At this point, I would recommend either a compression test on cylinder 3 or a combustion leak test or "bubble test" to determine if you are have a head gasket leak. I'm a bit puzzled by the ceramic being gone off the spark plug (?), But the combination of the antifreeze going missing, misfire on 1 cylinder, and parts replaced all kind of point toward a head gasket or a crack in the head.
If you have access to a compression tester, it screws into the hole for the spark plug and tells you if that cylinder is losing compression via a leak.
An easier method is the bubble test. Most of your parts stores carry a "block tester" which basically looks like a large eyedropper. You fill it with a test fluid, usually blue in color. You remove the radiator cap and start the engine After your thermostat opens and allows circulation of your coolant through the engine (usually withing a few minutes of starting), you place the tester into the cap opening on your radiator and draw air into it. If combustion gases are present, as they would only be with a cracked head, cracked block, or blown head gasket, the fluid will change color - usually yellow.
If this sounds like a bit more than you want to tackle, I would recommend a pressure test from your local repair shop.
P.S. - This may sound ridiculous, but have you smelled that spark plug? If it smells like antifreeze, I would definitely move on to the pressure test.
Monday, October 13th, 2008 AT 7:44 PM