For automotive technicians and DIY enthusiasts alike, navigating engine driveability issues can be daunting without the right tools and knowledge. Fortunately, the OBD2 scanner has become an indispensable asset in modern vehicle diagnostics. Among the plethora of data points available, understanding the EQ_RAT (Equivalence Ratio) and its normal range is crucial for accurate assessments. This article delves into how to effectively utilize your OBD2 scanner, focusing on the EQ_RAT parameter to pinpoint and resolve engine performance problems.
Deciphering OBD2 Scan Tool Data: Your Diagnostic Starting Point
When faced with a vehicle exhibiting driveability symptoms, a systematic diagnostic approach is essential. While advanced factory scan tools offer comprehensive capabilities, a generic OBD2 scanner provides a surprisingly robust foundation for most diagnostic tasks. In fact, around 80% of driveability problems can be effectively narrowed down or even solved using the generic parameters available on an OBD2 scanner, many of which are accessible with tools priced under $300.
The evolution of OBD2 standards has significantly enhanced the value of generic scan data. Early OBD2 systems provided around 13 to 20 parameters, but revisions, particularly for CAN-equipped vehicles, have expanded this to over 100 potential parameters. This wealth of information, as illustrated by data from a 2005 Dodge Durango (Fig. 2), offers a deeper insight into engine operation. To effectively leverage this data, understanding key parameters like EQ_RAT is paramount.
Fuel Trim and EQ_RAT: Cornerstones of Engine Health
Regardless of the specific driveability issue, Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT) should be your initial focus. Fuel trim acts as a window into the Powertrain Control Module’s (PCM) fuel delivery adjustments and adaptive strategies. Displayed as a percentage, the ideal range for STFT and LTFT is within ±5%. Positive percentages indicate the PCM is enriching the mixture to compensate for a lean condition, while negative values signal leaning to counteract a rich condition. While STFT fluctuates rapidly, LTFT remains more stable. Deviations exceeding ±10% warrant further investigation.
To assess the condition across different engine loads, check fuel trim at idle, 1500 RPM, and 2500 RPM. For instance, high LTFT at idle that corrects at higher RPMs suggests a vacuum leak. Conversely, issues persistent across RPM ranges often point to fuel supply problems like a failing fuel pump or clogged injectors.
In bank-to-bank fuel control engines, fuel trim can isolate problems to specific cylinder banks. For example, a significantly negative LTFT on Bank 1 and near-zero LTFT on Bank 2 indicates a Bank 1 specific issue.
Essential OBD2 Parameters Beyond Fuel Trim
Several other parameters complement fuel trim analysis and offer broader diagnostic insights:
- Fuel System Status 1 & 2: These should ideally be in “Closed Loop” (CL). Open loop operation may compromise fuel trim accuracy.
- Engine Coolant Temperature (ECT): Aim for a normal operating temperature of 190°F (88°C) or higher. Low ECT can artificially enrich the fuel mixture.
- Intake Air Temperature (IAT): Should reflect ambient or underhood temperature. For cold engine checks (KOEO), IAT and ECT should be within 5°F (-15°C) of each other.
- Mass Airflow (MAF) Sensor: Measures incoming air mass for fuel calculations. Verify MAF readings across RPM ranges, including Wide Open Throttle (WOT), against manufacturer specifications. Ensure correct unit settings (grams per second or pounds per minute) on your scan tool to avoid misdiagnosis.
- Manifold Absolute Pressure (MAP) Sensor: Measures manifold pressure to determine engine load. Displayed in inches of mercury (in./Hg). Remember MAP is different from manifold vacuum; vacuum can be calculated using barometric pressure (BARO) and MAP.
Oxygen Sensors: Monitoring Fuel Mixture and Catalyst Efficiency
Oxygen Sensor Output Voltages (B1S1, B2S1, B1S2, etc.) are vital for PCM fuel mixture control and catalytic converter monitoring. A basic sensor test involves checking voltage fluctuations: sensors should swing above 0.8V and below 0.2V rapidly during throttle changes. Graphing scan tools enhance oxygen sensor diagnosis, but data grids can still be used. Remember that OBD2 generic data has a limited sample rate, especially when monitoring multiple parameters simultaneously. For detailed sensor analysis, a lab scope offers real-time measurements.
EQ_RAT (Equivalence Ratio): Understanding the Commanded Air/Fuel Mix
EQ_RAT (Commanded Equivalence Ratio) is a crucial parameter for understanding the PCM’s target air/fuel ratio. For traditional oxygen sensor systems in closed-loop, EQ_RAT should ideally read 1.0, indicating stoichiometric combustion (14.7:1 for gasoline). In open-loop, it reflects the PCM’s commanded ratio for conditions like power enrichment or deceleration. Wide-range oxygen sensors provide EQ_RAT readings in both open and closed loop.
To calculate the commanded Air/Fuel (A/F) ratio, multiply the stoichiometric A/F ratio by the EQ_RAT. For gasoline (stoichiometric 14.64:1), an EQ_RAT of 0.95 results in a commanded A/F of approximately 13.9:1 (14.64 x 0.95 = 13.9).
Understanding the normal range for EQ_RAT is essential for diagnosis. In closed loop operation under normal driving conditions, an EQ_RAT close to 1.0 is expected. Deviations from this value, especially in conjunction with fuel trim abnormalities, can indicate underlying issues. For example, an EQ_RAT consistently below 1.0 suggests a commanded rich mixture, potentially due to sensor inaccuracies or intentional enrichment strategies by the PCM. Conversely, values above 1.0 indicate a commanded lean mixture.
New OBD2 Parameters: Expanding Diagnostic Horizons
Modern OBD2 systems offer a wealth of enhanced parameters, including:
- FUEL STAT 1/2: Provides more detailed fuel system status information beyond just “Open Loop” or “Closed Loop,” such as “OL-Drive” (open-loop during power enrichment) or “CL-Fault” (closed-loop with sensor fault).
- ENG RUN TIME: Tracks engine run time since start, helpful for diagnosing intermittent issues appearing after a specific duration.
- DIST MIL ON: Indicates distance traveled with the Malfunction Indicator Lamp (MIL) active, useful for assessing problem persistence.
- COMMAND EGR/EGR ERROR: Displays commanded and actual EGR valve position as percentages, aiding EGR system diagnostics.
- EVAP PURGE: Shows commanded EVAP purge percentage, crucial for diagnosing fuel trim anomalies potentially caused by EVAP system operation.
- FUEL LEVEL: Provides fuel tank level percentage, important for meeting monitor enabling conditions for systems like misfire and evaporative emissions.
- WARM-UPS: Counts warm-up cycles since DTCs were cleared, useful for verifying monitor completion requirements.
- BARO: Displays barometric pressure, aiding diagnosis of MAP/MAF sensor issues.
- CAT TEMP B1S1/B2S1: Shows catalyst temperature, valuable for assessing catalyst performance and overheating risks.
- CTRL MOD (V): Displays PCM voltage supply, crucial for identifying voltage-related issues.
- ABSOLUT LOAD: Normalized air mass per intake stroke, reflecting engine load and pumping efficiency.
- TP-B ABS, APP-D, APP-E, COMMAND TAC: Parameters related to throttle-by-wire systems, facilitating diagnostics of electronic throttle control.
Mastering Your OBD2 Scan Tool for Effective Diagnostics
Generic OBD2 scan data has evolved into a powerful diagnostic resource. To effectively utilize your scan tool:
- Familiarize yourself with available parameters: Explore the data your scanner provides and understand their meanings.
- Analyze parameter relationships: Don’t view parameters in isolation. Understand how they interact and influence each other (e.g., EQ_RAT and fuel trim).
- Utilize graphing and recording features: If possible, choose a scan tool with graphing capabilities to visualize dynamic data and recording to capture intermittent faults.
- Consult vehicle-specific information: Remember OBD2 generic specifications may have variations. Always refer to vehicle service information for accurate specifications and procedures.
By mastering your OBD2 scanner and understanding key parameters like EQ_RAT and fuel trim, you can significantly enhance your diagnostic capabilities and efficiently resolve a wide range of engine driveability issues.
