Decoding 1991 Buick OBD2 GM Codes: Understanding Error Code 42

Navigating the complexities of your 1991 Buick’s engine can be daunting, especially when the check engine light illuminates. While 1991 Buicks were on the cusp of OBD2 implementation (more commonly featuring OBD1 or early GM diagnostic systems), understanding the diagnostic codes is crucial for effective car repair. Among the various GM codes, code 42 (and sometimes E23 in certain models) often causes confusion. This article, tailored for your 1991 Buick and other similar GM vehicles, will demystify code 42, focusing on its causes, symptoms, and how to troubleshoot it effectively.

Understanding GM Code 42/E23 and the EST System

GM code 42, or E23 in some earlier systems, signals a fault within the Electronic Spark Timing (EST) circuit. The EST system is a cornerstone of your 1991 Buick’s engine management, designed to optimize engine performance, fuel efficiency, and reduce emissions by precisely controlling spark timing. This system has been a feature in GM vehicles for many years, appearing in both High Energy Ignition (HEI) distributor setups and Distributor-less Ignition Systems (DIS).

The computer, often referred to as the ECM (Engine Control Module) or PCM (Powertrain Control Module), is the brain behind the EST system. It calculates the ideal spark advance based on several engine parameters:

• Manifold Absolute Pressure (MAP) Sensor: Indicates engine load. Lower MAP (less load) prompts more spark advance, while higher MAP (greater load) calls for less.
• Coolant Temperature Sensor (CTS): Monitors engine temperature. A cold engine requires more spark advance, whereas a warm engine needs less.
• Engine RPM: Higher RPM generally necessitates more spark advance, and lower RPM less.

How the EST System Works: Circuits and Functions

The EST system in your 1991 Buick is comprised of the ignition module, the ECM/PCM, and their connecting wiring. These components communicate through four key circuits, each with a specific role:

• Reference Circuit: This circuit is vital for providing the ECM with engine RPM information. If this circuit is disrupted – either open or shorted to ground – the engine will likely fail to start. This is because the ECM needs the reference signal to trigger the fuel injectors and manage engine operation. As the saying goes in automotive circles, “no reference, no run.”

• Reference Ground Circuit: This circuit ensures a stable ground connection between the ignition module and the ECM. It’s grounded at both ends to minimize voltage drop, which can negatively impact engine performance and potentially trigger false codes, including code 42/E23. An open in this ground circuit can sometimes lead to a no-start condition.

• Bypass Circuit: The ECM uses this circuit to take control of the spark timing from the ignition module. By applying 5 volts to the bypass circuit, the ECM signals the module to relinquish timing control. If this circuit is open or shorted to ground, code 42 will be set, and the engine will revert to running on base timing, plus a small amount of advance pre-programmed into the ignition module.

• EST Circuit: This is the circuit through which the ECM triggers the ignition module to fire the spark plugs at the calculated optimal time. The ECM operates based on the reference signal as a starting point. It advances or retards the spark timing from this base. Crucially, the ECM doesn’t directly “know” the actual timing; it dictates the desired timing relative to the reference. Therefore, if the base timing (adjustable in some older vehicles) is incorrectly set, the entire spark advance curve will be skewed. Faults in the EST circuit – opens or shorts to ground – will trigger code 42 and force the engine to run on the ignition module’s timing.

Common Causes of 1991 Buick OBD2 GM Code 42

Code 42 in your 1991 Buick signals a communication breakdown between the ECM and the ignition module regarding spark timing. This “failure to communicate” can stem from various issues. When the engine is running above approximately 400 RPM, the ECM and ignition module are in constant communication, with the ECM dictating the timing. If this communication is interrupted, or if false signals interfere with the circuits, the ECM defaults timing control back to the ignition module and sets code 42. Possible culprits include:

• EST Circuit Issues: An open or short to ground in the EST circuit wiring itself.
• Bypass Circuit Problems: Open or grounded bypass circuit wiring.
• Reference Ground Circuit Faults: An open in the reference ground circuit or a short to battery voltage (+).
• Poor Ground Connections: A faulty engine-to-computer ground can disrupt the entire system.
• Defective Ignition Module: A failing ignition module may not properly communicate or respond to the ECM’s signals.
• Faulty ECM/PCM: In rare cases, the ECM itself can be the source of the problem.
• Electromagnetic Interference (EMI): Spark plug wires positioned too close to EST circuit wiring can induce interference.
• Connector Issues: Oil contamination or oversized terminals in circuit connectors can cause poor connections.
• Wiring Damage: Chafing or shorts in the wiring of any of the EST circuits.

Diagnosing and Troubleshooting Code 42 in your 1991 Buick

When diagnosing code 42 on your 1991 Buick, a systematic approach is essential. Start with a visual inspection:

1. Spark Plug Wire Routing: Check the routing of your spark plug wires. Ensure they are not running too close to the EST circuit wiring, which could cause electromagnetic interference.

2. Connector Inspection: Carefully inspect the connectors associated with the ignition module and ECM. Look for signs of oil contamination or corrosion. Check if the terminals are properly sized and making good contact.

3. Wiring Examination: Thoroughly examine the wiring of the EST, bypass, and reference ground circuits for any signs of chafing, damage, or shorts to ground. Pay close attention to areas where wiring harnesses might rub against engine components.

For more in-depth troubleshooting, consider using a reliable online manual system like Alldata (mentioned in the original article), which provides detailed wiring diagrams, diagnostic procedures, and trouble trees specific to your 1991 Buick and other GM vehicles. These resources will guide you through testing the ignition module and ECM to pinpoint the faulty component or wiring issue.

By understanding the EST system and the potential causes of code 42, you can effectively diagnose and repair your 1991 Buick, restoring its performance and ensuring it runs smoothly. Remember to always consult a repair manual specific to your vehicle for detailed procedures and safety precautions.