The COP's are designed to handle reasonable amounts of moisture meaning moisture in the form of rain, snow, water blown over the engine from the cooling fan etc. When you force water through high pressure at various downward angles and from very close distances at them then you can have problems. I think what happens here is that over time the original COP's can simply fail from getting old or the boots deteriorate allowing moisture in some form into the sparkplug well. In the process of replacing these COP's the mechanic, vehicle owner, or whoever pulls the old boot looks inside (sometimes) maybe pulls the plug and changes it. Then slaps some dielectic grease (sometimes) on here and there, sticks the COP back in and figures its good to go and in most cases it will be good to go. However if there happens to be a little dirt that dropped down into the well preventing a good seal allowing moisture in or moisture that was not completely removed before the COP was reinstalled there is potential trouble brewing. It is a lot like putting a top on a pot of boiling water. The water evaporates rises to the lid on the pot cools and drops back into the pot. This same thing happens in the sparkplug well when the engine gets hot. The moisture becomes captive and eventually will rust the plug and distroy the COP. Sometimes this moisture will damage the COP within a very short time sometimes longer either way a miss developes. Auto manufacturers have no way of controlling how owners or mechanics service their vehicles after that vehicle leaves the factory. Most of the time if repeat failures are happening with COP's its because the sparkplug well was contaminated with dirt, water, or coolant when the new COP was reinstalled. It really doesn't take much captive moisture or dirt to cause problems as you can see. When changing the plugs or COP's you have to take a little time and clean the plug well out and make sure it is COMPLETELY dry. You wouldn't install a head gasket or any gasket without making sure the surfaces were completely clean and dry before the installation and there is really no difference when installing the COP's. A few extra minutes of preperation can save a lot of headaches later.
Last edited by DYNOTECH; 01-01-2011 at 11:28 AM.