Experiencing engine trouble like stumbles and poor performance, especially under load and below 3000 RPM? A common culprit could be a failing coil pack, and a diagnostic trouble code like P0302 might further point towards this issue. Replacing a coil pack isn’t overly expensive, especially for older vehicles, but the concern of misdiagnosing the problem and wasting money on a good part is real. Issues such as fouled spark plugs or damaged wires can mimic coil pack failure symptoms.
One quick and straightforward method to assess your coil pack’s health is the spark arc test. This test helps you visually check the strength of the spark generated by each coil pack output. Here’s how you can do it:
With the engine running, carefully disconnect one spark plug wire at a time. Using insulated pliers, hold the disconnected wire near a grounded metal part of the engine block. Observe the spark jumping from the wire to the block. A healthy coil pack should produce a strong, noticeable arc, approximately 1.5 to 2 inches long, as was observed with cylinder wires #1 and #4 in a recent test.
However, if you notice a weak or barely visible spark, like the short 1/4 inch arc observed with wires #2 and #3 in the same test, it could indicate a problem with that coil pack section. In many engines, a single coil pack often fires two cylinders simultaneously. Therefore, if two adjacent cylinders show weak spark, it strongly suggests a failing coil pack, as was the case with cylinders #2 and #3 which are paired on one part of the coil pack.
While this spark arc test isn’t foolproof and might not detect intermittent coil pack failures, it’s a highly reliable indicator when a coil pack is significantly degraded. A failed test strongly suggests the need for replacement, although it’s always wise to rule out other potential issues like spark plugs or wiring problems for complete certainty. If you’re unsure or the problem persists after replacing the coil pack, seeking professional diagnosis is recommended.
