How Tos / Ignition & Electrical / Tech

Reading 101: How to Read Your Spark Plugs

Listen closely because your spark plugs may be trying to tell you something.

Is your engine running too lean? Too rich? Is there an issue with oil control or ignition timing? Or is everything just fine with your engine? Like the mechanical version of the Magic 8-Ball, your spark plugs may have the answers for you. The trick is learning how to read your spark plugs.

With help from the spark plug experts at NGK, we’ll teach you how to diagnose minor tuning issues or potentially major engine problems by examining your spark plugs. Check out the images below, courtesy of NGK Spark Plugs, and get ready to do a little light reading the next time you pull your spark plugs.


Appearance: A light tan/gray or brownish color, along with very little electrode erosion, indicates optimal operation conditions, including a healthy engine and correct spark plug heat range.


Appearance & Symptoms: The electrodes—center and ground—are covered in an ashy coating. As a result of this masking of the electrodes, your engine may experience a misfire. This build-up of combustion deposits can eventually (but not usually) fill in the space between the two electrodes.

Possible Causes: Oil leaks, poor fuel quality.


Wet and Dry Fouling

Appearance & Symptoms: Dry fouling (top) appears as sooty, black build-up. Wet fouling (bottom) has a wet, sometimes oily appearance. Both conditions can create poor starting and misfiring.

Possible Causes: Depending on whether the spark plug is coated in oil or fuel, wet fouling can be symptomatic of a compromised head gasket, poor control from your pistons’ oil control ring, valvetrain problems, or an extremely rich condition. Dry fouling, or carbon fouling, is often caused by an overly rich condition, and the problem may lie with your air cleaner (clogged) or carburetor. Other possible causes could be low compression, vacuum leak, overly retarded timing, or improper spark plug heat range.

Lead Fouling

Appearance & Symptoms: Lead fouling can only occur in applications that use leaded gasoline, such as racing engines. Lead fouling generally shows up as yellowish brown deposits on the spark plug’s insulator nose. Lead fouling can cause your engine to misfire only at high-rpm and under hard acceleration.

Possible Causes: This condition commonly occurs when gasoline contains too much lead; however, because spark plugs are changed frequently in racing applications, lead fouling has become less common.


Appearance & Symptoms: The insulator around the center electrode may be broken (see left) or the ground electrode may be bent. Again, you will likely experience misfire and some power loss under these conditions.

Possible Causes: If the insulator is broken, it may be the result of sudden thermal expansion or thermal shock caused by extreme temperatures or temperature change. Detonation is often the culprit and can be caused by an extremely lean air/fuel mixture, drastically advanced timing, improper gasoline octane rating. A broken insulator can also be thermally triggered if cold fuel is sprayed on a hot insulator. Center and ground electrode damage is often the result of mechanical collision with internal engine components or excessive vibration. The mechanical collision occurs when a spark plug has too long of a reach. Vibration is generally the result of improper installation.


Appearance & Symptoms: In this case, the center or ground electrode is melted or scorched, and your engine may be experiencing some power loss.

Possible Causes: According to NGK, melting often results from loose installation, which prevents the plug from properly transferring heat from its tip. But melting may also indicate unusual heat or hot spots within the combustion chamber. This excessive heat is often the result of pre-ignition, which can be created by an overly lean condition, improperly advanced injection timing, or  improper heat range (too low).

Lead Erosion

Appearance & Symptoms: The tip of the ground electrode looks chipped and its surface may be thinned.

Possible Causes: Another condition unique to leaded gasoline, this condition is caused by lead compounds that react chemically with the electrodes at high temperatures. This makes the electrode material (nickel alloy) weak and brittle. This is caused by too much lead in your gasoline.

Erosion, Corrosion & Oxidation

Appearance & Symptoms: A plug with a combination of erosion, corrosion, and oxidation will have pitted and rough electrodes and may even have a green cast if the oxidation is heavy. These conditions can result in increased, improper spark plug gap and yield poorer performance.

Possible Causes: Typically, these conditions occur over time as lead in the gasoline reacts with the electrode materials.


Appearance & Symptoms: The insulator will have a glazed white appearance and may have small black deposits. There may also be abnormal electrode wear, and you will likely notice a loss of power at high speeds or under high engine load.

Possible Causes: Overheating may occur from over-advanced ignition timing, poor cooling system efficiency, lean air/fuel mixture, vacuum leak, or wrong spark plug heat range (too low).


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  10. Russ Cochran says:

    ok, 1970 351c 2v block fitted with 4v quench heads, bored .030 and decked .010, fresh rebuild car cranked, but sounded like crap. I pulled all the plugs, #5 (front right looking at car) looks like it hit top of piston, collapsed on top of electrode. All other plugs high carbon deposits, but no damage, how can one plug it and not the others, or is there something I am missing here.

    • mic tdc on all pistons at tdc and make sure #5’s tdc measurement is the same. had a jd motor out of a 89 front loader that had to have the pistons milled thousands off cause they slapped the head on tdc.

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  17. 1 out of the 6 spark plugs soaked with oil. Oil in exhaust. All other plugs dry soot. No smoke coming out of exhaust. 2-3 hrs running lost 3 quarts of oil.

  18. I have a Nissan b13 model with a GAv15 engine. plug number 2 has been dry fouling and I have replaced the valve seals and the piston rings but the same problem. what could be wrong?

  19. Joseph Shemanski says:

    IF you foul a plug & clean it up,will it work or once a plug is fouled is still good after cleaning it?

    • If it’s visibly damaged on the exterior, then it’s pretty certain to be bad and need replacement. If it’s just dirty & has some dry soot or dry deposits that can be cleaned up with a brush that won’t damage anything & using some contact cleaner, brake clean, carb/tb clean, alcohol, etc. and you’ve fixed the problem that caused it to get that way in the first place, then it “should” be alright to re-use. However, unless it’s a really expensive, high-performance plug, I’d just replace it or all of them. Most plugs aren’t that expensive so it’s probably best to just fix the fault and replace all the plugs at the same time, unless again, it’s a really expensive or rare/hard to find plug type.

  20. Dude if a plug is fouled. Throw it in the trash and buy new ones! Dang man if ya too damn cheap to buy a spark plug you prolly shouldnt be pullin a wrench on anything. Period! Just my opinion….. plugs are cheap……engines are not! Thats a fact

    • Dude. Not my horse, and dont know you but turned a wrench many a time where I barely had bucks to see if it would turn over. Beyond that. Good info to know in case something happens out in bfe . Agreed change the plugs but, if u can get to the house where there Are tools, much easier/safer to do that with no tow bill, jack stands. etc..

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