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From Section 8 of the 1988 Fuel and Emissions manual. All engines except 2.5L and 4.3L (S/T)
(So basically all gasoline C/K engines.)
the pump does pump all the time when it's turning
Yes.
but when the valve is switched on, it pumps clean air from the filter (so it doesn't get plugged up with dust and dirt from inside the engine compartment),
No. The AIR pump never sources air "from" the air cleaner. It sometimes delivers air TO the air cleaner.
The pump has a built-in "centrifugal filter fan" that cleans the air before the AIR pump compresses it.
into the exhaust to add more oxygen to help the Cat fire off.
The air pumped into the exhaust manifold assists in burning the residual hydrocarbon and carbon monoxide in the exhaust stream. It
also helps light-off the catalyst.
That's a plastic valve and rubber hose, it wouldn't be able to withstand the heat from the exhaust if it were to back flow.
Of course. That's why there's check-valves at the exhaust manifolds--to prevent exhaust-gas flowing into the AIR system plumbing.
The diverter valve switches the output of the AIR pump between blowing air into the exhaust manifolds (cold, open-loop, when the computer is ignoring the O2 sensor) and blowing air into the air cleaner (warm/hot, closed-loop). (The air is diverted to the air cleaner primarily for noise reduction.)