BMW N52 Cylinder 6 Misfire After Swapping Parts: What Should I Test Next?

A BMW N52 can keep misfiring on cylinder 6 after a coil, spark plug, or injector has been changed. The next step is to prove what stayed on that cylinder, under the same operating condition, before opening the engine or buying another part. A 2008 X3 owner described a long-running cylinder 6 misfire after plug and coil work, injector replacement, and intake repairs on BimmerFest. That is a useful case history, not proof that another N52 has the same cause.

Why does my BMW N52 still misfire on cylinder 6 after swapping the coil, plug, and injector?

A swap only answers a narrow question: did the fault follow the part to another cylinder? If it did, that part or its installation deserves attention. If the misfire remains on cylinder 6, the test has ruled out less than many owners expect. A poor connector, damaged wiring, weak injector control, a cylinder sealing problem, or an intake leak can stay with the same cylinder.

The BimmerFest owner reported that a plug and coil change did not resolve the fault, two injectors had been replaced, and swapping an injector between cylinders did not move the misfire. The thread also mentions an idle-only symptom after a DISA repair and a smoke test the owner did not trust. Each detail changes the next test. It does not establish a diagnosis for other N52 cars.

BMW's SI B12 18 14 covers misfires on N51, N52, N52K, and N52T engines. BMW identifies ignition-coil failure as a common cause and asks technicians to verify the repair with a test drive. If rough running or misfire remains after basic ignition diagnosis, the bulletin directs further diagnosis. A coil swap is a useful branch, not an end-of-test certificate.

Confirm the fault before repeating parts swaps

Save the complete BMW fault record before clearing anything. Record the fault status, occurrence count, engine temperature, RPM, load, and whether the problem appears at idle, during acceleration, or only on a cold start. Watch the cylinder misfire counters while the symptom occurs. A stored fault from last month and an active misfire now are not the same evidence.

Repeat only the test that can answer a question. If the coil has not been tested in a known-good cylinder, swap it once and see whether the misfire follows. If the misfire stays on cylinder 6, return the coil to its original location and inspect the coil boot, plug well, plug gap and condition, and connector engagement. Oil or water in a plug well, a torn boot, or a terminal that does not grip can make a new part behave like a failed one.

Do not keep exchanging parts without tracking their original cylinders. Mark each component and record the result. If the symptom changes from idle-only to load-related after a repair, capture that too. The operating condition is part of the fault description.

Check the cylinder's electrical and fuel branches

Inspect the cylinder 6 harness and connector using the wiring information for the exact vehicle. Look for a loose lock, damaged insulation, pushed-back terminal, corrosion, or a conductor that opens when the harness moves. A visual check cannot prove the circuit works under load. Use the correct electrical test procedure and avoid probing sensitive DME circuits with an unsuitable test light.

For the injector branch, verify that the DME is commanding and driving the injector. A substitute injector cannot help if the connector, wiring, or driver circuit is not delivering the required signal. Use suitable test equipment and compare the affected cylinder with another cylinder under the same conditions. Do not infer a failed DME from a misfire code alone.

Read the spark plug as evidence, not a verdict. A wet plug may reflect fuel that did not burn, an injector problem, weak spark, or repeated cranking. Oil on the firing end and oil in the plug well are different findings. Record which one is present, and whether the plug condition changes after a controlled run.

Use mixture data to check for an air leak

A single-cylinder misfire can coexist with a broader mixture problem. Compare bank fuel corrections at warm idle and at a stable raised RPM, if the scan tool reports those values. A correction that changes substantially with engine speed can support an air-leak branch, but fuel trims do not point to one gasket by themselves.

Inspect the intake connections and test for leaks with an appropriate smoke procedure. Check any hose or fitting disturbed during recent work, including crankcase ventilation connections. The BimmerFest owner found a disconnected vacuum line during the repair history, but the discussion does not show that this line explained every later cylinder 6 symptom. Repeat the test with equipment and pressure appropriate for the intake system. Do not use an improvised method that could damage components or give false confidence.

When compression and leak-down belong next

If ignition, connector, injector control, and air-leak checks do not explain the active misfire, compare compression across all cylinders under the same test conditions. Follow the vehicle-specific procedure for disabling fuel and ignition and managing the throttle. A single reading without a comparison or repeat is weak evidence.

If cylinder 6 is materially different, a leak-down test can help identify where sealing is lost. Listen at the intake, exhaust, and crankcase while the cylinder is held at the required position. A borescope can add evidence about the cylinder and piston crown, but it does not replace compression or leak-down measurements. If readings are even and repeatable, do not jump to a broken camshaft or internal damage on the strength of a misfire code.

Stop running the engine if the misfire is severe, the warning lamp flashes, the engine makes new mechanical noise, or oil pressure or temperature warnings appear. A sustained misfire can damage the exhaust catalyst. Arrange diagnosis or recovery when the car is unsafe to operate.