Ford Festiva. Instruction — part 48

POOR FUEL ECONOMY



Verify intake air is not restricted.



Verify fuel pressure is within specification.



Check ignition module.



Check canister purge solenoid.



Check vane air temperature sensor.



Check vane airflow meter.



Check heated exhaust gas oxygen sensor.

EXHAUST SMOKE/ODOR



Check vane air temperature sensor.



Check vane airflow meter.



Check exhaust gas oxygen sensor.

FUEL ODOR/LEAKS



Check evaporative emission control components.



Check rollover vent valve.



Check by-pass air control valve.

FAILS EMISSION TEST



Check canister purge solenoid.



Check pressure regulator control solenoid.



Check heated exhaust gas oxygen sensor.



Check by-pass air control valve.

INTERMITTENTS

INTERMITTENT PROBLEM DIAGNOSIS

Intermittent fault testing requires duplicating circuit or component failure to identify problem. These procedures may lead to computer setting
a fault code (on some systems) which may help in diagnosis.

If problem vehicle does not produce fault codes, monitor voltage or resistance values using a DVOM while attempting to reproduce conditions
causing intermittent fault. A status change on DVOM indicates a fault has been located.

Use a DVOM to pinpoint faults. When monitoring voltage, ensure ignition switch is in ON position or engine is running. Ensure ignition
switch is in OFF position or negative battery cable is disconnected when monitoring circuit resistance. Status changes on DVOM during test
procedures indicate area of fault.

TEST PROCEDURES

INTERMITTENT SIMULATION

To reproduce conditions creating an intermittent fault, use following methods:



Lightly vibrate component.



Heat component.



Wiggle or bend wiring harness.



Spray component with water.



Remove/apply vacuum source.

Monitor circuit/component voltage or resistance while simulating intermittent. If engine is running, monitor for self-diagnostic codes. Use test
results to identify a faulty component or circuit.

Copyright 2009 Mitchell Repair Information Company, LLC. All Rights Reserved.

Article GUID: A00022699

Back To Article

E - THEORY/OPERATION

1991 ENGINE PERFORMANCE Ford/Mercury Theory & Operation

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Read this article before attempting
to diagnose systems with which you are not completely familiar.

COMPUTERIZED ENGINE CONTROLS

An electronic Control Assembly (ECA) receives and processes signals from various sensors and switches. See

Fig. 1

. It then generates signals

which control ignition timing, fuel injection functions and various emission control devices. The ECA has system diagnostic capabilities and
will store trouble codes for use by service technicians.

Fig. 1: Identifying Input Devices & Output Signals (Not All Shown)

Courtesy of FORD MOTOR CO.

ELECTRONIC CONTROL ASSEMBLY (ECA)

The ECA is located under the instrument panel on the driver side. It receives and processes data from sensors, switches and other components.
The ECA generates output signals to control fuel injection, spark timing, other engine functions and emission systems.

CEC INPUT DEVICES

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine input device usage of
a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article. The available input devices include the following:

BAROMETRIC PRESSURE SENSOR (BP)

This device senses changes in barometric pressure. The ECA uses this information in calculating fuel metering, ignition timing and idle speed.
On Festiva, BP is incorporated into the ECA. On Capri, BP is located on the passenger-side cowl.

BRAKE ON-OFF (BOO) SWITCH

This switch, located at the brake pedal, senses brake operation. The ECA uses this information in calculating fuel metering.

NOTE:

Components are grouped into 2 categories. The first category is CEC INPUT DEVICES, which control or
produce voltage signals monitored by the control unit. The second category is CEC OUTPUT SIGNALS,
which are components controlled by the control unit.

CLUTCH ENGAGE SWITCH (CES)

This switch is located on the instruction transaxle and informs the ECA of transmission operating conditions. The ECA uses this data for idle
speed control and canister purge valve operation.

CRANKSHAFT POSITION SENSOR (CPS) (FESTIVA)

This sensor is located in the distributor and sends engine RPM and crankshaft position data to the ECA. The ECA uses this data in calculating
fuel metering, ignition timing, idle speed control and canister purge valve operation.

CYLINDER IDENTIFICATION SENSOR (CID) (CAPRI)

This sensor is located in the distributor and provides crankshaft position data to the ECA. The ECA uses this data in calculating fuel metering
and ignition timing.

ELECTRICAL LOAD SWITCHES

Blower control, cooling fan, rear window defroster, air conditioning and headlight switches all send signals to the ECA. The ECA uses these
signals for idle speed control.

ENGINE COOLANT TEMPERATURE (ECT) SENSOR

This sensor is located in the intake manifold and sends coolant temperature data to the ECA. The ECA uses this data in calculating fuel
metering, idle speed control and canister purge valve operation.

EXHAUST GAS OXYGEN (EGO) SENSOR

This sensor is located in the exhaust manifold and senses oxygen concentration in the exhaust gas. The ECA uses this data in calculating fuel
metering, idle speed control and canister purge valve operation.

NEUTRAL SAFETY SWITCH (A/T)

This switch is located on the automatic transaxle and sends a signal to the ECA whenever the transaxle is in Neutral or Park ranges. The ECA
uses this signal in calculating fuel metering, idle speed control and canister purge valve operation.

POWER STEERING PRESSURE SWITCH (PSPS)

This switch is located on the power steering pump and sends data about power steering operation to the ECA. The ECA uses this data for idle
speed control.

THROTTLE POSITION SENSOR (TP)

This device is located on the throttle body and contains 2 switches. The Wide Open Throttle (WOT) Switch sends a signal to the ECA when
throttle valve opening is more than 70 degrees. The ECA uses this signal in calculating fuel metering.

The Idle Switch (IDL) sends a signal to the ECA when throttle valve opening is less than 1.5 degrees. The ECA uses this signal for idle speed
control and canister purge valve operation.

VANE AIRFLOW (VAF) METER

This sensor is located in the air cleaner housing and sends intake airflow data to the ECA. The ECA uses this data in calculating fuel metering.

VANE AIR TEMPERATURE (VAT) SENSOR

This sensor is located inside the vane airflow sensor and senses intake air temperature. The ECA uses this data in calculating fuel metering,
idle speed control and canister purge valve operation.

CEC OUTPUT SIGNALS

CANISTER PURGE (CANP) SOLENOID

See EMISSION SYSTEMS.

CHECK ENGINE LIGHT

See SELF-DIAGNOSTIC SYSTEM.

FUEL INJECTORS

See FUEL CONTROL.

IDLE SPEED CONTROL BY-PASS AIR (ISC-BPA) VALVE

See IDLE SPEED.

NOTE:

Vehicles are equipped with different combinations of computer-controlled components. Not all
components listed below are used on every vehicle. For theory and operation of each output
component, refer to indicated system.

FUEL DELIVERY SYSTEM

Major components of the fuel delivery system are an electric fuel pump, fuel supply and return lines, a pressure regulator, fuel rail and fuel
injectors. See

Fig. 2

or

Fig. 3

.

Fig. 2: Locating Fuel Delivery System Components (Festiva)

Courtesy of FORD MOTOR CO.

Fig. 3: Locating Fuel Delivery System Components (Capri)

Courtesy of FORD MOTOR CO.

FUEL PUMP

The fuel pump, located inside the fuel tank, includes a check valve at the outlet and a filter element at the inlet. A pressure relief valve limits
maximum fuel outlet pressure.

FUEL PUMP RELAY

This relay prevents fuel pump operation unless the engine is cranking or when a signal from the VAF confirms the engine is running. See

Fig.

4

. Internal circuitry holds the relay contacts closed during momentary loss of the VAF signal during rapid deceleration. The fuel pump relay is