I have a 2015 sedan with a persistent engine misfire that's been throwing a P0304 code for cylinder 4. I've already swapped the coil pack and spark plug with ones from a known-good cylinder, but the misfire didn't move, so it's not those components. The fuel injector clicks when tested, and a compression test showed normal results. I'm a decent home mechanic but stuck on what to check next—could it be a vacuum leak specific to that runner, a failing fuel injector driver in the ECU, or something else less obvious? Any guidance on systematic diagnosis would be appreciated.
You're chasing a stubborn cylinder. Here's a clean, repeatable diagnostic path you can follow:
1) Verify the misfire with live data—confirm cylinder 4 shows a misfire event and that short-term fuel trims swing oddly when it misfires.
2) Do an injector swap test: move the injector from cylinder 4 to another cylinder (say cylinder 1) and see if the misfire follows the injector. If it does, the injector or its wiring is the culprit; if not, the problem is likely the injector drive circuit or PCM.
3) Inspect the injector drive/wiring: check the connector on cylinder 4 for corrosion, and test continuity pin‑to‑pin and injector resistance to ensure it matches the others.
4) Check rail pressure and flow: attach a fuel pressure gauge and verify steady pressure when the injector is commanded on, and look for any drop under load or cranking.
5) Inspect for air leaks around cylinder 4: inspect the intake runner, manifold gasket, PCV hoses, and related plumbings. A smoke test or a propane/flow spray test can reveal leaks by the engine reacting.
6) If you still have nothing, do a leak‑down test on cylinder 4 to rule out valve seating or a dynamic compression issue under load.
If you want, tell me your engine family (do you have port fuel injection or direct injection, number of cylinders) and any other symptoms (idle quality, RPM ranges), and I’ll tailor the test steps.
Take a data-first approach and keep the steps concrete. A straightforward sequence to try:
1) Confirm misfire with live data and observe cylinder 4’s fuel trim signature when it misfires.
2) Swap the injector (as above) to see whether misfire behavior follows the injector or stays on cylinder 4; that tells you if the issue is injector/wiring versus the driver/PCM.
3) Check the injector circuit end-to-end: connector integrity, continuity, and compare injector resistance to the other cylinders.
4) Verify fuel pressure is steady when the injector is pulsed; a failing pump, clogged filter, or regulator can hide here.
5) Look for vacuum leaks around the cylinder 4 intake path with spray/propane or a smoke test; any RPM change points to a leak.
6) If it still won’t break, perform a leak-down test on cylinder 4.
Share specifics (engine type, modifications, any other codes), and I’ll sketch a precise, step‑by‑step plan you can run.
Short, practical checklist:
• Confirm misfire with live data (cylinder 4).
• Swap injector to test if the misfire follows injector vs. cylinder.
• Check injector drive wiring and resistance; compare to the others.
• Verify rail pressure stays steady when injector is energized.
• Check for vacuum leaks around cylinder 4 intake runner; try a smoke test.
• If unresolved, perform a leak-down test to assess valve seating.
If you want, share your engine type and any other symptoms and I’ll tailor the plan to your car.
What else would help? If you can share the engine family (Cylinders, turbocharged?), fuel system (port vs direct injection), and whether you’ve seen things like lean symptoms under boost or at idle, I can tailor a precise test order. In short: confirm misfire with data, test injector/circuit with a swap, verify fuel pressure, then hunt vacuum leaks or valve concerns. If you want, I can draft a one-page diagnostic checklist you can print and follow while you’re under the hood.