Fuel Systems & Air-Fuel Management
Stoichiometric ratio, rich and lean mixtures, short and long term fuel trim, throttle body operation, electronic throttle control, GDI vs port injection, and GDI-specific issues.
- State the stoichiometric air-fuel ratio for gasoline engines
- Explain the difference between rich and lean fuel mixtures
- Interpret short term fuel trim (STFT) and long term fuel trim (LTFT) values
- Describe throttle body function and electronic throttle control (drive-by-wire)
- Compare GDI and port fuel injection systems including fuel pressure and common issues
Lección 1
Stoichiometric Ratio & Fuel Trim (STFT/LTFT)
The Stoichiometric Ratio
The stoichiometric air-fuel ratio for gasoline engines is 14.7:1. This means 14.7 parts of air by weight for every 1 part of fuel. At this ratio, all the fuel and all the oxygen are completely consumed during combustion, producing maximum catalytic converter efficiency.
Rich vs Lean Mixtures
A rich fuel mixture means too much fuel, not enough air - the ratio drops below 14.7:1 (e.g., 12:1). A lean mixture means too much air, not enough fuel - the ratio rises above 14.7:1 (e.g., 16:1).
Rich Mixture (Below 14.7:1)
Too much fuel, not enough air
Black smoke from exhaust
Increased HC and CO emissions
Washes oil from cylinder walls
Can foul spark plugs
Lean Mixture (Above 14.7:1)
Too much air, not enough fuel
Higher combustion temperatures
Increased NOx emissions
Can cause detonation/knock
Potential engine damage
Short Term Fuel Trim (STFT)
Short term fuel trim (STFT) indicates the immediate adjustments the ECM is making to fuel delivery. STFT responds in real-time to the oxygen sensor signal, adding or subtracting fuel to maintain the stoichiometric ratio.
- Positive STFT (e.g., +10%) - ECM is adding fuel because the mixture is lean
- Negative STFT (e.g., -8%) - ECM is removing fuel because the mixture is rich
- Near 0% - fuel delivery is on target
Long Term Fuel Trim (LTFT)
Long term fuel trim (LTFT) indicates learned adjustments to fuel delivery over time. When STFT consistently corrects in one direction, the ECM programs that correction into LTFT so the short term trim can return to near zero.
Fuel Trim Interpretation
A positive fuel trim value indicates the ECM is adding fuel because the mixture is lean. If LTFT is +15% or higher, there is likely a significant vacuum leak, weak fuel pump, or dirty MAF sensor. Normal fuel trims stay within plus or minus 10%.
The stoichiometric air-fuel ratio is 14.7:1. A positive fuel trim means the ECM is adding fuel to correct a lean condition. STFT shows immediate corrections; LTFT shows learned corrections over time.