OnBoard Diagnostics: Also known as OBD. Mandatory since 1981 on U.S. cars.

There are two kinds of OBD systems. OBD1 is for cars made from 1981 to 1995. OBD2 is for 1996 and newer vehicles. Since the first one falls within the smog-era, that’s what we’re covering here, through ’83.

General speaking, OBD is a computer system that runs your engine for lower emissions. Several components entail the overall system which are…

Control Module. This is the computer brain of the system which make calculations and adjustments to the carburetor and distributor, based on what it sees in sensors. It has a full name of electronic control module (ECM), or powertrain control module (PCM). These units are mounted in a cool dry spot, such as inside fender or in an interior kick panel. The modules also store trouble codes that can be deciphered on the check engine light (see next).

Check Engine Light. Also known as the Service Engine Soon Light. When this illuminates, it means there’s a trouble code stored in the module ready to tell you what sensor or solenoid is defective.. By counting the flashes you can decode the issue. GM vehicle codes can be read by shorting two specific terminals on the OBD port under the dash. Fords back in the day were tripped by using a vacuum pump on a certain hose under the hood. Chryslers were tripped with a series of on-off motions with the ignition key.

Feedback Carburetor. These carburetors facilitate being adjusted by the module. There is a mixture control solenoid inside them that the computer can move around to make adjustments.

Feedback Distributor. Like the carburetor, this is a distributor that the module can make adjustments on, though they’re regarding spark timing here.

Throttle position Sensor. Also known as the TPS sensor. This sensor is inside the carburetor. It tells the module how much accelerator input is being requested.

Coolant Temperature Sensor. This sensor is located in the thermostat housing or one cylinder head. It tells the module how hot or cold the engine is.

Manifold Absolute Pressure Sensor. Also known as the MAP sensor. This sensor is located on top of the intake manifold usually off to one side or the other. It tells the module the amount of vacuum that the engine is producing.

Oxygen Sensor. Also known as the O2 sensor. This sensor looks for the amount of oxygen inside the vehicle’s exhaust so the module knows to richen or lean out the fuel mixture.

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Evaporative Emission Control: Also known as the EVAP system. Common starting in 1972.

Gasoline is always evaporating! The idea behind this system is to catch some of those vapors when they escape from your fuel tank. Early systems such as those found in smog-era cars catch only part of the evaporating fuel, but later systems incorporate a better fuel cap that completely seals the system.

The main component of the system is the vapor canister. It contains a replaceable charcoal-impregnated filter element to let outside air in, while keeping fuel vapors inside. The canister is round or squared in shape and is mounted near the radiator. This location keeps gas vapors safely away from ignition and exhaust components. This canister collects vapors that evaporate from your tank though a network of hoses and valves.

Once inside, the vapors wait to be pulled into the engine for a proper burning. The EVAP system keeps hydrocarbons from escaping into the outside air. Hydrocarbons are raw fuel and are obviously not good to breathe in.

The horsepower and weight penalty of this system is negligible. In theory, you might even get better fuel mileage by keeping the system in your car.

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Thermostatic Air Cleaner: Also called T.A.C. or Thermostatically-Controlled Air Cleaner. Started to be be common in 1972.

This system existed to heat the incoming air on a cold engine. It basically lessened the need for the carburetor’s choke to be engaged fully, effectively leaning out the air/fuel ratio. This process helped to control the levels of hydrocarbons exiting the exhaust. It also helped cars warm up faster and run better in cold temperatures.

The entire system consisted of a ‘heat stove’ on one exhaust manifold, a riser tube coming off the stove, a few vacuum lines, a valve in the air cleaner snorkel, and a thermostat valve tapped into the car’s cooling system and/or inside the air cleaner.

Basically, heated air came from the stove and went into the bottom of the air cleaner snorkel. When the system’s thermostatic valve got to a predetermined temperature, it would let engine vacuum open up the regular valve in the air cleaner snorkel. This let in cooler outside air once the engine.

The advent of electronic fuel injection negated the need for this system since fuel control became more precise. Replacing the system with a standard open filter type air cleaner, to this day, is still frowned on by emission inspectors. Just don’t scrap the original system if this is what you plan on doing.

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Air Injection Reaction: Also known as A.I.R. or ‘Air Injection Reactor’. Became common in 1974, but not all manufacturers utilized an A.I.R. system.

The A.I.R. system is probably the number one ugly thing that was done to engines of the smog-era. The core of the system is really not that ugly and is usually tucked out of view. It’s a belt-driven piece called an air pump (also know as smog pump). It’s designed to push fresh air into the exhaust manifolds. Sometimes the catalytic converter gets a supply line for air as well.

The ugly parts of the system are the yards of rubber air hose & vacuum line, 2 or 3 one-way check valves, diverter valve, diverter muffler, manifold tubing, and on some cars an electronic control box.

The overall process effectively adds oxygen to unburnt and partially burnt fuel. It completes the burning process where the car is the hottest— exhaust manifold and catalytic converter. This helps lower the carbon monoxide and hydrocarbons in the exhaust.

These systems help a great deal with controlling pollution, but are not very friendly to the necessity of having a clean engine compartment.

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Exhaust Gas Recirculation (EGR): Exhaust Gas Recirculation System. Became common by 1973.

On an EGR engine, there are extra exhaust ports inside the cylinder heads that exhaust gases flow back into the air/fuel intake stream. The flow of these gases is controlled with a vacuum actuated piece called an EGR valve. It completes a connection between the intake manifold’s EGR passages and air/fuel passeges.

The EGR valve itself sits on the top the intake manifold, and is connected to a manifold vacuum port. Controlling this connection is a coolant temperature sensor that will only let vacuum through when the engine is properly warmed up.

The advantage of this system is a lesser amount of ‘oxides of nitrogen’ (NOx) get released from the tailpipe. Some engines actually will actually run cooler with an EGR valve too.

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Catalytic Converter: Debuted in 1975. These devices get red hot on the inside and essentially burn the carbon monoxide and hydrocarbons left over from the engine’s combustion process. They are also known simply as a ‘catalyst’.

‘Pelletized’ was the early style of catalyst. Exhaust gases flow through thousands of pellets which are coated with platinum and palladium. This style genuinely snuffs horsepower, as much as 20 or more on the average 70s car.

‘Screen’ style was the later version which is still used on modern cars. Instead of pellets there are layers of screens inside. The screens themselves are coated with the platinum and palladium. This style creates less backpressure and doesn’t decrease horsepower much on a street car.

Leaded fuel will destroy a catalytic converter. This is why ’75 and up American cars also have a fuel inlet restrictor on the gas filler neck that allows the nozzle from only an unleaded fuel pump to fit.

Back in the day, it was a popular modification to remove the catalytic converter and fuel inlet restrictor on cars just to be able to save money by using cheaper leaded fuel. This change became obsolete when leaded fuel was discontinued in the U.S.

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Positive Crankcase Ventilation (PCV): Also known as a PCV valve. These became common around 1967. Simply put, it’s a one-way valve that keeps engine ‘blow-by’ gases from entering the outside air.

All engines, no matter how new or how efficient they are, allow some unburnt gases past the piston rings and into the oil pan. This creates pressurized blow-by that needs to go somewhere.

Before 1967, most engines just had ‘drag tubes’ that exited the valve covers to let these gases get sucked out and blown into the outside air. Not only did gases come out, but a microscopic mist of oil did as well.

On a PCV system, there is a PCV valve, usually on one valve cover. It’s fitted in via a grommet most of the time. The other end is connected to an intake manifold vacuum port. Coming out of another point of the engine is a tube connected to a small filter that allows the exchange of gases with the valve. This small filter was usually found inside the air cleaner assembly. The overall arrangement allows unburnt gases to come out of the oil pan and get burned inside the engine. This process saves a great deal of hydrocarbons from getting to the atmosphere due to these gases being mostly unburnt fuel and discarded motor oil.

Believe it or not, this is actually a good system to have. The extra vacuum inside the crankcase is said to help with piston ring sealing. This means more reliable performance and less oil being burned.

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