GM Metro/Sprint 1985-1993 Repair Guide

Electronic Fuel Injection (EFI) System


See Figure 1

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Fig. Fig. 1: EFI system schematic


The Electronic Fuel Injection (EFI) system is used on 1989-93 Geo Metro vehicles. The system, namely the electronic control type fuel injection system, consists of an ECM, an in-tank type electric fuel pump, a pressure regulator, fuel injectors, a manifold absolute pressure sensor, a throttle position sensor, an intake air temperature sensor, a coolant temperature sensor, an oxygen sensor, a vehicle speed sensor, a crankshaft angle sensor and others

Electronic Control Module (ECM)

The ECM is required to maintain the exhaust emissions at acceptable levels. The module is a small, solid state computer which receives signals from many sources and sensors; it uses these data to make judgments about operating conditions and then control output signals to the fuel and emission systems to match the current requirements.

Inputs are received from many sources to form a complete picture of engine operating conditions. Some inputs are simply Yes or No messages, such as that from the Park/Neutral switch; the vehicle is either in gear or in Park/Neutral; there are no other choices. Other data is sent in quantitative input, such as engine RPM or coolant temperature. The ECM is pre-programmed to recognize acceptable ranges or combinations of signals and control the outputs to control emissions while providing good driveability and economy. The ECM also monitors some output circuits, making sure that the components function as commanded. For proper engine operation, it is essential that all input and output components function properly and communicate properly with the ECM.

Since the control module is programmed to recognize the presence and value of electrical inputs, it will also note the lack of a signal or a radical change in values. It will, for example, react to the loss of signal from the vehicle speed sensor or note that engine coolant temperature has risen beyond acceptable (programmed) limits. Once a fault is recognized, a numeric code is assigned and held in memory. The CHECK ENGINE dashboard warning lamp will illuminate to advise the operator that the system has detected a fault.

In the event of an ECM failure, the system will default to a pre-programmed set of values. These are compromise values which allow the engine to operate, although possibly at reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is almost always affected when the ECM enters this mode.

Check Engine Lamp

The primary function of the dash warning lamp is to advise the operator that a fault has been detected, and, in most cases, a code stored. Under normal conditions, the CHECK ENGINE lamp will illuminate when the ignition is turned ON . Once the engine is started and running, the ECM will perform a system check and extinguish the warning lamp if no fault is found.

Additionally, the dash warning lamp must be used to retrieve stored codes after the system is placed in the Diagnostic Mode. Codes are transmitted as a series of flashes with short or long pauses.


If a fault occurs intermittently, such as a loose connector pin breaking contact as the vehicle hits a bump, the ECM will note the fault as it occurs and energize the dash warning lamp. If the problem self-corrects, as with the terminal pin again making contact, the dash lamp will extinguish after 10 seconds but a code will remain stored in the ECM memory.

When an unexpected code appears during diagnostics, it may have been set during an intermittent failure that self-corrected; the codes are still useful in diagnosis and should not be discounted.

Tools and Equipment

The 1989-91 Metro fuel injection system does not require the use of a scan tool during diagnosis. All diagnostic procedures are conducted with common shop tools. The 1992-93 Metro fuel injection uses a scan tool to read the data stream during diagnosis and is required in certain diagnostic tests. However, the diagnostic codes can be read without the use of the scan tool.

A digital voltmeter with 10-mega-ohm impedance is required for testing all systems. This type of meter will not place an additional load on the circuit it is testing; this is extremely important in low voltage circuits.

An ohmmeter will also be required during diagnosis. A particularly useful tool is the digital multimeter, such as J 34029-A or equivalent, combining the volt and ohm meters. The multimeter must be of high quality in all respects and be equipped with leads suitable for back-probing connector terminals. It should be handled carefully and protected from impact or damage. Replace batteries frequently in the unit.

Other necessary tools include an unpowered test light, a quality tachometer with inductive (clip-on) pick up and the proper tools for releasing GM's Metri-Pack, Weather Pack and Micro-Pack terminals as necessary.


See Figures 2 and 3

Diagnosis of a driveability and/or emissions problems requires attention to detail and following the diagnostic procedures in the correct order. Resist the temptation to perform any repairs before performing the preliminary diagnostic steps. In many cases this will shorten diagnostic time and often cure the problem without electronic testing.

The proper troubleshooting procedure for these vehicles is as follows:

Visual/Physical Underhood Inspection

This is possibly the most critical step of diagnosis. A detailed examination of connectors, wiring and vacuum hoses can often lead to a repair without further diagnosis. Performance of this step relies on the skill of the technician performing it; a careful inspector will check the undersides of hoses as well as the integrity of hard-to-reach hoses blocked by the air cleaner or other component. Wiring should be checked carefully for any sign of strain, burning, crimping, or terminal pull-out from a connector. Checking connectors at components or in harnesses is required; usually, pushing them together will reveal a loose fit.

Diagnostic Circuit Check

This step is used to check that the on-board diagnostic system is working correctly. A system which is faulty or shorted may not yield correct codes when placed in the Diagnostic Mode. Performing this test confirms that the diagnostic system is not failed and is able to communicate through the dash warning lamp.

If the diagnostic system is not operating correctly, or if a problem exists without the dash warning lamp being lit, refer to the specific "A" diagnostic charts. These charts cover such conditions as Engine Cranks but Will Not Run or No CHECK ENGINE Light.

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Fig. Fig. 2: Diagnostic switch location under the left side of the instrument panel

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Fig. Fig. 3: Diagnostic connector location under the hood near the ignition coil