Air Fuel Control

Air Fuel ControlThe Stoichiometric mixture of 14.7:1 is critical toemission control.14.7 grams of Air mixed with 1 gram of Gasolinecreates the most efficient air/fuel ratio (for the catalyst)The Stoichiometric mixture is critical for thecatalytic converter to operate efficiently to reduceHydrocarbons (HC) and Carbon Monoxide (CO)

Air Fuel ControlThe PCM will sense how much air enters theengine and match it with the proper amount of fuel.The oxygen sensor will report to the computer ifthe air/fuel mixture is too Rich or too Lean.The PCM will “Trim” how much fuel is added byturning on the fuel injectors for a longer or shorteramount of time.

Air Fuel ControlUsing a scan tool you can monitor “Fuel Trim” .Fuel trim is displayed in % (percentage)Numbers close to 0% mean the computer is sensingthe proper amount of air entering the engineand adding the proper amount of fuel.

Air Fuel ControlPositive fuel trim means the oxygen sensor isreporting too much oxygen in the exhaust.To correct this the PCM will add extra fuel.A fuel trim of 10% means the PCM has to add10% MORE fuel to keep the stoichiometric mixtureof 14.7 grams of air to 1 gram of fuel.

Air Fuel ControlA fuel trim of -10% means the PCM has to add10% LESS fuel to keep the stoichiometric mixtureof 14.7 grams of air to 1 gram of fuel.Fuel trim numbers over 10% or under -10%should be investigated to ensure all sensors areaccurate, fuel pressure is correct, and injectors arenot leaking or plugged.

Base Fuel ControlThe PCM calculates all fuel quantitiesbased on a specific fuel pressureThe Fuel pressure regulator must beaccurate to ensure proper fuel trim and fuelcontrol.The PCM will only monitor fuel pressure onthe newest fuel injection systems.

Base Fuel ControlFuel pressure that is too high too rich(fuel trim under -10%)Fuel pressure that is too low too lean(fuel trim over 10%)Correct fuel pressure is required for fuel trimto maintain the Stoichiometric air/fuel ratio

Vacuum controlled Fuel Pressure Regulatorpressure regulatedfuel supplylow pressurefuel return

Vacuum controlled Fuel Pressure Regulatorpressure regulatedfuel supplylow pressurefuel return

Check Fuel PressureTest fuel pressure with No Vacuum to thefuel pressure regulatorANDTest Fuel Pressure with 20” vacuumapplied to the fuel pressure regulatorThere should be 10 PSI difference

Fuel FiltersRestricted fuel filters may allow proper fuelpressureWith LOW fuel volume.This will cause engine to run leanEspecially under load

Replacing Fuel FiltersFuel filters are often under about 60 psieven when the engine is OFF!Be sure to relieve all fuel pressure beforereplacing fuel filtersBe sure to pressurize and carefully checkfor leaks at filter any time you haveremoved or replaced it.

Types of Fuel InjectionCISContinuous Fuel Injection(very old German cars)TBIThrottle Body Injection(early style of Electronic Fuel Injection)PFIPort Fuel Injection(Can be Group Fired or Sequential)

CISContinuous Injection SystemUsed by European (Robert Bosh) vehiclesMay be mechanical or computer controlledConstantly injects fuel at each intake port.Fuel pressure regulated by mechanicalintake air sensing plate.More air More Fuel Pressure

TBIThrottle Body InjectionFuel is injected above the throttle platePCM turns injectors on and offMore fuel is added by leaving injector on longerAll PCM calculations assume correct fuel pressure

PFIPort Fuel InjectionOne fuel injector for each cylinderFuel pressure regulator keeps the quantity ofinjected fuel constantAll PCM calculations assume correct fuel pressure

Port Fuel InjectionInjectors can be fired once per engine cycleSingle FireInjectors may be fired twice per engine cycleDouble FireInjectors are fired simultaneous or in groupsSequential fuel injection fires individualinjectors just before each intake stroke

Basic MaintenanceInjectors must be kept cleanLeaking ‘O’ rings can cause a lean misfireOrLeaking ‘O’ rings can cause an external fuel leakShorted injector windings can burn out the PCM

A leakin this‘O’ ringmaycause aleanmiss

A leakin this‘O’ ringleaksgasolineonto theengine

Cold EnrichmentWhy do cold engines need riches mixture?Less fuel will vaporizeWhat sensor is used to enrich cold engines?ECTEngine Coolant Temperature sensor

High Resistancewhen Cold!

Low Resistancewhen Hot!

ECTMost defects in the Engine CoolantTemperature sensing circuit make PCM thinkthe engine is colderThis will cause a rich mixtureCompare actual temperature to scan tool dataUsing the ODB-II room!

Cold IdleCold engines idle at a higher RPMThis is achieved by an IACIdle Air Control

Idle Air ControlMany systems will open the IAC while theengine is cranking or starting.This is why many engines have nomeasureable cranking vacuum

Main Fuel MeteringThe Stoichiometric mixture of 14.7:1 iscritical to emission controlPCM uses sensors measure the engine“load” to determine how much fuel to addLoad sensors are the MAF, the VAF, andthe MAP sensors

MAFMass Air Flow SensorMeasures volume, temperature andhumidity of incoming airWill be inaccurate if “hot wire” is dirty orcontaminatedWill be inaccurate if there are any air leaksbetween the sensor and the intake valve.

VAFVane Air Flow sensorSome systems measure the volume ofincoming air.This is less accurate as temperature andhumidity also affect the oxygen content of agiven volume of airAny air or vacuum leaks will fool theengine into running lean

Speed DensityDoes not directly measure incoming airUses a MAP sensor. Temperature sensor Throttle Position sensor and RPM sensorIndirectly calculates engine load

MAPManifold Absolute Pressure SensorMeasures manifold pressure (vacuum)Any vacuum leak will be measured andmore fuel will be addedIdle speed will increaseMay send a varying voltage, or frequency

Closed LoopFeedbackSensors are monitored and the correct fuelquantity is calculatedOxygen sensor is monitored to adjust“fuel trim”O2 (Oxygen) sensor provides the feedback

O2S & HO2SOxygen sensor must be hot to operateModern Oxygen sensors have a heating elementto keep them at operating temperatureSignal of 0.2V (200mV) to 1.1V (1100 mV)Any signal below 450mV is considered “lean”

O2S & HO2SAny air leak in the exhaust will send a “false”lean signalAny cylinder misfire will send unburned oxygeninto the exhaust and cause a lean signalA worn out O2S will send a lean signal

O2S & HO2SAny lean signal will have the PCM add fuel(positive fuel trim numbers)“False” lean signals will cause the vehicleto run rich

Closed LoopFeedbackWhen in closed loop the fuel control attempts tomaintain a stoichiometric mixtureThe Load sensor, ECT (Engine CoolantTemperature) sensor and O2 sensors are critical

Open LoopAt times the stoichiometric mixture is notwantedIdle, cold operation, high load (power) andaccelleration all call for extra fuel and areconsidered “open loop”When in open loop the feedback from theoxygen sensor is ignored by the PCM

TPSThrottle Position SensorThe TPS asks for extra fuel when it isquickly openedThe TPS asks for extra fuel when thethrottle is held wide openThe TPS asks for extra fuel when the engineis at idle (older systems)

TPSThrottle Position SensorMany Acceleration, Power and Idleproblems can be traced to a defective TPSCheck voltage signal with an oscilloscopeto catch small voltage dropouts

Air Fuel Control A fuel trim of -10% means the PCM has to add 10% LESS fuel to keep the stoichiometric mixture of 14.7 grams of air to 1 gram of fuel. Fuel trim numbers over 10% or under -10% should be investigated to ensure all sensors are accurate, fuel pressure is correct, and injectors are not leaking or plugged.