Internal combustion engines, such as those found in most cars, trucks, and SUVs, rely on spark plugs to set off combustion inside each of its cylinders. On some engines, the spark plugs reach the cylinders through passageways that extend through the valve cover. When this is the case, the passageways need to be sealed so that oil cannot escape the engine and contaminants cannot find their way inside.
A spark plug tube is a device that is pressed or threaded into the cylinder head. When the valve cover is in place, the tubes create passageways into each cylinder for the spark plugs to mount. The tubes are each sealed with gaskets, the spark plug tube seals, that are pressed into the valve cover around the openings for the spark plug tubes. On some valve covers, the tube seals can be removed and replaced, whereas on others, the entire valve cover must be replaced if the seals go bad.
When the spark plug tube seals fail, engine oil is able to enter the passageways and fill up where the spark plugs are mounted. This can cause an engine misfire or keep your engine from starting in the first place. Your check engine light would likely come on in that case as well. If you are driving, your engine could stall or fail to produce power. At the very least, fuel economy could suffer.
The spark plug tube seals on some engines are not replaceable apart from the valve cover into which they are pressed. On engines with serviceable spark plug tube seals, a technician must first remove the top engine cover and any other components - shields, guards, cables, emission control tubes, and the like - that prevent access to the valve cover(s). Then the valve cover needs to be removed carefully to prevent damage to the intake manifold or the mounting flange on the valve cover.
Once the valve cover has been removed, the seals are carefully pried out without damaging the seal seats. Typically, there are three to four seals in each valve cover. The new seals are lubricated and pressed into the valve cover in the proper orientation. The spark plug tubes are cleaned of any oil that intruded inside and the valve cover and other components are reinstalled and torqued to manufacturer’s specifications. Finally, the engine is started and checked for leaks.
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