CASE 011
Random Misfire P0300 After City Driving
advanced
22 min
Petrol
Vehicle
- ModelTraining Hatch 1.4
- Engine1.4 MPI
- Mileage118,000 km
Customer complaint
The engine stumbles at idle after a long period of short city journeys. The check engine light comes on intermittently.
History
Mostly short trips. Plugs and air filter are recent. EGR was cleaned a year ago.
Fault codes
P0300 Random/multiple cylinder misfireP0401 EGR flow insufficient
Live data
- Misfire countersScattered across cylinders
- Fuel trimsSlightly positive
- EGR feedbackBelow commanded
Measurements
- CompressionEven across cylinders
- EGR valve commandMoves but position lags
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Diagnostic case
Next step — Reasoning.
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Confirmed cause
Restricted EGR passages causing a scattered misfire at idle and an EGR flow code.
Reasoning
Random misfire across cylinders with even compression and slightly positive trims points away from a mechanical fault and toward a distribution or dilution problem. EGR feedback below commanded, with a stored EGR flow code, identifies restricted passages. Recirculated gas that cannot be metered makes idle combustion unstable.
Recommended repair
- Clean or replace the EGR valve and clear the passages.
- Re-test EGR flow with commanded tests.
- Clear codes, road test and confirm misfire counters stay at zero.
Related tools
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.