Do you know if your truck is a 49 state Federal (EPA) car or a CA compliant Truck? Your under-hood emissions sticker still visible??
Here is some descriptions from the manual and some pics:
The AIR system injects compressed air into the exhaust system, near enough to the exhaust valves to continue the burning of the normally unburned segment of the exhaust gases. To do this it employs an air injection pump and a system of hoses, valves, tubes, etc., necessary to carry the compressed air from the pump to the exhaust manifolds. Carburetors and distributors for AIR engines have specific modifications to adapt them to the air injection system. These components should not be interchanged with those intended for use on engines that do not have the system.
A diverter valve is used to prevent backfiring. The valve senses sudden increases in manifold vacuum and ceases the injection of air during fuel–rich periods. During coasting, this valve diverts the entire air flow through a muffler and during high engine speeds, expels it through a relief valve. Check valves in the system prevent exhaust gases from entering the pump.
The CCS system is a combination of systems and calibrations. Many of these are not visible or serviceable, but are designed into the engine. Originally the system was comprised of special carburetion and distributor settings, higher engine operating temperatures and a thermostatically controlled air cleaner. In later years, the thermostatically controller air cleaner was used independently of the other settings on some engines. Likewise, some engines used the special settings without Thermac. In 1970, the TCS system was incorporated and the entire system was renamed CEC in 1971. The name reverted to TCS in 1972. In 1973, EGR was also added to the system.
The various systems, TCS, CEC and EGR are all part of the Controlled Combustion System.
The 6–cylinder and 1975 Mark IV (big block) V8 EFE systems consist of an EFE valve mounted at the flange of the exhaust manifold, and actuator, a thermal vacuum switch (TVS) and a vacuum solenoid. The TVS is located on the right hand side of the engine forward of the oil pressure switch on 6–cylinder engines and directly above the oil filter on the Mark IV V8. The TVS is normally closed and sensitive to oil temperature.
The small block V8 and 1976 and later Mark IV EFE system consists of an EFE valve at the flange of the exhaust manifold, an actuator, and a thermal vacuum switch. The TVS is located in the coolant outlet housing and directly controls vacuum.
In both systems, manifold vacuum is applied to the actuator, which in turn, closes the EFE valve. This routes hot exhaust gases to the base of the carburetor. When coolant or oil temperatures reach a set limit, vacuum is denied to the actuator allowing an internal spring to return the actuator to its normal position, opening the EFE valve.
The EGR system and valve were introduced in 1973. Its purpose is to control oxides of nitrogen which are formed during the peak combustion temperatures. The end products of combustion are relatively inert gases derived from the exhaust gases which are directed into the EGR valve to help lower peak combustion temperatures.
The EGR valve contains a vacuum diaphragm operated by manifold vacuum. The vacuum signal port is located in the carburetor body and is exposed to engine vacuum in the off/idle, part–throttle, and wide open throttle operation. In 1974, a thermo–delay switch was added to delay operation of the valve during engine warm–up, when NOx levels are already at a minimum.
There are actually three types of EGR systems: Vacuum Modulated, Positive Exhaust Backpressure Modulated, and Negative Exhaust Backpressure Modulated. The principle of all the systems is the same; the only difference is in the method used to control how far the EGR valve opens.
In the Vacuum Modulated system, which is used on all trucks through 1976, and some models thereafter, the amount of exhaust gas admitted into the intake manifold depends on a ported vacuum signal. A ported vacuum signal is one taken from the carburetor above the throttle plates. Thus, the vacuum signal (amount of vacuum) is dependent on how far the throttle plates are opened. When the throttle is closed (idle or deceleration) there is no vacuum signal. Thus, the EGR valve is closed, and no exhaust gas enters the intake manifold. As the throttle is opened, a vacuum is produced, which opens the EGR valve, admitting exhaust gas into the intake manifold.
In the Exhaust Backpressure Modulated system, a transducer is installed in the EGR valve body, reacting to either positive or negative backpressure, depending on design. The vacuum used is still ported vacuum, but the transducer uses exhaust gas backpressure to control an air bleed within the valve to modify this vacuum signal. Backpressure valves are used on all light duty emissions California and High Altitude engines in 1976 and 1978, and on most engines in 1979. The choice of either a positive or negative backpressure valve is determined by measurement of the engine's normal backpressure output. Negative valves are used on engines with relatively low backpressure; positive valves are used on engines with relatively high backpressure. The choice of valve usage is made at the factory, and is nothing for the backyard mechanic to worry about; however, if the valve is replaced, it is important to install the same type as the original. The difference between the three valves (ported, positive, or negative) can be determined by the shape of the diaphragm plate; your Chevrolet or GMC dealer will be able to match the old valve to a new one.
On 6–cylinder engines, the EGR valve is located on the intake manifold adjacent to the carburetor. On small block V8 engines, the valve is located on the right rear side of the intake manifold adjacent to the rocker arm cover. Mark IV V8 EGR valves are located in the left front corner of the intake manifold in front of the carburetor.
EVAPORATIVE EMISSIONS SYSTEM Introduced on California vehicles in 1970, and nationwide in 1971, this system reduces the amount of escaping gasoline vapors. Float bowl emissions are controlled by internal carburetor modifications. Redesigned bowl vents, reduced bowl capacity, heat shields, and improved intake manifold–to–carburetor insulation serve to reduce vapor loss into the atmosphere. The venting of fuel tank vapors into the air has been stopped. Fuel vapors are now directed through lines to a canister containing an activated charcoal filter. Unburned vapors are trapped here until the engine is started. When the engine is running, the canister is purged by air drawn in by manifold vacuum. The air and fuel vapors are directed into the engine to be burned.
Positive Crankcase Ventilation (PCV) System - This system draws crankcase vapors that are formed through normal combustion into the intake manifold and into the combustion chambers to be burned. Fresh air is introduced to the crankcase by way of a hose connected to the carburetor air cleaner. Manifold vacuum is used to draw the vapors from the crankcase through a PCV valve and into the intake manifold.
The Thermostatic Air Cleaner (Thermac) is on all gasoline engines. This system uses a damper assembly in the air cleaner inlet, controlled by a vacuum motor to mix preheated and cold air entering the air cleaner. This is necessary to maintain a controlled air temperature into the carburetor. The vacuum motor is controlled by a temperature sensor in the air cleaner. The preheating of the air cleaner inlet air allows leaner carburetor and choke settings, which result in lower emissions, while maintaining good driveability.
Two different throttle return control systems are used. The first is used from 1975 to 1978. It consists of a control valve and a throttle lever actuator. When the truck is coasting against the engine, the control valve is open to allow vacuum to operate the throttle lever actuator. The throttle lever actuator then pushes the throttle lever slightly open reducing the HC (hydrocarbon) emission level during coasting. When manifold vacuum drops below a predetermined level, the control valve closes, the throttle lever retracts, and the throttle lever closes to the idle position.
The second TRC system is used in 1979. It consists of a throttle lever actuator, a solenoid vacuum control valve, and an electronic speed sensor. The throttle lever actuator, mounted on the carburetor, opens the primary throttle plates a preset amount, above normal engine idle speed in response to a signal from the solenoid vacuum control valve. The valve, mounted at the left rear of the engine above the intake manifold on the 6–cylinder, or on the thermostat housing mounting stud on the V8, is held open in response to a signal from the electronic speed sensor. When open, the valve allows a vacuum signal to be sent to the throttle lever actuator. The speed sensor monitors engine speed at the distributor. It supplies an electrical signal to the solenoid valve, as long as a preset engine speed is exceeded. The object of this system is the same as that of the earlier system.
Trapped Vacuum Spark -
This system is used to prevent a drop in vacuum to the distributor vacuum advance during cold engine operation, when the engine is accelerating. A thermal vacuum switch (TVS) is used to sense engine coolant temperature. A check valve is installed in the vacuum line to the distributor. The other side of the check valve has two connections: one to manifold vacuum (at the carburetor base), and the other to the thermal vacuum switch.
When the engine is cold, the TVSA vacuum ports are closed. Manifold vacuum is routed through the check valve to the distributor. The check valve keeps the vacuum to the distributor at a high vacuum level, so that when the engine is accelerated, the vacuum to the distributor does not drop. This results in a constant spark advance.
When the engine temperature reaches a predetermined value, the TVS ports open to allow manifold vacuum to the distributor, and the check valve operates only as a connector.