I just got my car checked for emissions again so that I can renew my tags. We're on a two year cycle here so that gives plenty of time to mess around and get it screwed up . I had my car running greatly and went straight to the emissions check station without doing anything to help it pass. It failed, of course, so I spent the next few hours dinking around with the idle air bypass and my fuel pressure and failed three more times. So I went home and realized that I was using my MSA AFM that I had bought new nine+ years ago and that it had always run rich. I don't know why, it just does. No matter how far I back out the idle air bypass I couldn't get it to lean out. I pondered drilling out the bypass passage to let more air past the blade but it's not an easy operation. So I pulled my last old AFM down from the shelf, an original 75/76 AFM, and installed it. Then I leaned out the idle air bypass until removing the oil filler cap caused it to stumble to barely running. I went back to the test center the next day and passed on the first shot through.
I realized as I was tuning that I could remove my oil filler cap and the engine would not stumble, when I had bad results. I had forgotten about that basic "running rich" test. Even though it was not noticeably rich, it wouldn't pass, and I could leave the cap off with no problems.
Anyway, the short story is that if you can remove your oil filler cap and your 280Z doesn't notice, you're probably going to fail the idle emissions test. So that's the tuning tip. If you're in CA with the 2500 RPM requirement the same test might still help.
And even though I could leave the filler cap off, it still only failed CO. Not HC. It was very close.