Experiencing a no-spark issue in your 1991 Subaru Legacy can be frustrating. While OBD2 diagnostic systems were still in their early stages around this time, understanding potential fault codes and knowing where to begin your diagnosis is crucial. This guide outlines a systematic approach to pinpointing the cause of a no-spark condition in your classic Subaru.
First, consider the possibility of mechanical issues. A compression test is an invaluable initial step. This simple test can quickly rule out significant problems such as a slipped timing belt, which, although potentially not severe enough to trigger specific OBD2 codes immediately, could still disrupt the engine’s combustion cycle and prevent spark. If the timing belt has jumped a tooth, it might not be obvious, but it can certainly lead to a no-start condition.
Next, move to the electrical components of the ignition system. A basic continuity tester is your best friend here. Start tracing the signal path from the spark coil back towards the engine control unit (ECU). The key components in this circuit are the coil itself, the ignitor (if your model has a separate ignitor module), and the ECU. Remember that ignitors can be grounded through their casing (typically 3-wire systems) or via a dedicated wire (4-wire systems). Corrosion on grounding points, especially in older vehicles like a ’91 Legacy, can disrupt the circuit.
The signal to fire the spark plugs originates from the crankshaft and camshaft angle sensors. These sensors provide crucial timing information to the ECU. If either sensor is faulty, or if the timing belt is significantly off, the ECU won’t send the necessary signal to the ignition system, and consequently, you’ll have no spark. In many cases, sensor issues or timing belt problems would trigger OBD2 fault codes. However, it’s worth noting that early OBD2 systems might not be as comprehensive as modern systems, and some subtle issues might not immediately register as a code.
Consider the ECU itself. The fact that your spark plugs are wet with fuel and the fuel pump is running suggests the ECU is partially functional – it’s controlling fuel delivery. The absence of OBD2 codes in this scenario is interesting. It’s less likely, but still possible, that the ECU is malfunctioning specifically in the spark signal output section.
Based on these observations, here’s a prioritized troubleshooting checklist for your ’91 Subaru Legacy no-spark issue:
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Compression Test: Rule out major mechanical timing issues. Even without OBD2 codes directly indicating timing slip, a compression test is a definitive check.
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Ignition Circuit Continuity: Thoroughly test the wiring and components from the coil back to the ECU. Focus on:
- Coil: Test coil resistance according to your service manual.
- Ignitor: If applicable, test ignitor input and output signals.
- Wiring: Check for breaks, shorts, or corrosion in the wiring harness. Pay particular attention to any wiring near the knock sensor, as these wires can become brittle and short to ground, although this might be less directly related to spark in all systems, it’s good practice to inspect.
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Ground Points: Clean and inspect all engine and chassis ground connections. Poor grounding is a common culprit in electrical issues and can be easily overlooked. This is especially pertinent in older vehicles where corrosion is more prevalent.
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ECU Reset: Perform a full ECU reset by disconnecting the battery for at least 30 minutes. This can sometimes resolve transient ECU glitches. While less likely to be the root cause, it’s a simple step to try.
While OBD2 codes can be a valuable starting point, especially in newer vehicles, diagnosing a no-spark condition in a 1991 Subaru Legacy often requires a more hands-on, systematic approach. By methodically checking these key areas, you should be able to isolate the cause of your no-spark issue and get your Subaru back on the road. Remember to consult your Subaru Legacy repair manual for specific component testing procedures and wiring diagrams relevant to your model year.
