FAQ
TL;DR: BMW N43 hard‑start/misfire often links to rail‑pressure anomalies (reported at 150 bar) and rough idle—"High pressure ... isn't it too high?" Check 5 bar feed, swap‑test injectors, and try the unplug test to localize faults. [Elektroda, Łysoldrift, post #16962426]
Why it matters: This FAQ helps BMW E87 N43 owners quickly isolate fuel‑system causes of hard starts and cylinder misfires.
Quick Facts
- N43B16 with direct injection; symptoms noted: hard start, misfires (often cyl. 4), rough idle. [Elektroda, Łysoldrift, post #16962426]
- Reported rail pressure: ~150 bar at idle, dropping near ~25 bar as RPM increases. [Elektroda, Łysoldrift, post #16962426]
- In‑tank (low‑pressure) pump: ~5 bar considered correct by users. [Elektroda, Łysoldrift, post #16962426]
- Unplugging rail‑pressure sensor or HPFP regulator forced ~5 bar and smoothed idle. [Elektroda, Łysoldrift, post #16966886]
- One owner resolved the issue by replacing all injectors with new units. [Elektroda, Łysoldrift, post #17669527]
What are the most common thread‑reported causes of hard starting and misfires on the BMW N43?
Users reported abnormal rail pressure, misfires starting on one cylinder then spreading, and injector issues. Timing was checked and confirmed correct by the OP. The thread centers on fuel delivery: low‑pressure pump at 5 bar, high rail around 150 bar at idle, and erratic behavior under load. Start with fuel pressure verification and injector swap tests to see if misfires follow a component. "High pressure ... isn't it too high?" captures the core concern. [Elektroda, Łysoldrift, post #16962426]
Is ~150 bar rail pressure normal at idle on the N43?
The OP observed about 150 bar at idle and questioned if it was excessive, with pressure dropping toward 25 bar as RPM increased. This sparked the diagnostic focus on the high‑pressure side. Treat 150/25 bar as a symptom to investigate, not a target spec. Correlate pressure readings with misfire counters and idle quality to decide whether to inspect the sensor, regulator, or injectors next. [Elektroda, Łysoldrift, post #16962426]
Does swapping injectors change which cylinder misfires?
The OP moved injectors between cylinders and saw no change in which cylinder misfired. That suggests the fault was not confined to a single injector in that test, and it kept attention on system‑level pressure control or multiple injectors. Use this as a screening step before purchasing parts. [Elektroda, Łysoldrift, post #16962426]
What fixed the original poster’s car?
The thread author reported the cure was replacing all injectors, stressing “Only new!” parts. This indicates multiple injectors were weak or out of spec, and a full set solved the misfire and start issues in that case. "All injectors to be replaced. Only new!" [Elektroda, Łysoldrift, post #17669527]
What if new injectors don’t solve it?
One user installed new injectors and still had issues during gentle driving, while high‑RPM operation was fine. This edge case points to load‑dependent control or pressure regulation problems rather than injector hardware alone. Check the rail‑pressure sensor signal and HPFP regulator behavior under light load. [Elektroda, radek2266, post #17670008]
Why does unplugging the rail sensor or HPFP regulator make the engine idle better?
The OP noted that unplugging either the rail‑pressure sensor or the HPFP regulator dropped pressure to ~5 bar and the engine ran smoothly. This forces the system into a fallback state, which can mask over‑pressure or erratic control. Use this as a diagnostic clue toward rail control or injectors. [Elektroda, Łysoldrift, post #16966886]
What low‑pressure pump value should I see on this car?
Users reported the in‑tank pump supplying about 5 bar, considered correct for initial checks. If feed pressure is far from ~5 bar, address the low‑pressure side first. Stable supply pressure avoids false conclusions about the high‑pressure circuit and injectors. [Elektroda, Łysoldrift, post #16962426]
How do I perform a quick rail‑pressure sanity test on the N43?
- With the engine idling rough, unplug the rail‑pressure sensor or the HPFP regulator.
- Observe rail pressure drop near ~5 bar and note idle quality changes.
- If idle smooths, investigate rail‑pressure control and injector leakage next. [Elektroda, Łysoldrift, post #16966886]
Can incorrect timing be the cause here?
Timing was checked after prior work and confirmed correctly aligned by the OP. Despite correct timing, hard starts and misfires persisted. This steered diagnostics away from mechanical timing and toward the fuel system and controls in this thread. Verify timing, but expect fuel‑system answers for these symptoms. [Elektroda, Łysoldrift, post #16963339]
Why do misfires start on one cylinder and then involve others?
The OP saw cylinder 4 misfires first, then faults appeared on all cylinders. That pattern suggests a shared‑system issue, such as rail pressure control or multiple injectors degrading. Track misfire counters during start and warm‑up to see how the spread develops. [Elektroda, Łysoldrift, post #16962426]
What is the rail‑pressure sensor on the N43?
It measures fuel pressure in the high‑pressure rail for the DME. In this thread, unplugging it forced a ~5 bar fallback and a smoother idle, which implicated pressure control and injector behavior. Use the unplug test only for diagnostics, not as a fix. [Elektroda, Łysoldrift, post #16966886]
What is the HPFP (high‑pressure fuel pump) regulator?
It is the control element on the high‑pressure pump that modulates rail pressure. The OP observed that unplugging the regulator dropped pressure to ~5 bar and improved idle, aiding diagnosis toward control or injector issues. Inspect wiring, connector fit, and commanded duty. [Elektroda, Łysoldrift, post #16966886]
Where can I find official BMW procedures or specs mentioned in the thread?
A participant pointed to BMW TIS resources for specifications and procedures. Use these to confirm pressures, replacement steps, and torque values before repairs. They complement forum observations with factory documentation and diagrams. [Elektroda, ozon79, post #16966945]
What is direct injection on the N43, in simple terms?
Direct injection means the injector sprays fuel straight into the combustion chamber, not the intake port. The OP explicitly states the N43 is a direct‑injection engine, which explains its high rail pressures and injector sensitivity. Precise fueling is critical for smooth starts. [Elektroda, Łysoldrift, post #16962426]
After cold nights, my misfire returned on cylinder 2 even with a new injector. What does that suggest?
A user reported improved cold starts after injector replacement, but the misfire reappeared on cylinder 2 after two weeks and cold weather. That suggests temperature‑sensitive issues like sealing, wiring, or rail control, not just a single injector. Re‑evaluate pressure data and swap components to confirm. [Elektroda, sparko86, post #18702089]
