Ask and ye shall receive! MY FSM says replace every 30,000 miles. That's for 93 Jeeps XJ and YJ, 2.5 and 4.0. Same thing for the rest of my Wrangler FSM's 98 - 04, replace plugs every 30,000 miles. I ain't sayin' I do it but that's what's recommended. <img src="/forums/images/graemlins/cheers.gif" alt="" />

I've changed spark plugs/igniters on everthing from F-16's to go carts and this is my professional opinion. I have seen plugs strip out the threads in aluminum heads. The official cause is somebody trying to remove the plug with the engine still hot. Let it cool down "completly" and use a small amount of anti-seize compound when installing the plugs and it will never happen. As far as how long to run a spark plug; the manual will not lead you wrong. On my rigs with out a distributor, I choose to pull plugs at 50k to inspect and regap them, then again at 100k to replace them. The only reason I pull mine at 50k is to keep the gap with-in specs; gap gets too large and it can cause undue stress or component failure in the ignition system. On my old 91 wrangler, with a distributor, I replace them at 30K-50K.

I kust checked the owners manual on the 20076 Trailblazer I just got. It says change the spark plugs every 100,000 miles. It doesn't say anything in the maintenance schedule about checking them more often. Does mention that the gap is .040" -- that's in a 5.3L V8.

Every spark plug that I replaced at 100k that had not been regapped at around 50k, had a gap burnt out to almost double the specs. That alone is why one will run so much better after the plugs are changed out; the gap is back with in specs.

99 Cherokee Sport, with the 4.0L and the Factory Owners Manual says 30K miles to Replace the Plugs. As does my Haynes repair manual.

John ...Mine does not run better... Using the word "better" implies the thing was misbehaving in the first place. With modern computers and coil setups the larger gap is taken into stride by most of these vehicles. A larger gap in an older ride will cause misfiring from lack of a strong enough coil. And yes, if the thing is acting up go hahead and change out the plugs. But if it is doing good leave it alone.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

Who cares if you chage them regularly or leave them in for 100k, the bottom line is that plugs wear and a responsible shop should never take the time to take out plugs and "regap" them, if you remove them...change them.

Look at owners manuals of rigs built 10-20 years ago, being as most here are not driving 07 trucks.

SD