APPEARANCE: This plug is typical of one operating normally. The insulator nose varies from a light tan to grayish color with slight electrode wear.  The presence of slight deposits is normal on used plugs and will have no adverse effect on engine performance.  The spark plug heat range is correct for the engine and the engine is running normally. 

CAUSE: Properly running engine. 

RECOMMENDATION: Before reinstating this plug, the electrodes should be cleaned and filed square.  Set the gap to specifications.  If the plug has been in service for more than 10-12,000 miles, the entire set should probably be replaced with a fresh set of the same heat range.

APPEARANCE: The firing end of the plug is covered with a wet, oily coating. 

CAUSE: The problem is poor oil control.  On high mileage engines, oil is leaking past the rings or valve guides into the combustion chamber.  A common cause is also a plugged PCV valve, and a ruptured fuel pump diaphragm can also cause this condition.  Oil fouled plugs such as these are often found in new or recently overhauled engines, before normal oil control is achieved, and can be cleaned and reinstalled.  

RECOMMENDATION: A hotter spark plug may temporarily relieve the problem, but the engine is probably in need of work.

APPEARANCE: The effects of high temperature on a spark plug are indicated by clean white, often blistered insulator.  This can also be accompanied by excessive wear of the electrode, and the absence of deposits. 

CAUSE: Check for the correct spark plug heat range.  A plug which is too hot for the engine can result in overheating.  A car operated mostly at high speeds can require a colder plug.  Also check ignition timing, cooling system level, fuel mixture and leaking intake manifold. 

RECOMMENDATION: If all ignition and engine adjustments are known to be correct, and no other malfunction exists, install spark plugs one heat range colder.

APPEARANCE: Carbon fouling is easily identified by the presence of dry, soft, black, sooty deposits. 

CAUSE: Changing the heat range can often lead to carbon fouling, as can prolonged slow, stop-and-start driving.  If the heat range is correct, carbon fouling can be attributed to a rich fuel mixture, sticking choke, clogged air cleaner, worn breaker points, retarded timing or low compression.  If only one or two plugs are carbon fouled, check for corroded or cracked wires on the affected plugs.  Also look for cracks in the distributor cap between the towers of affected cylinders. 

RECOMMENDATION: After the problem is corrected, these plugs can be cleaned  and reinstalled if not worn severely.

APPEARANCE: Spark plugs fouled by MMT (Methycyclopentadienyl Maganese Tricarbonyl) have reddish, rusty appearance on the insulator and side electrode. 

CAUSE: MMT is an anti-knock additive in petrol used to replace lead.  During the combustion process, the MMT leaves a reddish deposit on the insulator and side electrode. 

RECOMMENDATION: No engine malfunction is indicated and the deposits will not affect plug performance any more than lead deposits (see Ash Deposits).  MMT fouled plugs can be cleaned, regapped and reinstalled.

APPEARANCE: Glazing appears as shiny coating on the plug, either yellow or tan In color. 

CAUSE: During hard, fast acceleration, plug temperatures rise suddenly.  Deposits from normal combustion have no chance to fluff off; instead, they melt on the insulator forming an electrically conductive coating which causes misfiring. 

RECOMMENDATION: Glazed plugs are not easily cleaned.  They should be replaced with a fresh set of plugs of the correct type. If the condition recurs, using plugs with a heat range one step colder may cure it.

APPEARANCE: Ash deposits are characterized by light brown or white colored deposits crusted on the side or center electrodes.  In some cases it may give the plug a rusty appearance. 

CAUSE: Ash deposits are normally derived from oil or fuel additives burned during normal combustion.  Normally they are harmless, though excessive amounts can cause misfiring.  If deposits are excessive in short mileage, the valve guides may be worn. 

RECOMMENDATION: Ash-fouled plugs can be cleaned, gapped and reinstalled.

APPEARANCE: Detonation is usually characterized by a broken plug insulator. 

CAUSE: A portion of the fuel charge will begin to burn spontaneously, from the increased heat following ignition.  The explosion that results applies extreme pressure to engine components, frequently damaging spark plugs and pistons. 
Detonation can result by over advanced ignition timing, inferior petrol (low octane) lean air/fuel mixture, poor carburetion, engine lugging or an increase in compression ratio due to combustion chamber deposits or engine modification. 

RECOMMENDATION: Replace the plugs after correcting the problem.