tier 1, [which was the s15g, which did not require 100% of the fleet, but everyone went along with 100%] was ship dates Jan. 2003 ended dec 31, 2003. tier 2 started Jan 1 2004. tier 3 was jan 1, 2007.
there were about 6 months of a tier 2 (impco spectrum) system 4g63/4g64 engine powered sold before MCFA went to the K21, K25 manufacturing/shipping product in 2004. in Jan 2005, MCFA made a change from a relative pressure sensor to an absolute pressure sensor on the throttle body, that was not a "tier" change, but was made to improve performance of trucks that moved to different altitudes (ran great in the shop in the valley, but about 3000 feet, not so good, and vice vs)
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