Pulsation Damper

Fuel System Diagnosis

Fuel System RequirementsThe fuel system must be able to maintain a consistent supply of fuel to the engine If the engine is receiving either more or less fuel than required, engine performance will suffer

Air/Fuel RatioThe stoichiometric air/fuel ratio is 14.7:1A higher ratio (meaning more air or less fuel) is considered a lean conditionA lower ratio (meaning less air or more fuel) is considered a rich condition

Possible Causes of a Lean Condition (Carbureted)Low fuel pressure/volumeWeak fuel pumpBlocked fuel filterPinched fuel lineFaulty fuel pressure regulator (if used)Improper jetsImproper float levelDirty fuel circuits within the carburetor

Possible Causes of a Lean Condition (Fuel Injection)Low fuel pressure/volumeWeak fuel pumpBlocked fuel filterPinched fuel lineFaulty fuel pressure regulatorECM improperly controlling fuel due to:Faulty oxygen sensor (O2)Faulty mass airflow sensor (MAF)Faulty manifold absolute pressure sensor (MAP)Faulty throttle position sensor (TPS)Faulty engine temperature sensor (ECT, CTS)Faulty intake air temperature sensor (IAT)Faulty fuel injectorDirtyElectrically faulty

Possible Causes of a Rich Condition (Carbureted)High fuel pressure/volumeFaulty fuel pressure regulator (if used)Improper pumpBlocked air bleedsHigh float levelImproper jetsFaulty or misadjusted chokeLeaking gasketLeaking main well plugs (Quadrajet)

Possible Causes of a Rich Condition (Fuel Injection)High fuel pressure/volumePinched return fuel lineFaulty fuel pressure regulatorECM improperly controlling fuel due to:Faulty oxygen sensor (O2)Faulty mass airflow sensor (MAF)Faulty manifold absolute pressure sensor (MAP)Faulty throttle position sensor (TPS)Faulty engine temperature sensor (ECT, CTS)Faulty intake air temperature sensor (IAT)Faulty fuel injectorPintle leakingMechanically faulty

TestingFuel pressure testChecks fuel pump for proper pressureChecks fuel pressure regulator for proper operationFuel volume testChecks fuel pump, lines, filter ,etc. For proper flow rateEven if there is proper pressure at idle, at WOT there may be enough restriction due to a clogged filter etc. To cause a drop in pressureInjector balance test

Fuel Pressure TestRelieve fuel pressurePull fuel pump fuse and crankReplace fuseHook the fuel pressure gauge up to the pressure line of the fuel system (use a rag to catch any fuel spillage)Tee into the lineAttach gauge to the Schrader valveLook up testing procedures for particular vehicleKey on testRegulator test

Fuel Pressure Test (General Specs)

Injection SystemPressure SpecGM TBI9-13 PSIGM PFI40-47 PSIGM SFI60-66 PSIFord EFI35-45 PSIChrysler EFI43-53 PSIJeep MFI39-41 PSIJeep SFI45-55 PSIBMW49-58 PSIHonda/Acura38-48 PSI

Fuel Volume TestUsing the fuel pressure gauge, open the bleed valve while the engine is idling Measure the amount of fuel that is pumped through, and the amount of time it took to flowCompare to manufacturers specs1 pint in 30 seconds (rule of thumb)

Injector Balance TestUsed to compare the flow rates of each injectorHook fuel pressure gauge and cycle the ignition key to energize fuel pumpRecord the pressureCycle one injector and record the amount of pressure dropCycle the key to restore full pressure to the systemRepeat for each injectorInjectors should not deviate more than 1.5 PSI from the average drop

ECM ControlsOn fuel injected and feedback carbureted systems, the ECM (engine control module) controls fuel deliveryThe ECM uses various inputs to determine the amount of fuel that should be delivered

Engine Performance ECM Inputs (Sensors)Crankshaft position sensor (RPM)Oxygen sensor (O2)MAP sensorMAF sensorCTS IAT sensorTPS

Crankshaft Position Sensor (CKP)Lets the ECM know how many rpms the engine is running atLets the ECM know when a piston is at TDC (SEFI)May be located In the blockReluctor on crankshaftIn the distributor On the rear of the blockFlywheel/flexplate is the reluctor (Dodge)On the front of the blockReluctor on the harmonic balancerSeparate reluctor

CKP

MAP sensor (Manifold Absolute Pressure)Lets the ECM know what current manifold pressure (vacuum) isUses a strain gauge to produce a signal which the ECM can interpret as manifold vacuum

Oxygen Sensor (O2)Compares the amount of oxygen in the exhaust to the amount of oxygen in the atmosphereProduces its own voltageThe greater the difference in oxygen between the two, the more voltage is produced (0 volts 1 volt)ECM uses this signal as a rich/lean indicator

Oxygen Sensor (O2)

Throttle Position Sensor (TPS)Lets the ECM know how much the throttle is being depressedGenerally located on throttle bodyMay be located on the accelerator pedal and the throttle indirectly operated by the ECM (Corvette and 6.5L turbo diesel)Potentiometer

Throttle Position Sensor (TPS)

Coolant Temperature Sensor (CTS, ECT)Lets the ECM know the current engine coolant temperatureLocated in the head, intake, or thermostat housing

Coolant Temperature Sensor (CTS, ECT)

Intake Air Temperature Sensor (IAT)Lets the ECM know what temperature the incoming air isLocated in the throttle body, air cleaner, or intake manifold

Intake Air Temperature Sensor (IAT)

Mass Airflow Sensor (MAF)Used to tell the computer the mass of the air entering the engine per revolution.A heated wire is suspended in the incoming airstream, and the sensor monitors how much amperage is needed to maintain the temperature of the wire.The more airflow (mass), the faster the wire is cooled, which requires more amperage to be passes through the wire to maintain the temperature.The circuitry of the sensor calculates the airflow from this required amperage and sends a signal to the ECM telling it how much air is entering the engine.Various styles of mass airflow sensors are used.

Mass Airflow Sensor (MAF)

ECM ControlsTo determine the amount of fuel required, the ECM needs to know how much air is entering the engineTwo methods for measuring airflowSpeed-DensityMass Airflow

Speed-DensityThe ECM receives input from MAP (engine vacuum)TPS (throttle position)CTS (coolant temperature)IAT (air temperature)O2 (rich/lean indicator)CKP (engine RPM)

Speed-DensityBased on the engine vacuum, engine rpm, intake air temperature, and the pre-programmed volumetric efficiency of the engine, the computer calculates the amount of air entering the engineEngine operating temperature, throttle position, and rich/lean indications from the O2 are then used to calculate/adjust the required amount of fuel

Speed Density

ECMMAPCKPIATAirflowECMO2CTSTPSInjectors

Mass AirflowBased on the airflow data received from the MAF engine operating temperature, throttle position, and rich/lean indications from the O2 are used to calculate/adjust the required amount of fuel

Mass Airflow

ECMAirflowECMO2CTSTPSInjectorsMAFCKP