YAWA: SPARK PLUGS
10/18/2007
QUESTION: I’ve heard stories of how “in the old days,” lots of old mechanics would remember which spark plugs went with which vehicles. I think there are too many cars today, so how can I memorize which spark plugs go with which engines?
ANSWER: You don’t. Just follow the spark plug supplier application charts and you usually can’t go wrong. Spark plugs come in many different sizes, styles and electrode configurations. So follow the application guide and compare thread diameters, the length of the threads, plug reach and the plug seat (flat with a gasket, or tapered) on the old and new plugs to make sure they match.
WARNING: Installing the wrong spark plugs in an engine may damage the plug threads in the cylinder head, or the engine itself if the plugs protrude too far into the combustion chamber.
Spark plugs are pre-gapped at the factory. Even so, due to consolidation, the electrode gap should also be checked before the plugs are installed — unless the plugs have a special type of electrode that is non-adjustable (follow the spark plug manufacturer’s recommendations).
QUESTION: A friend of mine said his uncle’s Ford pickup truck blew out a spark plug. Why did this happen?
ANSWER: Ford has had problems with spark plugs blowing out of the cylinder heads on some of its Triton V8 truck engines. Though the spark plugs are deeply set in the heads and have a long reach to the combustion chamber, the spark plug hole is only threaded part-way up the hole.
If the spark plugs are not installed with the proper amount of torque, they can loosen up over time. After 30,000 to 50,000 miles, a loose plug can unscrew itself down to the last couple of threads in the hole. When this happens, pressure often blows the spark plug right out of the head and destroys the threads in the spark plug hole. The repair requires drilling out the spark plug hole and installing a thread repair insert, or replacing the damaged cylinder head.
To prevent this from happening, the plugs must be properly tightened when installed and rechecked every 20,000 to 30,000 miles to make sure they haven’t loosened up.
QUESTION: Our instructor said reading spark plugs could make us a better diagnostic technician. What exactly does “reading” the spark plugs mean?
ANSWER: “Reading” is visually inspecting the spark plugs after they have been removed to look for signs of trouble. This should always be done when spark plugs are changed.
All spark plugs get dirty with use. Combustion byproducts stick to the electrodes and typically cause the white ceramic insulator around the center electrode to discolor. Normal deposits are a light gray or tan color.
You can tell a lot by “reading” plugs. If the electrodes are coated with black fluffy carbon deposits, the engine is running too rich — or is not being driven long enough or fast enough for the plugs to get hot and burn off the deposits. If the electrodes are coated with black, oily deposits, the engine is burning oil. Switching to a spark plug with a slightly hotter heat range may help keep the plugs cleaner longer.
Spark plugs with a yellow or blistered center electrode are the result of either the spark plug or the engine running too hot. The plugs may be the wrong heat range for the engine, or the engine may have a cooling problem (bad thermostat, water pump, plugged radiator, low coolant or inoperative cooling fan).
For more examples on “reading” plugs, check out Service Advisor next month in the November issue of Tomorrow’s Technician.
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