For a while I had been kind of dissapointed with the PDI Firepower Ignition setup I had: Wires, Cap/Rotor, Distributor. But I think my blame was misplaced. It appears my plugs may have been the problem.
You can't find a person who recommends platinum plugs for YJs and TJs, so I never bothered with those. I had instead upgraded -- or at least what I thought was an upgrade -- to Iridium XP plugs when I installed the PDI setup. I figured fine Iridium electrode, better spark, made for the Jeep 2.5 4banger, enough said.
I even went so far as to gently gap these Iridium XP plugs to .060" figuring the PDI system could bridge the gap no sweat.
Quite a while passed, I had messed around with a few other engine modifications, but the ignition performance was always bugging me. I was thinking: "D@mn I played close to $200 for the full PDI kit and these Iridium plugs and I might has well have stayed with the stock setup.
I messed around with different fuel octane grades to see if this would help. No luck. It was as if the timing was retarded by something.
In the last few weeks I finally said: "Alright what plugs do most people use on the Wranglers, PDI system or no?" I saw Jeffy here was using copper core and a query at jeepforum returned the same thing: copper core champion, champion truck plugs or autolite copper core.
It seemed like a step backwards using old-school copper core plugs. But I was like alright, maybe these exotic meteorite plugs I had were less than ideal for an older Jeep. So I drop a mere 8 bucks and bought a set of 4 Champion copper core plugs.
I gap them to .060" and installed them. Once the engine finally adjusted, I noticed the Jeep was running a lot better than with the pricey Iridum plugs. Just seemed like it had better torque and I wasn't downshifting where I normally would on some roads and hills. I was happy.
With the ignition -- now with copper plugs -- seemingly working more like I expected, I considered that perhaps the spark was siginificantly hotter with these plugs at .060". With the iridiums, fuel octane didn't seem to make any difference. But with the copper core plugs, there did seem to be an improvment when I went from 87 to 89.
Now none of this is scientific of course. Iridiums might work fine at the stock gap of .030" or .035" in a YJ/TJ and with a stock or PDI system (I kept the plugs and can always mess around with them). And the engine may still have been adjusting to the plug change, not the change in fuel octane.
But anyway, If I was talking to somebody with a YJ/TJ Wrangler who just bought a PDI ignition, I'd say use copper core plugs or copper truck plugs gapped at .060" (This basically a ditto of Jeffy's PDI install write-up). Even though I have no scientific basis for avoiding Iridium or other exotic metal plugs in the older Wranglers, I'd still wouldn't be surprised if you should stay clear of them. And I'd also say you may or may not want to try a higher octane fuel when you are running your copper core plugs at a .060" gap.
