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how is the test run?<snip>
A copy of the SNAP test document - SAE J1667 - is attached.
The driver pulls into the test bay. Equipment is used to measure engine RPMs at idle.
Upon instruction, the driver depresses the accelerator to the floor and holds it until instructed to release; usually three or four of the longest seconds of your life. The RPM governor is the only thing between engine destruction.
There are a few preliminary "snaps," then the probe is inserted into the tail pipe and three SNAPs are measured for opacity.
In WA, diesels starting at MY 1993 are allowed to blow a 40. Prior 1993, they're allowed to blow a 60. Vehicles are required to be tested every other year.
The same vehicle tested twice in the same bay will score differently in back-to-back tests; but in my personal experience, not wildly differently.
Vehicles test better in the summer - think 90 degrees F - than they do in the winter.
Mack, for registration year 2014, take your test the August prior to expiration, well before the state sends you the love note indicating you need a test before you get your tabs for 2014.
The test results in WA are good for 365 days.
You pass "easier" in the summer because the ambient temp & summer blend fuel promote more complete combustion.
You can send for your tabs well-before they expire (6 months, I think); well before the test expires.
-Jon