Spark Plug Fouling

The spark plug is the “canary in the coal mine” of the combustion chamber. Electrodes and porcelain can reveal both short term and long term problems if you know where to look.

Most OE spark plugs have a service life of over 50,000 miles, thanks to electrodes containing precious metals such as platinum and iridium. OEMs consider spark plugs to be part of the emission system in most modern vehicles.

If the spark plug fails more quickly than the recommended interval, it is important to solve the problem before installing a new spark plug. If you replace the spark plug to solve the misfire problem, the car will come back.

But, first, what causes the spark plug to rot quickly? What does a dirty spark plug look like, and what is a dirty spark plug?

Carbon Impurity

If the spark plug looks solid black or gray, it could be carbon fouling — something usually caused by a fuel mixture that is too rich.

During normal combustion, most of the fuel is oxidized and turns into carbon dioxide and carbon monoxide gas. When there is more fuel than oxygen, the carbon in the unburned fuel polymerizes into carbon deposits. These molecules like to stick to hot spots in the combustion chamber, and this includes spark plug tips and insulators.

Curing the problem will usually point to the fuel system and how the engine breathes. If the fuel injectors are clogged or sticking open, excess fuel can cause carbon problems.

If the mass air flow sensor or oxygen sensor does not accurately report air entering the engine or oxygen content in the exhaust stream, it can cause rich running conditions which can cause carbon to foul the spark plugs.

Another factor is how air flows through the valve. If air is restricted or has to flow past carbon deposits on the intake valves, it will become turbulent and interfere with the flame front and the size of the fuel droplets in the combustion chamber. This means that the fuel injected into the intake port or combustion chamber will not burn completely.

Oil Impurities

Dirt oil on spark plugs usually produces a shiny black appearance. If there is enough oil in the combustion chamber, deposits can accumulate on the tip, porcelain or shell.

If you can’t tell if it’s carbon or oil, kiss the plug; it will smell like engine oil. The oil can come from the piston rings, valve stem seals, or the positive crankcase ventilation (PCV) system. Leaking piston rings can be diagnosed with a leak test. If one cylinder has oil fouling, a relative compression check can help assess mechanical problems with that cylinder.

A malfunctioning PCV system is a major cause of oil contamination in modern engines. This system has become much more than a spring valve. Modern systems can separate oil from crankcase vapors and regulate electronically when the engine absorbs steam.

Some PCV systems have heaters to ensure that the valve does not freeze under certain conditions when condensation occurs. If the valve freezes, it can cause higher than normal crankcase pressure. This can cause oil to be pushed past the valve seal.

If the PCV valve is stuck open, the excess steam and oil drips can quickly foul the spark plug.

A failed turbocharger can be another source of spark plug oil fouling. The seal on the turbine shaft is strong, but can fall victim to heat and poor oil quality. The oil that lubricates the shaft can enter the pressure intake and eventually into the combustion chamber.

OEMs have issued a TSB regarding excessive oil consumption. Most of these problems are related to cylinder deactivation and variable valve timing.

The main cause of this problem is the vacuum created in the cylinder which sucks engine oil past the rings and into the combustion chamber. In vehicles with deactivated cylinders, the deactivated cylinder is negatively pressured and draws oil droplets in the crankcase past the rings and eventually into the converter. This has happened to some GM and Honda machines.

In some vehicles with variable valve timing (usually on the exhaust and intake cams), the valve timing can generate higher-than-normal vacuum pressures which can suck oil past the rings. This was the case for some recent Toyota, Honda and GM models. Customers will report increased oil consumption that exceeds one liter every 1,000 miles.

Outside of the oil passing through the ring, oil trapped inside the ring can become carbonized and cause damage to the cylinder wall. This can cause more damage and more oil consumption. In some cases, oil consumption results in a low oil condition which will cause damage to the bearing surfaces.

Cooling Problem

Internal coolant leaks can foul the spark plug and cause it to misfire. The problem could be a leaking intake manifold or head gasket, and a dirty plug may be located in one or two adjacent cylinders. Burning coolant leaves ash, white deposits on electrodes and insulators, creating hot spots that can cause premature ignition and set misfire codes.

When the plug is pulled, it may show chalk on the strap and center electrode. Modern coolants don’t cause this type of buildup as quickly, due to reduced phosphates, zinc, and other additives that can contaminate the catalytic converter.

In the past, the converter would clog and stop the engine before significant damage was done.

Unfortunately, the formulation means that a driver can run a vehicle with a coolant leak for several thousand miles, while the spark plug gets dirty slowly.