Petrol Injection Systems
From port injection to gasoline direct injection and the faults each introduces.
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What you will learn
- Compare port and direct injection
- Explain fuel trim and closed-loop control
- Relate misfire codes to fuel, spark and compression
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Early electronic petrol injection sprayed fuel into the intake port (PFI). Gasoline direct injection (GDI) sprays fuel directly into the cylinder at high pressure, improving efficiency and control but introducing new failure modes such as injector deposits, high-pressure pump wear and intake valve carbon build-up.
The ECU continuously corrects fuelling using oxygen sensor feedback: short-term trim is the immediate correction, long-term trim is the learned average. Positive trims mean the ECU is adding fuel (lean), negative trims mean it is removing fuel (rich).
Typical training example: a steady long-term fuel trim beyond about +10% often justifies searching for unmetered air or a weak fuel supply, but the acceptable band is manufacturer specific.
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Key points
- GDI runs at high pressure and can build intake valve deposits
- Fuel trims reveal the direction of the error
- A misfire has three possible families of cause
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
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