Reading spark plugs
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The spark plug is a window into the combustion chamber. To a well trained eye, the plug can tell a volume of information about the engine's condition, air/fuel mixture, timing, fuel distribution through the intake tract, and more. | The spark plug is a window into the combustion chamber. To a well trained eye, the plug can tell a volume of information about the engine's condition, air/fuel mixture, timing, fuel distribution through the intake tract, and more. | ||
| − | No single article will be able to bring someone from a beginner to a pro at anything, and reading plugs is no exception. But the basics can be conveyed well enough to get a leg up on how to read a plug. | + | No single article will be able to bring someone from a beginner to a pro at anything, and reading plugs is no exception. But the basics can be conveyed well enough to get a leg up on how to read a plug beyond the common practice of looking at a plug with thousands of miles, as well as recent stop and go driving on it, to tell what is going on inside the engine. |
| − | The following article draws heavily on an article written by Brian Odom. It | + | While that can give a general idea of conditions on a very broad scale, there are more precise ways of reading a plug. |
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| + | The following article draws heavily on an article written by Brian Odom. It's as complete, if not more, so than most articles on the subject as well as including photographs to supplement the text. | ||
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| + | [[File:FLASHLIT.jpg|right]] | ||
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| + | ==Magnifier== | ||
| + | It's imperative that a light source with a magnifier be used to read plugs, because the plug insulator at the deepest point is one of the things that is looked at. If the light source isn't uniform from reading to reading, the slight coloring differences can be misread. The idea is to have repeatable conditions. | ||
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| + | Many plug magnifier/lights use 10X magnification. The choice then becomes LED or incandescent light bulb(s). If possible try both to see which is preferred. | ||
==Tuning an engine by reading the spark plug== | ==Tuning an engine by reading the spark plug== | ||
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Look for a colored ring at the base of the porcelain. The color will vary depending on the type of fuel: Unleaded E10 pump gas will leave very little color because of the additives like detergents, as well as the methanol. If using 108-110 octane race gasoline, these fuels will leave a tan color while the C17 and higher octane fuels will leave an almost gray fuel ring. | Look for a colored ring at the base of the porcelain. The color will vary depending on the type of fuel: Unleaded E10 pump gas will leave very little color because of the additives like detergents, as well as the methanol. If using 108-110 octane race gasoline, these fuels will leave a tan color while the C17 and higher octane fuels will leave an almost gray fuel ring. | ||
| − | If you study the ring you will see that it starts to show color just below the base of the ground strap, this is caused by the shielding | + | If you study the ring you will see that it starts to show color just below the base of the ground strap, this is caused by the shielding effect of the strap leaving that portion cooler thus showing the most color. The ring will develop from each side and form a full circle of correct color as you get closer to the correct tune up. |
If you look down into the bottom of the porcelain and you detect a thin oily deposit you are seeing oil from the combustion chamber caused by either worn or improperly seated rings. This may also be seen with a new engine that hasn't fully seated the rings yet, just don't confuse this with the fuel ring. | If you look down into the bottom of the porcelain and you detect a thin oily deposit you are seeing oil from the combustion chamber caused by either worn or improperly seated rings. This may also be seen with a new engine that hasn't fully seated the rings yet, just don't confuse this with the fuel ring. | ||
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[[File:Rich18.jpg]] | [[File:Rich18.jpg]] | ||
| − | Black or brown | + | Black or brown specks: This is an indication of detonation which is caused by ignition timing that is too advanced for the air/fuel ratio or octane of the fuel. Detonation (an undesirable, uncontrolled combustion of the air/fuel mixture after the ignition event) causes the boundary layer of air within the cylinder to be disrupted, which in turn causes the components and surfaces within the cylinder to absorb heat from the combustion process and increase in temperature. The black spots are the result of oil deposits bypassing the piston rings during detonation and being fused onto the porcelain by the increased cylinder temperatures. Tiny beads of grey/silver material apparent on the porcelain are melted piston. Reduce the ignition timing, richen the mixture, increase the octane of the fuel or a combination of all of these to prevent this situation. |
| − | The black spots are the result of | + | |
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{{Warning}}Do not underestimate the damage this condition can cause your engine. Correct this by whatever means necessary. | {{Warning}}Do not underestimate the damage this condition can cause your engine. Correct this by whatever means necessary. | ||
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==General spark plug info== | ==General spark plug info== | ||
===Description=== | ===Description=== | ||
| − | A spark plug is a device for delivering electric current from an ignition system to the combustion chamber of a spark | + | A spark plug is a device for delivering electric current from an ignition system to the combustion chamber of a spark ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. |
| − | A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. | + | |
| − | The | + | The center electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto. The spark plug's metal shell is screwed into the engine's cylinder head and thus electrically grounded. |
| − | The | + | The center electrode protrudes through the porcelain insulator into the combustion chamber, forming one or more spark gaps between the inner end of the central electrode and usually one or more protuberances or structures attached to the inner end of the threaded shell, designated the "side", or "ground" electrode. |
| − | Spark plugs may also be used for other purposes; in Saab Direct Ignition when they are not firing, spark plugs are used to measure ionization in the cylinders - this ionic current measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire measurement function. | + | Spark plugs may also be used for other purposes; in the Saab Direct Ignition when they are not firing, spark plugs are used to measure ionization in the cylinders - this ionic current measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire measurement function. |
===Operation=== | ===Operation=== | ||
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