Engine Starts Then Dies When Cold
The engine starts but stalls shortly after, especially when cold. This suggests a fuel delivery, air intake, or sensor issue common on BMW N20/B46 engines.
Diagnosis Details
Possible Causes
Faulty high-pressure fuel pump (HPFP) – common on BMW 3 Series with turbo engines, causes hard cold starts and stalling.
Dirty or failing fuel injectors – can cause lean misfire and stalling on cold start.
Crankshaft position sensor or camshaft sensor failure – disrupts ignition timing, leading to start-and-die behavior.
Vacuum leak (e.g., intake manifold gasket, PCV system) – causes lean condition and stalling when cold.
Faulty engine coolant temperature sensor – sends incorrect cold-start enrichment signal.
Weak battery or poor electrical connections – voltage drop can cause ECU reset or fuel pump relay dropout.
Related OBD-II Codes
Not satisfied with this diagnosis?
Try Another DiagnosisWhat To Do Next
What You Should Do
Check for diagnostic trouble codes (DTCs) using an OBD-II scanner.
Inspect fuel pressure (low pressure and high pressure).
Test crankshaft and camshaft sensor signals.
Check for vacuum leaks with smoke test.
Verify coolant temperature sensor readings with live data.
If HPFP is suspected, perform fuel pressure leak-down test.
Related Repairs
Share this diagnosis
Help others with similar symptoms
Disclaimer: This AI-generated diagnosis is for informational reference only. Always consult a qualified mechanic for an in-person inspection. AutOwner is not responsible for decisions made based on this information.
