Are they really more highly recommended than properly-gapped Champion copper core plugs for our coil-railed XJ's? Don't mean to hijack, just curious.
Yes. The OEM recommendation switched from (IIRC) Champions to NGKs a few years ago.
FWIW, the most troublesome plugs I've ever used across a range of vehicles have been Champions; about the only place I've ever seen them work even reasonably well is in air-cooled VWs. Their truck plugs were one of the worst things I ever installed in my XJ.
Back on track: just in case heat soak is the issue here, the only reliable method I've found to deal with it is as follows:
- Start the XJ. Check the temperature gauge once the engine is running.
- If there is no stumbling, drive normally.
- If there is stumbling, chances are the temperature gauge will be reading above 210degF (but hopefully
not in the oh-crap-it's-overheating zone). If this is the case, let the XJ idle - and I mean idle; don't touch the accelerator - until the gauge gets back down to or just under 210degF; by this stage, the stumble should have cleared itself and it's OK to drive again.
Trying to drive it while it's stumbling will almost certainly result in a code being thrown and the CEL coming on. Just let the heat soak work itself out and you should be OK, assuming no further problems exist.