Experiencing a car engine that’s running rich can be frustrating. Symptoms like poor fuel economy, a strong fuel smell, and sluggish performance are common indicators. Modern vehicles equipped with OBDII systems provide valuable insights into engine operation, and examining live data, particularly the air intake temp, can be crucial in diagnosing such issues. Let’s delve into a real-world scenario and explore how OBDII live data can shed light on a rich running condition.
One car owner encountered a perplexing situation after replacing their Throttle Position Sensor (TPS). Despite installing a new Mopar TPS and verifying its voltage readings, their engine continued to run poorly and rich once warmed up. Suspecting a thermostat issue, they checked and confirmed the presence of a factory-spec 195°F thermostat, eliminating coolant temperature as the immediate cause.
To isolate the problem, they employed a clever method: disconnecting each fuel injector, one at a time, while the engine was running. Interestingly, upon unplugging any injector, the engine would initially stumble, but the engine control unit (ECU) would quickly compensate, smoothing out the idle. After reconnecting the injector, the engine would idle smoothly and the air-fuel mixture would lean out significantly. An OBDII scanner confirmed this, showing the oxygen sensor readings shifting from a rich .8-1.0V range to a lean .02-.08V range. Furthermore, Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values plummeted from the high 30s to 0.0, and the scanner displayed “NA” instead of “CL” (Closed Loop) or “OL” (Open Loop), indicating an unusual operating state.
This temporary fix, achieved by cycling any injector, would last only until the engine was turned off and restarted. The rich running condition would then return. This behavior raises questions about what could be causing the engine to run rich in the first place and why interrupting the injectors, even briefly, temporarily resolves it.
While the original troubleshooting steps didn’t explicitly focus on air intake temp, this parameter, readily available in OBDII live data, is vital for correct air-fuel mixture calculation. The ECU uses the air intake temperature sensor (along with other sensors like mass airflow sensor or manifold absolute pressure sensor) to determine the density of the incoming air. If the air intake temperature sensor is malfunctioning and reporting a falsely low temperature, the ECU might incorrectly assume the incoming air is denser than it actually is. Consequently, it would command the fuel injectors to deliver more fuel, resulting in a rich running condition.
In the described scenario, the act of unplugging and replugging an injector might be momentarily resetting or recalibrating some ECU parameters, possibly related to fuel trim or sensor readings. However, without further investigation using OBDII live data, including a detailed look at the air intake temp reading alongside other sensor values like coolant temperature, MAF/MAP readings, and oxygen sensor data, pinpointing the root cause remains challenging.
Therefore, when faced with a rich running engine, especially after component replacements like the TPS, a systematic approach using OBDII live data is essential. Monitoring air intake temp is a crucial step, alongside examining fuel trims, O2 sensor readings, and other relevant parameters. This data-driven approach allows for a more accurate diagnosis and effective repair strategy, moving beyond guesswork and towards precise problem-solving in modern automotive systems.
