How to Guides

Know What Your Dirty Spark Plugs Mean?

Zaki Moulvi posted this on Jun 9, 2014

Changing out your vehicles spark plugs is driveway mechanics 101, but did you know that you should not just toss the old ones aside?  After the job is done, you should carefully examine each dirty, or fouled, plug one by one.  How they look and smell will tell you a lot about the over-all health of your engine and could alert you to any impending problems.

Motor Oil Fouled

A spark plug that has been shorted out by exposure to engine oil will feel greasy when you touch the tip and smell, well of engine oil.  Worn piston rings or cylinder walls is the most likely cause but it can also be attributed to worn valve stem seals.  If not addressed, oil and oil vapors can make their way into the combustion chambers.  If you notice your old plugs seem to have been exposed to engine oil, you will need to do some digging into your engine to find the cause.

Carbon Fouled

One or two spark plugs that contain black carbon around the wick is indicative of sticking valves, bad wires or a cracked distributor cap.  If the whole set looks this way then the air to fuel ratio is probably too rich.  Look for a faulty choke or a clogged air filter to be the culprit.  If those look alright then your fuel injectors could be causing the problem by injecting too much gas.

Insulator Glazing

This type of dirty spark plug will have bright yellow or tan deposits at its top.  They have most likely been left there by normal deposits that have melted onto the part due to excessive heat.  Using leaded fuel can cause a drastic increase in temperature on acceleration and will leave behind these types of deposits. 

Mechanical Damage

If any of your spark plugs look like the firing end has been hit with a hammer, you most likely have a foreign object inside the combustion chamber.  That object can go from cylinder to cylinder so you will need to explore them all to find the culprit.  This is why it is important to keep the throttle body openings and spark plug holes covered when you are doing work on the engine.

Overheated Spark Plugs

The white insulator nose of a spark plug will be blistered if it has been consistently exposed to high temperatures.  The first thing you should check is the engine cooling system to make sure that it is operating properly.  If that isn’t the cause then you may have a leaking intake manifold.  If you still have trouble finding the source of the heat, consult your owner’s manual.  It may be that you are using spark plugs that are not right for your vehicle.

Splash Fouled

This looks like dots of debris melted onto the insulator nose.  The most likely cause is an overdue tune up.  After normal combustion temperatures were restored those deposits were thrown off of the piston and ended up on the spark plug. 

Ash Fouled

White ash built up on the firing end of the spark plug usually means that fuel and oil additives are being burned along with gasoline during combustion.  While this does not indicate a problem with your engine, continued use of these products could cause the spark plugs to misfire.


If your spark plugs center electrode appears melted you have a problem with heat in the combustion chamber.  Again you will want to take a look at your engines cooling system or the spark plugs may have a heat range that is too high for your vehicle.


A fractured insulator nose is a sign of the spark plug being exposed to too much pressure.  Lean air to fuel ratios can cause that condition, faulty ignition timing or gasoline that that has an octane rating that is too low for your vehicle. 

Gap Bridging

Gap bridging is typically caused by the same engine problems that cause splash fouling. Only in this case the deposits build up between the electrodes.  Your engine has been misfiring as this spark plug was rendered useless by the build up.

Worn Spark Plugs

A worn spark plug usually just means that it has long stopped being very useful and should have been changed miles ago.  The voltage needed to fire it has probably doubled and you have been wasting fuel to make up for its poor performance.

As you see, changing the spark plugs is only half the battle.  You are setting yourself up for future repairs if you don’t take a few moments to see what the old ones have to tell you.  If you can find and fix the cause you will extend the life of your new set of spark plugs, and possibly other important engine parts.