Automotive Air Pump (Function and Diagnosis)
Article by Mark Trotta
Introduced in the 1960's, automotive AIR pumps, also known as smog pumps, are powered by an engine-driven belt along with other accessories, such as alternator, power steering pump, and air-conditioning compressor. Early versions were driven by a single V-belt.
Some later versions were electrically operated.
Pictured: belt-driven smog pump
What Does An Air Pump Do?
The function of an AIR pump is to help lower tailpipe emissions. It does this by adding clean air into a car's exhaust until the engine reaches operating temperature. Once the exhaust gets hot enough, the extra air isn't required, and a valve opens to divert the air away from the exhaust.
Problems With Smog Pumps
They can and do go bad. Once moisture gets inside them, they will seize up. When this happens, the pulley stops turning and it 'throws' the belt.
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Symptoms Of a Bad Smog Pump
Check Engine Light illuminates
Vehicle fails its emission inspection
Engine runs rough, stalls, hesitates, or idles too low
Reduced engine power and poor acceleration
A 'howling' noise from under the hood
Although a bad unit may reduce engine power, give poor acceleration, or cause stalling, rough-running, hesitation, or rough idle, these symptoms are the same as several other engine problems (carburetor, tune-up, etc.)
A diagnostic check with a Code Scanner can pinpoint the general cause of the problem. But before you do that, check for a leaking or clogged hose (or a missing belt).
If the cause is indeed the smog pump, it can be replaced with either a new or remanufactured unit.
Does My Car Need an Air Pump to Run?
Unless other damage was done while removing, a car will still run without one, but may not run correctly, as the engine was designed to run with one attached and working.
NOTE: Once a belt is removed, but the rest is still hooked up to the exhaust, it's now sitting dormant, and more apt to freeze up.
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AIR Pump Removal
Today, we know that a functional air pump robs very little power from an engine. But because most pre-1980's cars didn't have a serpentine belt, many have been eliminated.
Smog Pump Eliminator Bracket
For off-road racing fans, you can fabricate your own eliminator bracket, or buy a kit specifically for your year, make, and model.
Pictured above is National Parts Depot # M-8604-A50 smog pump eliminator bracket. It fits many 1979 through 1993 Ford models (including Mustang) when catalytic converters are not used. This competition style air pump bracket is advertised as off road use only and is a direct bolt on replacement for stock air pumps with serpentine belt systems.
For racers who are concerned about every ounce of weight, removing a smog pump will save a few pounds.
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Potential Issues With Smog Pump Removed
Aside from legalities...
Without the pump, extra air is no longer leaning out the air-fuel mixture, so your O2 sensor may keep the system in an open loop longer without the smog pump.
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Missing Air Pump
Many cars from the late sixties and seventies are missing the factory-installed air pump. There are basically two reasons smog equipment is removed off old cars.
The most common reason is that the part was not working, or perceived to be not working.
The second reason people remove an air pump is for looks. I've heard more than classic car owner say, "I wanted to clean up the engine compartment".
If your old car is missing the air pump, it's probably also missing the exhaust manifold tubes, check valves, diverter valve, mounting bolts and brackets, as well as hoses and clamps.
Those missing parts can be hard to find, but can often be taken from a similar year and model donor car.
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By the early eighties, car manufacturers were producing more efficient fuel-injected engines along with improved ECM's, which regulated engine functions precisely. Soon after, the amount of unburned fuel into the exhaust was greatly reduced, and the need for air pumps faded.
Collector Value
Classic cars are worth more in factory-stock condition--which means having the correct emissions equipment installed and operative.
De-Veining a Smog Pump
There are quite a few owners who have removed the innards of the pumps, then re-installed them to keep the factory look.
Some time ago, MotorTrend published an informative article on How to De-Vein a GM Smog Pump so it will spin freely but not pump. It also details the various ways of plugging the exhaust ports.
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