I finally got around to replacing my spark plugs on Saturday. Mileage 67000 - I said I wouldn't do it until 100K but now that the Denso recommended plugs are Iridium Denso IKBH20TT rather than the original Platinum plugs and are available for £42 something a set I decided to plunge in. I also replaced the surge tank to manifold gaskets (3 off) - I bought them from RockAuto for about £19 including postage.
I've watched the video on YouTube but haven't been able to find a workshop manual type guide for this job, so I've put one together so you can print it off and keep it handy whilst doing the job.. It uses the Lexus instructions for removal of the coils (which then gives you access to the plugs), an exploded diagram of the relevant part of the intake system and some notes which might help. You have to remove the air surge tank (incidentally this is not the inlet manifold, as frequently mis-stated, here and elsewhere).
Most of the excess time for the job goes on a few frustrating things - the stupid inaccessible bolt on the back of the surge tank, the clip for the main power feed from the battery and a couple of other things. I'm sure that if it weren't for them the job could take an hour, but as it is, allow an afternoon!
At least I have the satisfaction of having saved a wedge of money compared with dealer charges and I know the job has been dome properly.
Incidentally I wondered whilst doing the job if the surge tank had really been removed only a couple of years ago when the fuel pressure sensor seal was replaced - or was it? I began to wonder if the dealer had just used some sort of sniffer, decided the seal was OK and done nothing. They had the car for a suspiciously short time and did a video of various safety checks too. It just felt as if no-one had been in there for 12 years, not just two years ago. Might be my imagination!
IS250 spark plug replacement.pdf