My 2001 F-150 (big V8) has approx. 300,000 miles and is giving me no problems whatsoever. It doesn’t burn oil, leak, slip, misfire or anything. I was considering having a tune-up, but am afraid if I start messing with the thing at this stage in its life, I might cause more problems than good. I can just see getting new plugs and coils and having the computer and ignition system go crazy on me. My thought is to leave well-enough alone. Anyone else have a thought? I did put new tires on it.

As long as you use genuine OEM motorcraft replacement parts you should be fine. Id say go ahead.

Your fears are ungrounded. You’ll need to replace the spark plugs at some point or the truck will run like crap. At 300, 000, they should have been replaced a few times already. Worn plugs and bad wires will lead to coil failure, so delaying maintenance does cause more expensive repairs in the long run.

Your owner’s manual will tell you if the spark plugs are due or not. Are they?

Change 'em. But if you can, let some penetrating lubricant soak around them before you try. After 300,00o miles, they may be reluctant to come out.

Replace the plugs now

If you don’t, they will blow out, and take the threads and coils with them

Your engine is known for doing this

Don’t let that happen to you