A few months ago I changed the spark plugs on my 2000 explorer 4.0 OHV 88k miles. The #4 plug looked slightly different than the others, but nothing major. Within about a month, the check engine light came on. I read the code and it was P0304 (#4 cyl misfire). I pulled that plug and it looked orange. I figured it was either a bad plug or bad wires, so I replaced all the plugs (again) and the wires.

Ran great until last week. Check engine light on again with the same code and it ran like crap. I pulled the plug again, same wierd color. I changed the #4 and #5 wires to try to isolate that, but no difference. I installed a new plug in #4, and it runs perfect (which I would guess eliminates bad coil or injector), but I wonder what the heck is causing that cylinder to keep misfiring. My fear is a slight head gasket leak that over time is fouling the plug since the threads are a little rusty. The oil looks fine so no water leaks into the crankcase, but my coolant level was quite low. There is no oil or carbon build up on the plug and it is not super white like a lean condition.

Any ideas??? Thanks!!!

 

Try a compression test. There is also a cylinder balance test which drops the spark from each cylider one at a time. If it's an electrical issue, this test should support that. Most of the scanners above $50 should be able to run this test.

 

Correct me if I'm wrong, but I swear that 4.0's are known to have intake manifold gasket leaks.

Losing coolant points to that scenario....coolant is not a real good pal to the lower end of an engine. Would I sit around waiting for something to happen? No.

 

By 2000, hell by 2004, Ford adressed the intake manifold leaks with new gaskets with rubberized edges. But there is still a continued issue with the manifold bolts coming loose. Upper manifold bolts specifically. but this would cause a lean condition which has not bee reported yet, right?

 

Well, back about 6 months ago the check engine light came on and I scanned the codes. It was "lean bank 1" and the other code was "lean bank 2", both occuring at the same time. When I realized the original spark plugs were still in the engine, I replaced them, and that code never popped up again. The gap on the originals had increased over time to over .090" and they were all consistent on the gap and their color.

 

Maybe showing lean because of mis fire then. still worth it to check the torque of the intake bolts. But are you still having a #4 cylinder problem?

 

When I changed all the plugs about 6 months ago, the lean bank 1 and bank 2 codes were reset and they have not returned. BUT, since then, the code P0304 has been showing up and I've replaced the #4 cyl plug another 2 times because the engine was running like crap. Each time it was really orange in color and when a new plug was installed, the engine ran great. I can't figure out what is breaking down those plugs over time in that cylinder.

I will check the torque on the intake bolts.

 

You mentioned the rust color before. Coolant leaks inside the engine don't usually do this - not that I've seen. They usually just get white and you alos get a lot of white smoke out the exhaust. Do you have any bubbles in you radiator white the car is running? I mean a lot like the air pumps in a fish tank.

Then I was thinking if you have a coolant leak to the outside, it could be resting in the spark plug socket at the head. maybe even enough to get into the spark plug boot and cause a ground to the head on the outside, not through the electrode inside the combustion chamber. This might also give you rusted threads since the water is just puddling there. Just a thought.