My 99 4x4, 4.0L Explorer keeps going through Spark plugs on the # 4 cylinder. This is the 3rd time this has happened in less than two years. The plug will cause miss-firing and then the Engine Service Light goes on. When the plug is pulled it lookes extremely black and dirty.. as if it was two year old but actually it only 6 or 7 months old. The other plugs appear normal. The Fuel Injector system was cleaned less than two months ago. Any suggestions?

Regards,
Vega

had a customer that had the same problem,#4 cyl with bad plug.he drove it like that and would come in every 3 months and we would screw a new plug in it. then he traded it off.never got to fully diag.you could have a bad (leaking) injector.weak #4 wire or coil,or a cyl that is using a bunch of oil.I dont think you are alone,have seen a few people on differant message boards with the same problem.

Sounds to me like a new coil, or maybe a compession test is nessasary.

There are two things that foul a plug, oil and gas.

If it's oil it's a bad valve seal on the intake valve , or a bad ring. You would do a compression test or leakdown test to see if it's rings. Oil fouling ususally leaves a black wet looking plug, much like a plug in a two stroke engine (chainsaw or motorcycle).

You could also have a bad fuel injector with a slow leak. This floods the cylinder and when starting causes a lot of deposits to load up on the plug. You would probably notice hard starting after your car sits overnite if you have this condition. (As fuel leaks, the fuel system losses pressure, and when you crank the engine in the morning, it takes the fuel pump a couple of seconds to restore pressure. In this situation the other cylinders are too lean because of a lack of fuel pressure.)

Starting after the engine is warmed up is OK because you haven't had the engine shut off long enough to lose fuel pressure.

Had this situation on my 99, and a new injector fixed it. They are about $90.00 apiece

My nieghbor had the same problem with #2 cylinder ruining plugs all the time.

What we figured was a bad injector, it certainly wasn't bad valve seal and he wasn't losing ANY oil. To test that theory we traded the #2 injector with a different # injector. The problem then stopped at the #2 cylinder and started at the injector we traded with. Problem diagnosed. We then bought a new injector and installed it. Problem solved and hasn't returned.