In the early '70s, Mopar changed the concept of a tune-up with the introduction of the breakerless, capacitor-discharged ignition system. Thus began the evolution of electronic engine controls. since then spark plug technology has advanced as well, but spark plugs remain the core of any ignition system. Let's face it, unless you're running a diesel engine, your engine has them. They have a great bearing on your car's performance, and can reveal many symptoms of an engine's condition when they are properly "read."
"We recommend spark plug replacement at OE specified intervals or sooner if spark plug problems are suspected. When you pull the old plugs out, the color and condition of the plug's tip can provide a good indication of what's going on in the combustion chamber," says Matt Hallis, product technical support specialist with the Bosch Corporation.
Being able to read spark plugs has always been considered "black magic" of sorts for many, but properly reading a spark plug is a must for any kind of performance tuning. Reading spark plugs can also give you an early insight into any problem that may be just beginning to rear its ugly head.
In order to properly read the plugs and use the readings to improve performance, it's important to keep them in order with the cylinders they were removed from. In a good running and properly tuned engine, the plug tips should all be very close in coloration. A tan, slightly reddish deposit on the spark plug is normal. Other color or condition on the tip may point to fuel, ignition, vacuum, or larger, more fatal engine problems.
If your spark plugs are covered with black and fluffy soot on the insulator, this indicates an over-rich fuel condition, which may be caused by an out-of-adjustment carburetor. If the plug has small, white deposits on the insulator, nine times out of ten this may indicate oil consumption. If that's the case, now you can look deeper for possible problems with a cylinder's valve seals, worn valve-guides, a badly worn or broken piston ring, or even a badly scored cylinder.
A very light tan or, even worse, white, blistered insulators may point to a mixture that is too lean. If proper carb tuning doesn't seem to help, a vacuum leak may be the cause of this condition. Other spark plug conditions include wet fouling, which indicates no spark is getting to the cylinder, or a physically damaged electrode, which indicates serious internal engine problems.
Not all spark plugs are created equal. We may like to think that the spark plugs hanging in the automotive section of Wal-Mart are as good as any plug on the market, but that's simply not true. Buying a quality spark plug means doing a little research. Say you are using a set of Edelbrock heads. Edelbrock has done extensive testing with their heads, and you can bet that different plugs have been part of that testing. Sure, there may be some variables in your engine that dictate a different heat range, but their recommendation will get you close, if not right on.
 Although this is an extreme...  Although this is an extreme circumstance, "bridging" (or the accumulation of deposits on the firing end of the plug) is influenced by oil leakage, fuel quality, and the engine's operating duration. Light brown deposits encrusted on the ground and/or center electrodes indicate ash deposits. When the deposits are found on only one side of the spark plug's nose, it's considered to be a problem within the cylinder head. This could mean either valve stem seals leaking or worn valveguides. When the deposits are found on both sides of the spark plug, it's considered to be a problem with piston ring sealing. This condition can smother the spark, and in some cases, cause it to misfire.
Possible causes: Check for worn valveguides and valve stem seals. If they appear in good condition, check the piston rings. Also, make sure the spark plug used is the correct heat range for the application. If the wrong heat range is used, it may not be a victim of the engine's condition, but the cause of it. Another thing that may add deposits to the nose of the plug is fuel additives. |
 This image shows what is called...  This image shows what is called "flashover." Flashover is when the voltage sent to the spark plug does not fire through the spark plug and out the electrode, but rather follows the outside of the plug and shorts between the metal shell and the terminal nut on the plug's exterior causing the engine to misfire.
Possible causes: This can occur when the air is highly ionized (such as on a stormy day), or when the insulator is dirty. |
 Ever wonder what makes up...  Ever wonder what makes up the plug's "heat range?" According to this chart from Bosch, we can see the larger the insulator nose, the hotter the spark plug's heat range. |