been in there? Are the ignition leads in good condition?

For reasons best known to Ford, their engines don't seem to like certain brands and types of spark plugs. Nothing against these plugs: it's just a peculiarity of the engines. For best results, stick with Motorcraft plugs and wires.

Also, make sure you use silicon grease on the plug boots, for two reasons. First, it'll make it a whole lot easier to remove them the next time you need to, and second, it helps to seal the connection against moisture entry.

Finally, check the coil pack. If everything else is good, you might try replacing it. Again, I recommend using a genuine Ford part rather than an aftermarket one.

Nov 04, 2010 | 1999 Ford F150 Regular Cab

2 Answers

Ceramic spark plug failuer


no it is not a common problem just make sure you have the correct plugs for that car

Sep 25, 2009 | 2002 Nissan Altima

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