1988 454 belt

[PlayingWithTBI]

there's a diverter valve that switches the air output from the exhaust manifold (engine cold--open loop when the O2 sensor reading doesn't matter--helping to light-off the catalyst) to the dirty side of the air filter in the air cleaner housing.

That's not the way I see it, the pump does pump all the time when it's turning 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), into the exhaust to add more oxygen to help the Cat fire off. 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. Anyway, that's the way I see it...

 

[Schurkey]

X8836_1988_GMC_Light_Duty_Truck_Fuel_and_Emissions_Including_Driveability

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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.)

 

[PlayingWithTBI]

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.

I stand corrected, just read section 8 you posted and see it switches back and forth like you said. After seeing it, that makes sense since it would be dead heading the pump in my scenario. Thanks for that.

Anyway, it doesn't pump air into the exhaust during Closed Loop (when working properly) and throw off the O2 sensor.

 

[johnjr]

I just wanted to follow up that the belt for a 91’without air pump works. The only downside is I ordered it from Amazon (Gates) and it was probably 20 years old and made in the USA. Hopefully it will last. The ad on Amazon says made in Chin, but the vendor probably bought up old stock.