I recently had my 86 runner smogged and it passed every category easily except the idle hydrocarbon (passed by 1 pt!). I had it tested at the end of the holiday weekend after 500 miles of driving, then waiting in line 20min (engine off), tank was maybe 1/8 (chevron 91, as usual, but from out of town, so no ethanol/mtbe). I replaced the plugs w/ Denso, cleaned TB, and checked timing 5K mi ago. Cat was replaced and CA smog legal header installed 55K mi ago. For the other categories (CO, NOx at idel, CO/NOx/HC at 25mph), the vehicle easily passed (numbers are 50% of the allowable). Any ideas on what would lead to a high HC idle reading? I'm afraid next time, I might not be so lucky.

Engine is a 22re has 183K mi on it, new cam/rocker arms/valve seals/exhaust guides at 125K mi, head pulled and cleaned then, too.

Some possible causes.