Audi S5 4.2 FSI (CAUA) Long Crank — Diagnostic Workflow

The problem

"Long crank" on an Audi S5 with the CAUA 4.2 FSI means the starter turns the engine longer than normal before combustion catches and the engine runs on its own. The ECM will not enable injection until two thresholds are met — a minimum crank RPM and a minimum rail pressure — so anything that delays either one produces a long crank that is not necessarily a fuel fault.

This page is the problem-specific path for a CAUA long crank. It is narrower than the CAUA engine hub and deeper than the generic long-crank symptom guide, which covers the symptom across all engines.

Vehicle-specific verified specification required for every threshold on this page; crank-enable and injection-enable values are engine-code, year, and software specific.

Differentiate the condition first

The temperature and history at which the long crank occurs is the first evidence:

• Long crank hot (after a run, warm soak) → vapour lock, hot sensor failure (crank sensor), pressure bleed-down when hot, hot-start enrichment.
• Long crank cold (first start of the day) → fuel-pressure bleed-down overnight, cold-enrichment / coolant-sensor, low crank speed (cold oil drag).
• Long crank after standing (left for hours/days) → pressure bleed-down, priming, check-valve / residual-pressure faults.
• Crank-no-start → crank/cam sync, immobiliser / terminal-50, then fuel pressure, then injector command — a different fault class from long crank.
• Slow cranking → battery, grounds, starter; a slow crank takes longer to reach both enable thresholds.
• Normal cranking speed but delayed combustion → injection/fuel-pressure/timing evidence, not starter.

Capture which of these the customer actually means before testing.

Evidence areas

• Battery / system voltage — low or unstable voltage delays injection enable and destabilises sensors. Confirm supply voltage and grounds first.
• Cranking speed — the ECM enables injection at a minimum crank RPM; a slow crank delays it. Slow cranking is a battery/ground/starter question, not a fuel question.
• Rail-pressure build — does rail pressure reach command during cranking? A pressure that does not build points to low-side supply, HPFP, or control — in that order.
• Residual-pressure reasoning — pressure that bleeds away after shutdown means the pump must rebuild from zero on the next start: a long crank. A pressure hold test after shutdown confirms it.
• Crankshaft signal — the G28 crank sensor; RPM in live data does not prove signal integrity. Scope when the fault is intermittent or hot.
• Camshaft signal / synchronisation — cam sensor and crank/cam correlation; both signals can look fine individually but correlate wrong. Scope the pair.
• Ignition — present and consistent; a missing spark on a hot start is a different fault from a fuel fault.
• Injector command / fuelling — is the ECM commanding injection once the enable thresholds are met? Immobiliser / terminal-50 blocks injection command before any fuel fault exists.
• Mechanical condition — compression and timing; a mechanical cause is a hypothesis requiring confirmation, not a first jump.
• Temperature-dependent evidence — the temperature at which the fault occurs is the single most narrowing piece of evidence.

Do not jump directly to one part

Resist the reflex to go straight to the fuel pump, crank sensor, injectors, or timing:

• A fuel pump replacement will not fix a slow crank, a bad ground, or a hot crank-sensor dropout.
• A crank sensor replacement will not fix pressure bleed-down or a weak battery.
• Injectors will not fix a pressure that bleeds away overnight (that is a check-valve / residual-pressure fault).
• Timing is the most invasive; confirm with crank/cam correlation on a scope and with compression before condemning it.

Each is a hypothesis to be confirmed by the evidence above, not the first answer.

Scan and live-data interpretation

With VCDS (Ross-Tech), capture before clearing anything:

• Freeze-frame for any crank/cam, fuel-pressure, or supply-voltage codes — the condition that set the code is where you reproduce.
• Cranking RPM — confirm the engine reaches the injection-enable threshold at normal speed.
• Requested vs actual rail pressure during crank — the core fuel evidence; does pressure reach command while cranking?
• System voltage during crank — rule out a voltage sag destabilising sensors and control.
• Crank and cam signal status where supported — sync and presence.

Do not publish proprietary or universal channel/group numbers.

Relevant tests

• Pressure hold test after shutdown — confirms bleed-down (overnight or hot).
• Pressure build during crank — confirms whether pressure reaches command while cranking.
• Crank and cam scope — confirms signal integrity and correlation, especially for a hot intermittent.
• Battery / ground voltage drop — confirms supply integrity under cranking load.
• Compression / relative compression — confirms mechanical condition only when fuel, sensor, and electrical causes are exonerated.

Next-best-test logic

1. Confirm which condition (hot / cold / after standing / slow crank) the customer means.
2. Check system voltage and grounds under cranking load.
3. Log requested vs actual rail pressure during crank and run a pressure hold test after shutdown.
4. Scope crank and cam if the fault is intermittent or temperature-dependent.
5. Only then form the injector / mechanical hypothesis and confirm with balance and compression.

Confirmation criteria

Confirm the cause only when the evidence isolates one path: voltage and grounds good, crank speed normal, pressure build and hold confirmed, crank/cam sync confirmed, and the long crank reproducibly explained by one element. Act on confirmatory evidence, not on the first named part.

Repair verification

After repair, clear fault memory, and start the engine under the condition that failed (cold morning, hot restart, after standing). The repair is verified when the engine starts promptly under the reproducing condition and live data shows pressure reaching command during crank.

CTA

Already have scan data or live values from this engine? MechanIQ can help organise the evidence, rank the diagnostic hypotheses, and identify the next best test.

Trademark disclaimer

MechanIQ is independent software. Audi, VAG, CAUA, FSI and ODIS are trademarks of Volkswagen AG. VCDS is a trademark of Ross-Tech, LLC. MechanIQ is not affiliated with or endorsed by any of these companies. All specifications are guidance only; vehicle-specific verified specification required.