The 1987 Mustang GT does not have an OBD2 system. OBD2 (On-Board Diagnostics II) became a standard in vehicles starting in the 1996 model year. The 1987 Mustang GT utilizes an earlier, proprietary Ford EEC-IV (Electronic Engine Control IV) system for diagnostics. This system relies on a self-test mode activated by grounding a specific test connector, often referred to as the STI (Self-Test Input) or STO (Self-Test Output) connector.
Understanding the 1987 Mustang GT Diagnostic System
Instead of an OBD2 scanner, diagnosing issues in a 1987 Mustang GT involves retrieving trouble codes through a flashing check engine light. This is done by connecting a jumper wire or paperclip between specific pins on the self-test connector located under the hood. Once connected, the check engine light will flash a series of codes corresponding to specific faults detected by the EEC-IV system.
During the self-test, you might notice the idle control motor (ICM) activating and the engine revving to a higher RPM (around 3000). This is normal as the system tests various components, including spark advance and the throttle position sensor (TPS). If the engine stalls during this test, it’s crucial to retrieve and interpret the trouble codes first.
The system will cycle through current codes, often related to water temperature, EGR (Exhaust Gas Recirculation), and air temperature if the engine isn’t at operating temperature. After two cycles, it will display stored codes from the vehicle’s memory. Codes are displayed in a sequence of flashes, for example, three flashes, a pause, then two flashes would indicate code 32.
Decoding the Shift Light
The 1987 Mustang GT, especially those equipped with a manual transmission, often feature a shift light. This light isn’t related to engine performance or protection but rather fuel efficiency. It illuminates based on RPM, vacuum, and throttle position. Typically, it activates around 3000 RPM under light throttle (high vacuum) conditions, signaling the optimal point for upshifting to maximize fuel economy.
The activation point of the shift light can vary depending on driving conditions. Light acceleration might trigger it at a lower RPM (around 1500), while heavier acceleration might delay it to 4500-5000 RPM. Generally, with moderate throttle, expect the shift light to activate in the 3000-3500 RPM range.
Fuel Efficiency in the 1987 Mustang GT
A well-maintained 1987 Mustang GT, with a properly functioning EEC-IV system, can achieve reasonable fuel economy. Factors like transmission type (4-speed or 5-speed manual) and driving habits significantly impact MPG. With everything running optimally, expect fuel economy in the mid-30s or even as high as 42 MPG in some cases.
Conclusion
While a 1987 Mustang GT lacks OBD2, its EEC-IV system offers valuable diagnostic capabilities. Understanding how to retrieve and interpret trouble codes through the flashing check engine light is crucial for troubleshooting and maintaining these classic vehicles. The shift light, a unique feature focused on fuel efficiency, further highlights the technology of its era. By leveraging the available diagnostic tools and understanding the vehicle’s systems, owners can keep their 1987 Mustang GT running smoothly.
