I am looking some help with a plug issue I am having. 1998 F-150 4.6L with 100k on it. The plugs on the passenger side keep getting the spark plug electrode closed by something. It always happens while towing a snowmobile trailer. It has happened 4 times in the last year & 3 times in the past two months. It runs fine when we put new plugs in it. The engine doesn't rap. They were the original plugs until this happened. The first time it happened we figured a piece of carbon let loose and closed it. Now it keeps happening and everyone I talk to is scratching their head. Did anyone hear of this before?

 

First question is what plugs are being used.
Compare with your plug spec in the owner manual.
It sounds like the nose projection is too long on the plugs being used.
Your living on the edge of engine destruction if a plug tip breaks off and does piston damage.
Only way out is to use a plug with shorter tip projection.
Good luck.

 

Bluegrass,
It happened to 3 of the stock original plugs from factory after 100,000 miles. I think there is something else causing the electrode closure but I have no idea what. I agree with your solution to the problem, I just can't figure out why the problem occurring with the factory plugs.

 

What is the SPECIFIC part number of the plug you are using?

Which SPECIFIC cylinder(s) has this occurred in?

Does it happen "all at once" or are they occurring one at a time?

 

Take a close look at the ground strap.
Is there any evidence of piston contact?
Does the strap look like it has been to hot? A whitish to clear look.
Things to think about is pre-ignition, Carbon buildup, running to Lean etc. These all raise combustion temperatures.
Under high loads the EGR never opens to cool the combustion temperatures.
Normally the piston clearance is more than enough to preclude that contact.
Try a set of colder plugs for trial to see if it makes a difference.
There are the lower cost copper tipped plugs that will work very well.
I have a set in now for about 40,000 miles in a 262,000 mile 4.6.
A plug that runs to hot can permote pre-ignition and a run-away situation to furthers higher cylinder temps.
You might look at the fuel pressure for being low, the Long Term Fuel Trims for being shifted Lean etc.
Their's always a reason these things happen.
Good luck.