Took our 1994 Toyota Land Cruiser for its annual emissions test. It passed the tailipipe emissions but failed on the gas cap pressure test. Drove 3 miles to a parts store, got a new cap and returned for a retest. Second test was on the same dynamometer with same technician. As you can see the results are very different. I'm pretty sure the gas cap plays no role in tailpipe emissions, just in gasoline vapor emissions. Anyone have any explanation? Emissions testing not reliable at all?

Limits:
HC--2.0
CO--20.0
NOX--4.0

Test one

HC--.7660
CO--16.5365
NOX--3.1477

Test two

HC--.4176
CO--7.6637
NOX--1.7369

Test one passes but pretty close on some numbers. Test two is clean as a whistle. Engine was at same temperature in both cases. No other differences besides a new gas cap and 6 additional miles.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13