Ford Mid-Size Cars 1971-1985 Repair Guide

Emission Control Equipment


1971-1973 MODELS

Emission controls between 1971 and 1973 are fairly simple compared to later systems. The controls listed are common to, but not necessarily found on, all models: air pump, closed Positive Crankcase Ventilation (PCV), calibrated carburetor and distributor, dual vacuum advance on the distributor, deceleration valve (6-cylinder), heated air cleaner, vapor control system canister storage, fresh air intake tube to the air cleaners, electronic distributor modulator, various vacuum check valves and Exhaust Gas Recirculation (EGR).


Late 1973 and 1974 models use an Exhaust Gas Recirculation (EGR) system to control oxides of nitrogen. On V8 engines, exhaust gases travel through the exhaust gas crossover passage in the intake manifold. A portion of these gases is diverted into a spacer which is mounted under the carburetor. The EGR control valve, which is attached to the rear of the spacer, consists of a vacuum diaphragm with an attached plunger which normally blocks off exhaust gases from entering the intake manifold. The EGR valve is controlled by a vacuum line from the carburetor which passes through a ported vacuum switch. The EGR ported vacuum switch provides vacuum to the EGR valve at coolant temperatures above 125ºF (52ºC). The vacuum diaphragm then opens the EGR valve permitting exhaust gases to flow through the carburetor spacer and enter the intake manifold where they combine with the fuel mixture and enter the combustion chambers. The exhaust gases are relatively oxygen-free and tend to dilute the combustion charge. This lowers peak combustion temperature thereby reducing oxides of nitrogen.

All models with a 5.8L Cleveland (C), 6.6L, or 7.5L engines use the new Delay Vacuum Bypass (DVB) park control system. This system provides two paths by which carburetor vacuum can reach the distributor vacuum advance. The system consists of a spark delay valve, a check valve, a solenoid vacuum valve, and an ambient temperature switch. When the ambient temperature is below 49ºF (9ºC), the temperature switch contacts are open and the vacuum solenoid is open (de-energized). Under these conditions, vacuum will flow from the carburetor, through the open solenoid, and to the distributor. Since the spark delay valve resists the flow of carburetor vacuum, the vacuum will always flow through the solenoid when it is open (this is the path of least resistance). When the ambient temperature rises above 60ºF (16ºC), the contacts in the temperature switch (which is located in the door post) close. This passes ignition switch current to the solenoid, energizing the solenoid. This blocks one of the two vacuum paths. All distributor vacuum must now flow through the spark delay valve. When carburetor vacuum rises above a certain level on acceleration, a rubber valve in the spark delay valve blocks vacuum from passing through the valve for 5-30 seconds.

After this time delay has elapsed, normal vacuum is supplied to the distributor. When the vacuum solenoid is closed at temperatures above 60ºF (16ºC) the vacuum line from the solenoid to the distributor is vented to the atmosphere. To prevent the vacuum that is passing through the spark delay valve from escaping through the solenoid into the atmosphere, a one-way check valve is installed in the vacuum line from the solenoid to the distributor.

In order to meet 1974 California emission control standards, all 1974 Ford cars sold in that state are equipped with a Thermactor® (air injection) system to control hydrocarbons and carbon monoxide. The EGR system is retained to control oxides of nitrogen.


All mid-size Ford Motor Co. cars are equipped with catalytic converters. Some California models are equipped with two converters, while models sold in the 49 states have only one unit.

Catalytic converters change noxious emission of hydrocarbons (HC) and carbon monoxide (CO) into harmless carbon dioxide and water. The units are installed in the exhaust system ahead of the mufflers and are designed, if the engine is properly tuned, to last100,000 miles (80,400 km) or more before replacement.

In addition to the converters, most 1975 Ford and Mercury cars are equipped with the Thermactor® air pump (air injection system) previously mentioned. the air injection system, which afterburns the uncombusted fuel mixture in the exhaust ports, is needed with the converters to prevent an overly rich mixture from reaching the converter, and to help supply oxygen to aid in converter reaction.

Other emission control equipment for 1975 includes a carry-over of the Positive Crankcase Ventilation (PCV) system, the Fuel Evaporative Control (FEC) system, and Exhaust Gas Recirculation (EGR) system.

Emission control related improvements for 1975 include standard solid state (breakerless) Ignition, induction hardened exhaust valve seats, exhaust manifold redesign, vacuum operated heat riser valves, and improved carburetors with more precise fuel metering control and a mechanical high speed bleed system.

All cars equipped with the 7.0L engine use a Cold Start Spark Advance (CSSA) system in 1975 to aid in cold start driveability. Basically, the system will allow full vacuum advance to the distributor until the coolant temperature reaches 125ºF (52ºC).


For 1976, the complexity of emission control equipment was reduced on Ford products. The average number of emission control components was reduced from 25 to 11 on most cars. All 1976 models have catalytic converters. In addition, a new proportional exhaust gas recirculation system has been introduced. Exhaust backpressure regulates the EGR valve spark port vacuum signal to modulate the recirculation of gases, matching EGR flow to engine load.

1977-79 MODELS

Most emission controls are carry-over from 1976. One exception, however, is the (Electronic Engine Control) EEC-II system. It is installed on all Mercury vehicles with the optional 5.8L engine.

The system is based on EEC-I control assembly used on earlier Ford products, but certain components have been changed to improve performance and reliability, and to reduce complexity and cost. EEC-II controls spark timing, EGR, and air/fuel ratio (mixture). A solid state module incorporating a digital microprocessor and other integrated circuits interprets information sent by seven sensors, calculates spark advance, EGR flow rate and fuel flow trim, and sends electrical signals to control the ignition module, EGR valve actuator, and an electric stepper motor in the carburetor. EEC-II also controls purging of vapors in the storage canister to prevent overly rich mixtures, high altitude fuel mixture adjustments, Thermactor® (air pump) air flow, and cold engine (fast idle) functions. Because the throttle idle position, ignition timing and mixture are controlled electronically, these functions cannot be adjusted in the conventional manner.


The major change in the emission control system for 1980 is in the EEC. The new system, EEC-III, performs the same functions as EEC-II but uses a new electronic control module. The EEC system computes information and makes any necessary changes about 30 times a second, controlling the air/fuel mixture, EGR, ignition timing and the air flow to the exhaust emission system.

1981-85 MODELS

The application of EEC-III is continued and EEC-IV and TFI-IV are introduced. Components include an oxygen sensor, a variable mixture carburetor, a three-way oxidation/reduction catalytic converter, an air pump, and a computer module.