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Spark Plug Question...


Active Member
February 25, 2006
Reaction score
City, State
South Dakota
Year, Model & Trim Level
02 XLT
Changed the plugs yesterday in my 02 4.0. Not a big deal took me about an hour start to finish.

My question is that the old plugs on the left side were gapped at about .90+. But the right side was only at about .75-.80.
Is this common to find with old plugs? I guess I have never come across it before and was just curious as what the cause might be in the difference from one side to the next.

The X has 100,000 on it now and I bought it 2 years ago with 92,000 so I don't know if they are the original plugs or not.

One thing to remember is that half of the spark plugs fire from the top of the plug down, and jump the gap to ground, and the other half fire backwards, from the bottom up. This is probably the reason why some have different wear patterns than others, or may be worn further. This, like many DIS systems is a "waste spark" system, where two cylinders get a spark at the same time from the same coil. One cylinder gets its spark on the compression stroke where it should, and the "wasted" spark goes to the cylinder that's on its exhaust stroke, where it does nothing. I'd have to sit and think about the firing order for a while to determine if it's possible that all the plugs that fire backwards happen to be on the left side of the engine. The 4.0's firing order is 1-4-2-5-3-6, and Ford numbers their cylinders differently than GM and Chrysler...Cylinders 1, 2, and 3 are on one bank, and 4, 5, and 6 are on the other. If I thought about it long enough, I'm sure I could visualize which cylinders are on their exhaust stroke when the other one on the same coil is on its compression stroke and conclude this.

The reason for a waste spark system is simplicity...A V6 will need only 3 coils instead of 6. GM has been doing it for over 20 years, and most domestic and a few imports have followed suit. Only way this really won't work is with coil-on-plug systems. On these, each coil fires in the correct polarity, from the top down.

Thanks Cornburner you learn something everyday around here.