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#1 plug fouls - 1998 Mercury Grand Marquis

Why does my #1 plug keep fouling out ?

oil getting into the cylinder via bad oil contrtol rings, or maybe even valve guide seals. I’m sure others might have some more ideas.

1 Like

I was waiting for more knowledgeable folks to reply, but it has been 14 hours so I will give you my GUESS.
First, is your check engine light on? That may give you some clues.
Bad/weak coil for that plug… If your coils are from 1998, replace the entire set. A compression test might also be beneficial prior to replacing the coils. That cylinder may have other issues that would require expensive repairs.

Agree with @PvtPublic as the most likely. It could be a bad coil as @Purebred says.

If the plug is oily, rings or valve seals in that cylinder are bad. I wouldn’t spend money on a 20 year old car, just keep changing the plug.

If the plug is black but not oily and/or the check engine light is on, change the coil.

I suspect you don’t work on the car yourself and a mechanic keeps telling you the plug is fouling, right? If that is true, it isn’t a coil or he would have replaced it by now.

Fouling usually means oil fouled. The car is 20 years old with ??? miles on it.

A compression should be run but a quick test with a vacuum gauge could tell you whether or not the compression test is necessary.
The guy who is changing the fouled plug should be the one telling you what is going on. It’s not that difficult.

5014999_rnb_42009_pri_larg would this help?

Install a hotter spark plug in that cylinder to prevent it from fouling.