Air-Fuel Ration Diagnostics... What's so hot about A/F testing?

A 14.7 to 1 air fuel ratio is needed for all gasoline engines to work properly. This has nothing to do with make or model, is a function of the gasoline fuel, as a chemical compound. All modern fuel control systems thrive for this small windows a air to fuel mixture. Some lean burning engines will switch to a leaner mix once on cruise, but the main operational parameter is the same. Learn what's involved in proper air fuel diagnosis and repair, right here...




The following procedures should be used to diagnose a lean/rich condition or a high/low fuel trim scanner reading. The steps should be followed in the sequence presented to avoid diagnostics errors.

1. Determine if the ECM is in control. While pre-loading the engine, look at these PIDs—O2 sensor , fuel trims, and MAF or MAP readings and at the same time make sure that the O2 sensor keeps switching at all times except deceleration and WOT. The use of a graphing display is highly recommended because of the ease of graphical visual recognition. Check for any codes present.

2. Check for proper fuel pressure and volume. A low fuel supply can upset the vehicle’s fuel injection performance in ways that might make other engine components appear faulty. Check fuel pump, fuel filter, and the voltage/ground reaching the pump. A resistance in the fuel pump circuit ( less than battery voltage reaching the pump ) can greatly alter the fuel pump’s rotational speed ( RPM ) and fuel flow rate.

3. Determine if proper spark is present. Check for high firing line KV demand, spark line KV, and spark duration. Typical firing line KV are in the range of 8 to 14 KV, spark line KV at around 2 to 3 KV, and spark duration at 1.3 mS minimum for 2 air gap systems ( distributor ), 1.0 mS for a 2 air gap DIS waste spark system and 1.5 mS minimum for 1 air gap systems ( COP). As a rule of thumb, the spark should jump a minimum of an inch.

4. Determine if EGR is stuck open. An EGR stuck open will skew the MAF sensor reading, tricking the ECM into reacting as if there is less air coming into the engine and creating a lean condition. In case of a MAP system, the ECM will react as if there is less vacuum or a bigger load present, therefore, increasing fuel pulsation. An EGR stuck open will make a MAF engine run lean and a MAP engine run rich because of the way these sensors respond to changing conditions.

5. Check the Fuel quality. Certain additives, water, alcohol, etc in the mixture will alter the O2 sensor’s response and have an adverse effect on emissions.

NOTEWhen the ECM is not in control ( O2 not switching ) and the problem is always present ( Idle ) look for primary fuel control sensors like ECT, IAT,TPS off range, and O2 sensor stuck ( lean or rich ). MAP & MAF sensors can also cause idle fuel problems if the reading are off range. A bad air intake duct hose, vacuum hose or clogged vacuum port can also be the culprit since they will upset the MAP/MAF readings. If the problem only happens upon acceleration then the MAP, MAF or TPS sensor are the most likely cause since they are responsible for fuel control at that time. Always check for vacuum and/or fuel leaks.


Fuel problems, whether lean or rich, are usually a result of the following order of possibilities.

a) ECM not in control.( MAF/MAP/TPS/ECT problems, intake air duct hose, MAP vacuum hose, PCV, etc. )

b) Improper pressure & volume.( fuel flow restrictions, clogged injector or filter, kinked fuel hose, defective fuel reg. )

c) Exhaust leaks.( air leaking into the exhaust before the O2 sensor. )

d) Vacuum leaks.( intake gaskets, brake booster, PCV, etc )

e) Fuel leaking into the engine.( Bad fuel reg., injector, etc. )

f) Diluted engine oil.

g) Water in fuel or bad fuel.


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Sensors                 O2-Sensor                  APP-Sensor                CAM-Sensor               CRK-Sensor               ECT-Sensor               IAT-Sensor                 Knock-Sensor              MAF-Sensor                MAP-Sensor                TPS-Sensor                 VSS-Sensor                FRP-Sensor                AFR-Sensor              

Actuators               Fuel-Injectors            Ignition-Coil           Leak-Detection Pump  GM-Series Alternator   Idle-Control Valve      EGR-System            EVAP-System      

Repair Strategies Current-Ramping      Lack of Power           Fuel-Flow Volume     Ignition Waveform       Ignition Testing          A/F Ratio Diagnosis    Minimum Air Rate      No Fuel-Pressure       No Injector Pulse      No-Start/No-Spark     General No-Start      

ECM/PCM Modes Cranking Enrichment   Warm-up Cycle         Open-Loop            Closed-Loop            Accel. Enrichment Deceleration Leaning   Idle Control             Low-Voltage Correction Clear Flood Mode     Selective Inj. Cut-Off   Limp-In Mode           Exhaust Variable Valve

 OBD-2 Codes

Generic DTCs            GM Cars DTCs           GM Truck DTCs        Ford DTCs                  Ford Trucks DTCs        Dodge DTCs                BMW DTCs                 Honda/Acura DTCs       Hyundai DTCs              Isuzu DTCs                  Jaguar DTCs                Kia DTCs                     Land Rover DTCs          Mazda DTCs                Mercedes DTCs           Mitsubishi DTCs           Nissan/Infinity DTCs     Saab DTCs                  Subaru DTCs               Toyota DTCs                Volvo DTCs                  VW/Audi DTCs        

 Code Setting Criteria

Dodge CSC                 Ford CSC                    GM CSC                     Honda/Acura CSC        Hyundai CSC               Isuzu CSC                   Kia CSC                      Mazda CSC                 Mitsubishi CSC            Nissan/Infinity CSC      Subaru CSC                Toyota/Lexus CSC

SRS Airbag DTC

GM SRS Airbag Code   GM Truck SRS Code    Ford SRS Airbag Code  Ford Truck SRS Code   Dodge SRS Code        Dodge Truck SRS Code Acura/Honda SRS Code Isuzu SRS Codes         Mazda SRS Codes        Subaru SRS Codes Infinity/Nissan SRS       Kia SRS Codes     Hyundai SRS Codes     Mitsubishi SRS Codes Lexus/Toyota SRS

How to Get SRS Codes

Retrieving Dodge SRS   Retrieving Ford SRS     Retrieving GM SRS      Retrieving Honda SRS   Hyundai/Kia/Mitsu SRS Isuzu/Mazda/Subaru     Retrieving Toyota SRS  Nissan/Infinity SRS

SRS-Airbag Repair Guide

Deleting SRS Codes   Dodge SRS Location       Dodge SRS Operation     Ford SRS Location          Ford SRS Operation        GM SRS Location            GM SRS Operation           Honda SRS Location       Honda SRS Operation     Isuzu/Mazda/Suba Loc.   Isuzu/Mazda/Suba Ope Kia/Hyun/Mitsu Location Kia/Hyun/Mitsu Oper.       Nissan/Infinity Location   Nissan/Infinity Operation Toyota/Lexus Location    Toyota/Lexus Operation 


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