Car systems
Fuel and air: how it works and what fails
The engine needs just the right amount of air and fuel: the computer measures the air coming in, decides how much fuel to inject and corrects the mixture with the oxygen sensors. A vacuum leak, a dirty mass air flow sensor or low fuel pressure upset that mixture and show up as jerking, a rough idle and higher fuel consumption.
Reviewed by Daniel Morales, founder of Reprorace SL, on
How it works
Air comes in through the filter, passes the mass air flow sensor or a manifold pressure sensor, and the electronic throttle sets how much gets through according to the pedal. The pump in the tank and, on direct injection and diesel engines, a high-pressure pump deliver the fuel to the injectors.
With those readings and the temperature, the computer calculates the injection time and adjusts it on the go according to what the oxygen sensors read (the fuel trims). If it has to correct too much, it stores a lean or rich mixture code. The idle is held by the throttle itself or by an extra idle air valve.
The most common faults
- Dirty or faulty mass air flow sensor: it measures less air than there is: jerking, lack of power, black smoke on diesels and a lean mixture.
- Vacuum leaks: a leaking hose or intake manifold gasket lets in unmeasured air; the mixture goes lean and the idle hunts.
- Dirty or faulty throttle body: an unstable idle, stalling when you lift off the pedal or limp mode with little power.
- Low fuel pressure: pump, filter or regulator: jerking on acceleration, hard starting and, on diesels, rail pressure codes.
- Fuel injectors: one that drips or sticks: smoke, a fuel smell, high consumption or a cylinder that does not pull its weight.
A guide, most likely first: a repair shop confirms the diagnosis of each car with its own data.
Symptoms that can come from this system
Car loses power: what limp mode is
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Rough idle: why the engine shakes or hunts when stationary
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Car stalls by itself: causes and what to do
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Black smoke from the exhaust: causes and what to do
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Smell of gasoline or diesel in the car: what to do
Stop as soon as it is safe
Car uses more fuel than normal: causes and what to check
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RPM slow to drop when you lift off the gas: why
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Fuel gauge reads wrong: what it could be
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Diesel will not start after running out of fuel: what to do
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Car jerks when accelerating: causes and what to do
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Engine cranks but will not start: causes and what to do
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The most relevant trouble codes
- P0171Mixture too lean (bank 1)
- P0172Mixture too rich (bank 1)
- P0101Mass or volume air flow sensor A, circuit: range/performance
- P0100Mass or volume air flow sensor A, circuit: malfunction
- P0113Intake air temperature sensor 1 (bank 1), circuit: signal high
- P0087Fuel rail/system pressure: too low (bank 1)
- P0088Fuel rail/system pressure: too high (bank 1)
- P0093Fuel system: large leak detected
- P0190Fuel rail pressure sensor (bank 1), circuit: malfunction
- P0201Cylinder 1 injector A, circuit: malfunction
- P0121Throttle/pedal position sensor/switch A, circuit: range/performance
- P2135Throttle/pedal position sensor/switch A/B voltage: correlation fault
- P0505Idle control system: malfunction
- P0507Idle control system RPM: higher than expected
- P2187System (bank 1): too lean at idle
Generic OBD-II/EOBD codes; each manufacturer adds its own. The same symptom can come from another system.
Code blocks of this system
- P02xxfuel and air metering (injector circuit)
- P05xxvehicle speed, idle control and auxiliary inputs
- P00xxfuel and air metering and auxiliary emission controls
- P21xxthrottle, accelerator pedal and EGR
- P01xxfuel and air metering
- P25xxturbo control
- P20xxparticulate filter, AdBlue and NOx
- P22xxNOx sensors, turbo and fuel pressure
See what your car is saying, system by system
The ECO-LITE plugs into the OBD port and shows the codes and live data your car publishes on your phone, explained; when needed, you share the session with your repair shop.
Presale price: €129 · US$129
Frequently asked questions
What is a lean or rich mixture?
Lean is too little fuel for the air coming in; rich, too much. The computer corrects it on its own within a margin and, when that is not enough, stores P0171/P0174 (lean) or P0172/P0175 (rich).
Can the mass air flow sensor be cleaned?
Some can be cleaned with a dedicated spray without touching the sensing element; if that does not help, it is replaced. First rule out vacuum leaks, which give similar symptoms.
Which live data help here?
The air flow, the manifold pressure, the throttle position, the short- and long-term fuel trims and, on diesels, the actual rail pressure against the requested one.