Car Will Turn But Not Start: Here’s the Fix

Car Will Turn But Not Start
Car Will Turn But Not Start

Car Will Turn But Not Start: Mercedes-Benz A-Class W177 ; Timing Fault Case Study

When a Mercedes-Benz A Class W177 turns over but won’t start, the culprits often hide in timing control, sensor signals, or fuel/ignition delivery. This real workshop case shows how an exhaust camshaft + camshaft adjuster fault on the M260 Engine created a no-start despite good cranking and how a structured diagnosis led to a clean fix.

Vehicle & Complaint

  • Model: Mercedes-Benz A-Class W177
  • Engine: M260
  • Customer complaint: “Car will turn but not start.”
Car Will Turn But Not Start
Car Will Turn But Not Start

Diagnostic Path (Step-by-Step)

1. Short Test & DTCs

  • Faults logged for exhaust camshaft position → points to timing/VVT control or signal integrity.

2. Power & Ground to Sensor

  • – Exhaust camshaft position sensor power supply: OK
  • Grounds: OK → sensor is powered correctly.

3. Signal Integrity

  • Sensor signal abnormal/unstable during crank.
  • Note: Bad timing can make a good sensor look bad.
Car Will Turn But Not Start
Car Will Turn But Not Start

4. Wiring & Resistance Checks

  • Harness continuity/resistance within spec → wiring not the root cause.

5. Mechanical Timing Verification

  • Opened for inspection.
  • – Finding: Misalignment between exhaust camshaft and exhaust camshaft adjuster (phaser).
  • – Result: ECU can’t reconcile crank/cam correlation → no start even with healthy cranking.
Car Will Turn But Not Start
Car Will Turn But Not Start
Car Will Turn But Not Start
Car Will Turn But Not Start
Technician verifying exhaust cam timing on Mercedes A-Class W177 M260 during no-start diagnosis.
A-Class W177 No-Start – Exhaust Cam Timing Check

Root Cause

Mechanical misalignment/failure of the exhaust camshaft & exhaust adjuster (VVT phaser) on the M260 disrupting cam/crank sync and preventing start authorization/fuel timing.

Car Will Turn But Not Start
Car Will Turn But Not Start
Car Will Turn But Not Start
Car Will Turn But Not Start

Repair Procedure

Replaced:

  • Exhaust camshaft (OEM)
  • Exhaust camshaft adjuster/phaser (OEM)

Reassembly & Setup:

  • Set timing using WIS alignment references.
  • Verified chain tension and timing marks.
  • Cleared DTCs and performed cam/idle adaptations.

Post-repair tests:

  • Crank → Immediate start
  • No DTCs returned
  • Stable cam actual vs. specified positions on live data
New exhaust camshaft adjuster installed on Mercedes M260 engine to fix cranks-but-no-start.
Cam Phaser (Adjuster) Replacement – M260

Quick Reference Tables

Symptoms → Likely Causes

SymptomLikely Cause
Cranks, won’t startCam/crank correlation off (timing/phaser)
DTCs for cam positionSensor signal skewed by mechanical misalignment
Intermittent sync during crankSticking/failed cam phaser or worn sprocket

What We Tested

TestResultMeaning
Sensor power/groundOKSensor powered; look at signal/mechanics
Signal waveformUnstableCorrelation problem likely
Harness continuityOKNot a wiring fault
Mechanical timingOffRoot cause confirmed

Camshaft Adjusters (Phasers) : Fast Primer

  • – Role: Advance/retard cam timing (VVT) for power, economy, emissions.
  • – How: Hydraulic (oil-pressure) phaser controlled by ECU via solenoid; feedback via cam sensor.
  • – Failure modes: Wear, internal sticking, oil sludge, solenoid faults → timing misalignment, no start, misfires.

Prevention:

  • Use MB-approved oil, change on time, keep oil passages clean; inspect/replace phaser if adaptation values drift.

Camshaft Position Sensor ; Location & Notes (General)

  • Typically on the cylinder head near the cam; varies by engine (front/side/rear).
  • A failed sensor can cause hard/no-start, but mechanical mis-timing can mimic sensor failure.

Takeaways

  • Don’t stop at a sensor DTC validate mechanical timing when the engine cranks but won’t start.
  • Power/ground OK + bad signal = suspect timing/phaser, not just the sensor.
  • Always finish with adaptations and a correlation check in live data.

Prevention & Maintenance

ActionWhy it Helps
Follow MB oil spec & intervalsKeeps VVT hydraulics clean and responsive
Inspect timing marks if correlation DTCs repeatCatches early phaser/chain issues
Replace filters with OEMEnsures correct oil flow/pressure to phasers
Record live data baselinesEarly detection of drift in cam actual vs. specified

If you’re facing similar starting problems, explore our full troubleshooting hub:
Mercedes No Start Issues: Causes, Fixes & Case Studies : a complete guide covering electrical, fuel, and ECU-related no-start scenarios across multiple Mercedes models.

FAQ

Q1: Why does the car crank but not start if the sensor has power?
Because the mechanical timing can be off so the ECU sees invalid cam/crank correlation and withholds start.

Q2: Can a bad camshaft adjuster alone cause a no-start?
Yes. A stuck or misindexed phaser can push timing far enough out to prevent combustion during crank.

Q3: Should I replace the cam sensor first?
Not before confirming timing alignment. Replace the sensor only if power/ground/signals remain faulty with verified timing.

Author Bio

Written by Mercedes Expert
With years of hands-on experience diagnosing and repairing Mercedes-Benz systems, he brings technical depth and practical case studies to help car owners, technicians, and enthusiasts troubleshoot complex automotive issues. His work focuses on clear repair guides, OEM-level procedures, and knowledge-sharing to empower both professionals and drivers.
Last update: October 2025