Technical Description of OBD-II Trouble Code
Correlation Between Engine Coolant Temperature and Fuel Temperature
What does that signify?
This generic powertrain diagnostic trouble code (DTC) is applicable to various OBD-II vehicles, including those from Mercedes-Benz, Chevrolet, Sprinter, GMC, Ford, etc.
When your OBD-II equipped vehicle displays a service engine soon lamp and stores a code P008F, it indicates that the powertrain control module (PCM) has detected a discrepancy in the signals correlating engine coolant temperature (ECT) and fuel temperature.
Engine coolant temperature and fuel temperature are typically compared only when the engine is at ambient temperature.
ECT sensors consist of a thermal resistor that is dipped in a hard resin and sealed in a metal or plastic housing. Brass is commonly used as the housing material for ECT sensors. The ECT housing is designed to be threaded into a coolant passage in the engine intake manifold, cylinder head, or block. As warm coolant flows through the passages and across the ECT sensor, the thermal resistance in the ECT decreases. When engine coolant temperature decreases, resistance increases, resulting in a reduction in ECT sensor circuit voltage. The PCM interprets these fluctuations in resistance (which cause voltage variations) as changes in engine coolant temperature. Input data from the ECT sensor is crucial in calculating fuel delivery and ignition timing strategy.
The fuel temperature sensor also utilizes a thermal resistor, but it is of the low voltage type. It is often integrated into the fuel level sending unit and provides the PCM with a voltage signal reflecting fuel temperature whenever the ignition is in the ON position. Fuel temperature can be particularly important for drivability in vehicles with diesel engines. Fuel temperature is also more of a concern in extremely cold climates.
A code P008F will be stored and a malfunction indicator lamp (MIL) may illuminate if the PCM detects a discrepancy (between the ECT sensor and the fuel temperature sensor) that exceeds the maximum allowable parameters over a set period of time and under specific circumstances.
What is the severity of this DTC?
Since ECT and fuel temperature are crucial for fuel delivery and ignition timing strategy, a stored code P008F should be addressed with some level of urgency.
What are some of the symptoms of the code?
Symptoms of a P008F engine code may include:
What are some of the common causes of the code?
Possible causes for this engine code may include:
What are some P008F troubleshooting steps?
If there are any stored ECT sensor related codes, it is important to ensure that the engine is not overheating. It should have sufficient coolant and operate within the acceptable temperature range. If the engine is not overheating, the next step is to visually inspect the wiring and connectors of the ECT sensor and fuel temperature sensor system.
Before attempting to diagnose a code P008F, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), an infrared thermometer with a laser pointer, and a reliable source of vehicle information. Connect the scanner to the vehicle diagnostic port and retrieve all stored codes and freeze frame data. Take note of this information as it may be helpful during the diagnostic process. Clear the codes and test-drive the vehicle to see if the code is reset.
Obtain wiring diagrams, connector pin out charts, component testing specifications, and connector views from a reliable vehicle information source. This information will assist in testing individual circuits and sensors. To prevent damage to the controller, test individual system circuits using the DVOM only after disconnecting the PCM (and all related controllers). Connector pin-out charts and wiring diagrams will help effectively test individual circuit resistance and/or continuity.
Use the DVOM (and the reliable vehicle information source) to test the ECT and fuel temperature sensors. Follow component testing specifications, disconnect the sensor to be tested, and set the DVOM to the ohms setting. Sensors that do not comply with manufacturer’s specifications are considered faulty.
If the P008F code is immediately reset, probe the reference circuit pin of each individual sensor connector using the positive test lead of the DVOM. Use the negative test lead to probe the ground pin.
Test for reference voltage (typically 5 volts) and ground at the individual sensor connectors with the key on and the engine off (KOEO).
If reference voltage and ground are present on the respective connector pins, reconnect the sensor. Now, probe the signal circuit of each sensor with the positive test lead of the DVOM (negative probe connected to a known good engine ground). Use the infrared thermometer to measure actual coolant and fuel temperature.
Refer to the temperature to voltage chart (found within the vehicle information source) to determine if each sensor is functioning properly by comparing actual voltage with desired voltage. If either of the sensors fails to reflect the correct amount of voltage (according to actual coolant temperature and fuel temperature), suspect that it is faulty.
If the individual sensor signal circuits reflect the correct voltage, use the DVOM to test the signal circuit (for the sensor in question) at the PCM connector. If there is no sensor signal at the PCM connector but it is detected at the sensor connector, there is an open circuit between the two components.
Suspect PCM failure or a PCM programming error only if all ECT and fuel temperature sensors and circuits are within specifications.
