For a full explanation of emissions system, read this article.

This is how to fix a seized air pump for nearly nothing:
1) break the three 10mm pulley bolts
2) remove the belt
3) remove the pulley bolts
4) loosen the two hose clamps (5/16", 8mm, flathead) and push the hoses back
5) remove the 9/16" (14mm) upper bolt
6) remove the 1/2" lower rear bolt
7) pry the pump case forward to open the clamp sleeve around the upper bolt hole, and remove the pump
8 ) reinstall the pulley bolts finger-tight
9) spray penetrating oil liberally into the hose nipples & the two 3/16" vent holes in the back of the case, rolling the pump to distribute the oil thoroughly; allow the oil to soak in for several minutes
IF ANY METAL CHUNKS OR FILINGS FALL OUT, replace the smog pump
10) use a propane torch to warm the case just so it's hot to the touch; don't melt any plastic or set fire to the oil
11) holding the assembly by the pulley, strike the large boss running across the bottom of the pump back & forth (sideways) to break the impeller loose of the corrosion inside; apply more oil as necessary
12) continue rotating the pump at least a full turn with the hammer; when it's loose enough, spin it by hand until it turns smoothly
13) dump as much oil as possible out of the hose nipples
14) if you plan to ford deep water, plug the vent holes with silicone rubber or RightStuff

UPDATE: After several months & a few tanks of gas, it's still working fine.
UPDATE: After a few years of intermittent use, being sandblasted & painted, it's still spinning smoothly.
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See also:
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Owner caption November 2009 · Photo 2996 of 5000