Experiencing a persistent engine misfire can be frustrating, especially when it occurs right from startup. If your OBD-II scanner is showing a P0304 code, it indicates a Cylinder 4 Misfire. This means your car’s computer has detected that cylinder number 4 isn’t firing correctly, leading to a rough idle, reduced engine power, and potentially further engine damage if left unaddressed. Let’s delve into diagnosing and troubleshooting this common issue.
One of the initial diagnostic steps involves checking basic engine health. A compression test and vacuum test can provide valuable insights. A healthy engine should exhibit consistent vacuum readings at idle. In this case, a solid 20 inches of mercury (in-Hg) at idle is a good sign. However, fluctuations during throttle application, as noted with a +/- 3-5″ swing, might suggest a mechanical issue or vacuum leak affecting engine performance, although in this scenario, the idle vacuum was stable. Similarly, a compression test is crucial. Readings between 200-220 psi across all cylinders, achieved within a few engine cranks, generally indicate good cylinder health. Consistent readings across all cylinders, as described with pressures around 140, 170, 200, and finally 215 psi within four cranks, further reinforce this.
With compression and vacuum seemingly in the normal range, the next logical step is to investigate the ignition system, a frequent culprit in misfire codes. Checking the ignition coil’s primary resistance is a standard procedure. Specifications typically call for a certain resistance range between specific pins. An ‘open’ reading (often displayed as ‘oL’ on a multimeter), where no resistance is detected, would typically indicate a faulty coil. However, the perplexing situation of encountering the same ‘open’ reading on a known good coil from a neighbor’s car highlights the complexity of diagnostics. This suggests the issue might not solely reside within the coil itself, or there might be a misunderstanding of the testing procedure or coil specifications.
To further isolate the misfire to cylinder 4, a cylinder isolation test is invaluable. This involves disconnecting spark plug wires one at a time while the engine is idling and observing the engine’s response. When disconnecting cylinders 1, 2, and 3, a noticeable roughening of the idle is expected, confirming they are contributing to the engine’s operation. However, if disconnecting cylinder 4’s spark plug wire produces no change in idle, it strongly suggests that cylinder was already misfiring and not contributing to the engine’s power. Further examination of the spark quality at each cylinder wire is crucial. A strong, visible spark, capable of jumping a significant air gap (like 1 inch), indicates a healthy ignition system for those cylinders. In contrast, a weak spark, barely visible even when grounded, and intermittent misfires, specifically observed at cylinder 4, firmly points towards an ignition issue on that cylinder.
Considering these diagnostic steps, the focus should now be heavily on the ignition components of cylinder 4. While the coil resistance test was inconclusive, the weak spark and lack of idle change during cylinder isolation strongly implicate a fault in the ignition system affecting cylinder 4. Further investigation should include:
- Spark Plug Inspection: Examine the spark plug for cylinder 4. Look for fouling, damage, or excessive wear. Replace the spark plug with a new one to rule out a faulty plug.
- Spark Plug Wire/Boot: Inspect the spark plug wire and boot for cylinder 4 for damage, cracks, or carbon tracking. Ensure a secure connection at both the spark plug and distributor/coil.
- Ignition Coil (Re-evaluation): While initial coil tests were confusing, consider re-evaluating the ignition coil specifically for cylinder 4. It is possible that the coil is failing intermittently or under load, even if static resistance tests are inconclusive. Swapping the coil with a known good one (even if the resistance readings were similar) might still be a worthwhile step to definitively rule out coil failure.
- Wiring and Connectors: Inspect the wiring and connectors leading to the ignition coil and spark plug for cylinder 4. Look for corrosion, damage, or loose connections.
While less likely given the initial compression and vacuum tests, other potential causes for a P0304 code, if ignition components are ruled out, could include fuel delivery issues to cylinder 4 (such as a faulty fuel injector) or more serious mechanical problems within cylinder 4 itself (valve issues, etc.). However, based on the described symptoms and diagnostic steps, addressing the ignition system, particularly for cylinder 4, is the most logical next step in resolving the P0304 misfire code.
