All 1967 models equipped with automatic transmission, and all 1969 and later models (regardless of other exhaust emission control equipment) are equipped with the Improved Combustion (IMCO) system. The IMCO system controls emissions arising from the incomplete combustion of the air/fuel mixture in the cylinders. The IMCO system incorporates a number of modifications to the distributor spark control system, the fuel system, and the internal design of the engine.
Internal engine modifications include the following: elimination of surface irregularities and crevices as well as a low surface area-to-volume ratio in the combustion chambers, a high velocity intake manifold combined with short exhaust ports, selective valve timing and a higher temperature and capacity cooling system.
Modifications to the fuel system include the following: recalibrated carburetors to achieve a leaner air/fuel mixture, more precise calibration of the choke mechanism, the installation of idle mixture limiter caps and a heated air intake system.
Modifications to the distributor spark control system include the following: a modified centrifugal advance curve, the use of dual diaphragm distributors in most applications, a ported vacuum switch, a deceleration valve and a spark delay valve.
Heated Air Intake System
DUCT AND VALVE ASSEMBLY
- Either start with a cold engine or remove the air cleaner from the engine for at least half an hour. While cooling the air cleaner, leave the engine compartment hood open.
- Tape a thermometer, of known accuracy, to the inside of the air cleaner so that is is near the temperature sensor unit. Install the air cleaner on the engine but do not fasten its securing nut.
- Start the engine. With the engine cold and the outside temperature less than 90°F (32°C), the door should in the HEAT ON position (closed to outside air).
- Operate the throttle lever rapidly to 1 / 2 - 3 / 4 of its opening and release it. The air door should open to allow outside air to enter and then close again.
- Allow the engine to warm up to normal temperature. Watch the door. When it opens to the outside air, remove the cover from the air cleaner. The temperature should be over 90°F (32°C) and no more than 130°F (54°C); 105°F (41°C) is about normal. If the door does not work within these temperature ranges, or fails to work at all, check for linkage or door binding.
- If binding is not present and the air door is not working, proceed with the vacuum tests given below. If these indicate no faults in the vacuum motor and the door is not working, the temperature sensor is defective and must be replaced.
- Check all the vacuum lines and fittings for leaks. Correct any leaks. If none are found, proceed with the test.
- Remove the hose which runs from the sensor to the vacuum motor. Run a hose directly from the manifold vacuum source to the vacuum motor.
- If the motor closes the air door, it is functioning properly and the temperature sensor is defective.
- If the motor does not close the door and no binding is present in its operation, the vacuum motor is defective and must be replaced.
Dual Diaphragm Distributor Advance and Retard Mechanisms
- Connect a timing light to the engine. Check the ignition timing.
- Remove the retard hose from the distributor and plug it. Increase the engine speed. The timing should advance. If it fails to do so, then the vacuum unit if faulty and must be replaced.
- Check the timing with the engine at normal idle speed. Unplug the retard hose and connect it to the vacuum unit. The timing should instantly be retarded 4-10°. If this does not occur, the retard diaphragm has a leak and the vacuum unit must be replaced.
- Check the routing and connection of all vacuum hoses.
- Attach a tachometer to the engine.
- Bring the engine up to normal operating temperature. The engine must not be overheated.
- Note the engine rpm, with the transmission in neutral, and the throttle in the curb idle position.
- Disconnect the vacuum hose from the intake manifold at the temperature sensing valve. Plug or clamp the hose.
- Note the idle rpm with the hose disconnected. If there is no change in rpm, the valve is good. If there is a drop of 100 or more rpm, the valve should be replaced. Replace the vacuum line.
- Check to make sure that the all-season cooling mixture meets specifications, and that the correct radiator cap is in place and functioning.
- Block the radiator air now to induce a higher-than-normal temperature condition.
- Continue to operate until the engine temperature or heat indicator shows above normal.
- If engine speed by this time has increased 100 or more rpm, the temperature sensing valve is satisfactory. If not, it should be replaced.
- Detach the vacuum line from the distributor at the spark delay valve end. Connect a vacuum gauge to the valve, in its place.
- Connect a tachometer to the engine. Start the engine and rapidly increase its speed to 2000 rpm with the transmission in neutral.
- As soon as the engine speed is increased, the vacuum gauge reading should drop to zero.
- Hold the engine speed at a steady 2000 rpm. It should take longer than two seconds for the gauge to register 6 in. Hg (20.3 kPa). If it takes lass than two seconds, the valve is defective and must be replaced.
- If it takes longer than the number of seconds specified in the application chart for the gauge to reach 6 in. Hg (20.3 kPa), disconnect the vacuum gauge from the spark delay valve. Disconnect the hose which runs from the spark delay valve to the carburetor at the valve end. Connect the vacuum gauge to this hose.
- Start the engine and increase its speed to 2000 rpm. The gauge should indicate 10-16 in. Hg (33.75-54 kPa). If it does not, there is a blockage in the carburetor vacuum port or else the hose itself is plugged or broken. If the gauge reading is within specification, the valve is defective.
- Reconnect all vacuum lines and remove the tachometer, once testing is completed.
REMOVAL & INSTALLATION
Heated Air Intake System
- Remove the hex-head cap screws which secure the air intake duct and valve assembly to the air cleaner.
- Remove the air intake duct and valve assembly from the engine.
- If inspection reveals that the valve plate is sticking or the thermostat is malfunctioning, remove the thermostat and valve plates as follows:
- Detach the valve plate tension spring from the valve plate using long-nose pliers.
- Loosen the thermostat locknut and unscrew the thermostat from the mounting bracket.
- Grasp the valve plate and withdraw it from the cut.
- Install the air intake duct and valve assembly on the shroud tube.
- Connect the air intake duct and valve assembly the air cleaner and tighten the hex-head retaining cap screws.
- If it was necessary to disassemble the thermostat and air duct and valve, assemble the unit as follows:
- Install the locknut on the thermostat, and screw the thermostat into the mounting bracket.
- Install the valve plate tension spring on the valve plate and duct.
- Install the vacuum override motor (if applicable) and check for proper operation.
- Drain about one gallon of coolant out of the radiator.
- Tag the vacuum hoses that attach to the control valve and disconnect them.
- Unscrew and remove the control valve.
- Install the new control valve.
- Connect the vacuum hoses.
- Fill the cooling system.