Experiencing a no-spark condition in your 1991 Subaru Legacy can be frustrating. While ’91 models predate the widespread adoption of OBD2, understanding diagnostic approaches similar to those used with OBD2 systems can still guide your troubleshooting process. Let’s explore a step-by-step method to diagnose why your ’91 Legacy might be lacking spark, drawing on principles relevant to interpreting OBD2 era diagnostic codes and system checks.
First, consider mechanical issues. A slipped timing belt, though sometimes triggering OBD codes in later models, might not always do so definitively in a ’91 Legacy. A compression test is a straightforward way to rule out significant timing belt issues or other internal engine problems affecting compression. This test offers a clear yes/no answer regarding basic engine mechanical health, eliminating guesswork.
Next, electrical continuity testing becomes crucial. Start tracing the ignition signal from the spark coil back towards the Engine Control Unit (ECU). Key components in this circuit include the coil itself, the ignitor (if equipped – some systems integrate this into the ECU or coil), and the ECU. Remember to check the grounding of the ignitor, especially in 3-wire configurations where grounding relies on the case; corrosion can disrupt this. Four-wire ignitors typically have a dedicated ground wire.
Signal input for spark generation primarily originates from the Camshaft Position Sensor (CMP) and Crankshaft Position Sensor (CKP). If either sensor fails or the timing belt is severely misaligned, the ECU won’t receive the necessary signals to trigger spark, potentially leading to a no-spark condition without always setting readily apparent ’91 Subaru Legacy Obd2 Codes – or rather, OBD1 or pre-OBD codes, which might require specialized tools to read.
Consider the ECU’s basic functionality. If your spark plugs are wet with fuel and the fuel pump is engaging, it suggests the ECU is partially operational, controlling fuel injection. The absence of stored diagnostic trouble codes (DTCs), even basic ones, and no apparent wiring damage preventing code transmission to a diagnostic port (even if pre-OBD2), makes a complete ECU failure less likely, though not impossible.
Based on these observations, the no-spark issue likely stems from one of these primary areas:
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Slight Timing Belt Slip: A minor slip might not be severe enough to trigger codes but could disrupt spark timing. The irregular engine cranking sound mentioned, along with a “weird hit,” warrants a compression test to investigate further.
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Wiring or Component Fault in Ignition Circuit: A damaged wire or component between the ECU and coil could interrupt the spark signal. A common issue is brittle wiring, such as a knock sensor wire grounding against its shielding. Similar wiring faults in the ignition circuit could exist without setting detectable ’91 Subaru Legacy OBD2 codes (or OBD1 equivalents).
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Grounding Problems: Inspect and clean all ground connections throughout the engine bay, particularly those for the ignition system and ECU. Poor grounds are notorious for causing intermittent and perplexing electrical issues.
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ECU Glitch: While less probable, an ECU malfunction cannot be entirely ruled out. Attempt a full ECU reset by disconnecting the battery for at least 30 minutes. This may resolve temporary ECU anomalies.
Beyond these steps, further in-depth diagnostics might be required. Remember to systematically check each possibility, starting with the simplest and most accessible, to efficiently pinpoint the cause of the no-spark issue in your ’91 Subaru Legacy. Let us know what you discover!
