BMW Long Crank Diagnosis — Cold, Hot & Restart Evidence

Long crank is a condition, not a single fault

"BMW long crank" means the starter cranks for an extended time before the engine fires. The diagnosis depends entirely on which long crank:

• Cold long crank — first start of the day, or after a long cool-down
• Hot long crank — restart when fully warm
• First-start-after-sitting — after the vehicle has stood for hours or days
• Short hot-soak restart — restart a few minutes after a hot shutdown

Each points to different evidence. This page is distinct from the cranks-but-won't-start guide: here the engine does eventually fire, so the question is why it takes longer than it should.

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Differentiate by condition

• Cold long crank, normal hot restart — glow system (diesel), cold fuel pressure build, cold sensor plausibility, battery state of charge.
• Hot long crank, normal cold start — heat-soak sensor drift, hot rail-pressure bleed-down, vapour/fuel handling, HPFP hot behaviour.
• First-start-after-sitting long, normal otherwise — fuel pressure bleed-down overnight; the rail loses prime and must rebuild on cranking.
• Short hot-soak restart long, normal otherwise — hot soak sensor drift or vapour, not mechanical.

Recording the exact condition is the first test.

Fuel-pressure build-up and retention

The most common long-crank cause is a fuel-pressure build delay. Compare rail pressure requested vs actual during the long crank:

• A slow rise of actual toward request → supply, HP pump, regulator, or bleed-down
• Requested itself staying low → DME/DDE not commanding start pressure (authorisation, crank/cam sync)
• Pressure that bleeds away after shutdown and must rebuild → pressure retention / leak-back

Do not state a universal pressure threshold. Compare actual to the DME/DDE's requested value and the verified start-enablement specification for the variant. A pressure-retention test (does the rail hold pressure after shutdown, and for how long) separates a bleed-down fault from a slow-build fault — but interpret it against the variant's verified retention specification.

Low-pressure supply vs high-pressure generation

Confirm low-pressure supply (pump, filter, lines) before condemning the high-pressure pump. A weak low-pressure supply starves the HP pump and produces a slow build that is not an HP pump fault.

Cranking speed

Low cranking speed itself delays rail-pressure build and injection enablement — especially on diesels. Confirm battery, terminals, starter, and grounds with voltage-drop testing before any fuel reasoning. A long crank caused by slow cranking is a battery/ground/starter fault.

Engine-speed signal

A marginal crank/cam signal that is slow to synchronise delays injection enablement and produces a long crank — without necessarily setting a DTC. Scope the crank and cam signals under the cold/hot fault condition. RPM in live data does not prove signal integrity.

Temperature-dependent faults

Sensor plausibility on cold soak (coolant vs oil temperature should agree; a mismatch flags a sensor fault) and heat-soak sensor drift (a sensor that reads correctly cold but drifts hot) both produce temperature-correlated long cranks. Compare sensor readings against engine state and against each other.

Injector / fuel delivery

A leaking or weeping injector can bleed rail pressure down after shutdown (long crank first-start-after-sitting) or alter hot-start mixture. Pair injector-correction values with the rail-pressure behaviour — a pressure bleed-down combined with correction on one cylinder points to that injector.

Start authorisation

A slow or marginal start-authorisation handshake can add a delay before injection is enabled. This is less common than fuel/signal causes but is checked after fuel and signal are confirmed. See the start-authorisation guide.

Evidence comparison: good vs bad operating conditions

The decisive technique on an intermittent long crank is to capture requested-vs-actual rail pressure and cranking speed on a good start and a bad start, then compare. The difference between the two is the evidence. A single snapshot on a good start proves nothing.

Verification after repair

Reproduce the original long-crank condition after repair — cold start, hot restart, hot-soak restart, first-start-after-sitting. A long crank that does not reproduce under the original condition is not verified fixed.

Diagnostic CTA

Diagnosing a BMW long crank on a real vehicle? Import your ISTA fault memory and live data into MechanIQ and let the evidence engine find the next best test.

Trademark disclaimer

MechanIQ is independent software. BMW, DME, DDE, CAS and ISTA are trademarks of BMW AG. MechanIQ is not affiliated with or endorsed by BMW AG. All specifications are guidance only; vehicle-specific verified specification required.