Why does my 1999 Ford Ranger 2.5 four cylinder engine need 8 sparkplugs?
Ford threw this into the design of a 4 cylinder simply to make a cleaner engine. 4 plugs ignite the gas as it is injected during the intake stroke and it creates the regular operation of the engine. The other 4 are there to burn off excess gas that did not get burned during the exhaust stroke. It makes a cleaner burning engine, and allows maximum efficiency of fuel. The downside is having to buy 8 spark plugs and wires and replacing them all, especially the ones under the intake manifold. Nissan and I think Toyota also designed 4 cylinder engines with 8 plugs.
