I have a 1998 Ford Explorer XLT 4WD with the 4.0 OHV "Cologne" engine with 249,955 miles. The engine seems to run fine and there is no visible smoke in the exhaust.

But, the #4 plug gets fouled every six months or so.

  1. Will a hotter plug help?
  2. Is the engine worn out?
  3. Is it the rings or head that's bad?
  • Do you have (or can you get) a side by side comparison of the "bad" plug and a "good" plug? Reading what exactly you mean by toast would be a great first step in telling you what's going on. I'd be looking for a detailed pic of the electrode end of things. Remember, too, there's a reason they call these vehicles "Exploder" ... just saying. Aug 29, 2016 at 14:30
  • A spark plug costs, what, 4 bucks? I wish every vehicle I owned would go to 250k and have this problem. You could do a compression test. That could help determine if the rings are shot and seeping oil into the cylinder. Then the other thing would be an injector that somehow runs rich (leaking? who knows). The latter would make the vehicle run poorly, though.
    – justinm410
    Aug 29, 2016 at 16:04
  • 1
    Bad oil control rings will not show up on a compression test.
    – Moab
    Aug 29, 2016 at 21:31
  • @Paulster2 My assumption is that plug fouling is caused by oil consumption and the excessive oil consumption was either the rings or the head. Rings would mean major engine work. The head may not be so bad to replace. Can you determine the cause by looking at a fouled plug? Aug 30, 2016 at 15:07
  • @justinm410 AFAIK the 4.0 OHV engine is actually really reliable. It's the bastardized 4.0 SOHC motor that caused so much trouble (and contributed to the "Exploder" term). Aug 30, 2016 at 15:09

1 Answer 1


Pull the plug out of that cylinder and inspect it.

If you don't know what to look for, go to any local parts store and find a Haynes repair manual. Look in the back for an in-color page with pictures of bad-burning sparkplugs. That should help you determine what's fouling that plug.

Most likely the rings in that particular cylinder have worn down quicker than the rest. This could be due to many reasons. Something as simple as a few grains of sand could've gotten in there and worn down the walls or rings. They don't have to be worn a lot, just enough to allow oil to blow by which will slowly foul that sparkplug.

It wouldn't hurt to try a spark plug spacer/adaptor. You screw the plug into the spacer/adaptor and then screw the adapter into the engine where the plug goes. It backs the plug up a little ways from its usual seating so whatever chemical keeps fouling it can't get to it as easily.

  • 1
    Another common source of plug fouling is oil leaking past the valve seals, which can be much cheaper to replace than the piston rings. You can do it with the head on the engine, although you'll need a spring compressor and an air compressor adapter that can thread into your spark plug holes.
    – TMN
    Aug 31, 2016 at 16:10
  • @TMN second that.
    – George
    Jan 25, 2019 at 2:19
  • You could also put an adapter on your spark plug so it doesn't sit as far down in the engine.
    – TRIGGA
    Oct 26, 2019 at 20:35

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