P008F Correlation between engine coolant temperature and fuel temperature

OBD-II DTC Datasheet

Correlation between engine coolant temperature and fuel temperature

What does this mean?

This generic powertrain diagnostic trouble code (DTC) typically applies to many OBD-II vehicles. That may include but is not limited to vehicles from Mercedes-Benz, Chevrolet, Sprinter, GMC, Ford, etc.

When your OBD-II equipped vehicle displays an Engine Service Soon lamp and a P008F code is stored, the powertrain control module (PCM) has detected a mismatch in the correlated signals between the perceived engine coolant temperature (ECT) and fuel temperature.

Engine coolant temperature and fuel temperature are usually only compared when the engine is at ambient temperature.

ECT sensors consist of a thermistor immersed in hard resin and housed in a metal or plastic case. Brass is the most common metal used as an ECT sensor body material. The ECT housing is designed to be screwed into a coolant passage in an engine intake manifold, cylinder head or block. As warm coolant flows through the passages and through the ECT sensor, the level of thermal resistance in the ECT decreases. When the engine coolant temperature decreases, the resistance increases and as a result, the voltage of the ECT sensor circuit decreases. These resistance fluctuations (which result in voltage fluctuations) are interpreted by the PCM as changes in engine coolant temperature. The ECT sensor input is critical in calculating the fuel delivery and ignition timing strategy.

The fuel temperature sensor also uses a thermistor, but it is low voltage. Most often it is built into the fuel level sending unit and provides the PCM with a voltage signal that reflects the fuel temperature when the ignition is in the ON position. Fuel temperature can be particularly important for the driveability of diesel vehicles. Fuel temperature is also a major concern in extremely cold climates.

A P008F code will be stored and the Malfunction Indicator Lamp (MIL) may illuminate if the PCM detects a mismatch (between the ECT sensor and the fuel temperature sensor) that changes more than the maximum allowable parameters for a set period of time and under certain circumstances.

What is the severity of this DTC?

Because ECT and fuel temperature are so important to fuel delivery strategy and ignition timing, a stored P008F code should be viewed with some degree of urgency.

What are some of the symptoms of the code?

Symptoms of a P008F engine code may include:

Reduced engine power
Reduced fuel efficiency
Excessively rich exhaust

What are some of the common causes for the code?

Reasons for this engine code may include:

Defective ECT sensor
Defective fuel temperature sensor
Open or short circuit in the wiring or connector
PCM or PCM programming error

What are some steps to troubleshoot the P008F?

Any stored codes associated with the ECT sensor would prompt me to make sure the engine is not overheating. It must be filled with coolant and operate within the permissible temperature limits. If the engine is not overheating, the next step should be to visually inspect the wiring and connectors of the coolant temperature sensor and fuel temperature sensor.

Before attempting to diagnose the P008F code, you will need a diagnostic scanner, a digital volt / ohmmeter (DVOM), an infrared thermometer with a laser pointer, and a reliable vehicle information source.

Now connect the scanner to the vehicle diagnostic port and get all stored codes and freeze frame data. Make a note of this information as it may be helpful as you continue to delve deeper into the diagnostic process. Then you need to clear the codes and test drive the car to see if the code has been cleared.

Use your vehicle information source for wiring diagrams, connector pinouts, component test specifications, and connector types. This information will assist you in testing individual circuits and sensors. To prevent damage to the controller, check individual system circuits using the DVOM only after disconnecting the PCM (and all associated controllers). Connector pinout diagrams and wiring diagrams help you effectively check the resistance and / or continuity of an individual circuit.

You can use the DVOM (and your source of reliable vehicle information) to test your coolant and fuel temperature sensors. Follow the component test specifications, disconnect the probe under test, and set the DVOM to the resistance setting. Sensors that do not meet the manufacturer's specifications are not suitable.

If the P008F is to be reset immediately, test the reference circuit contact of each individual sensor connector (ECT and fuel temperature) using the positive test lead from the DVOM. Use the negative test lead to test the ground pin.

Check the reference voltage (typically 5V) and ground at the individual sensor connectors with the key on, engine off (KOEO).

If both reference voltage and ground are present at the corresponding pins of the connector, reconnect the sensor. Now test the signal circuit of each sensor with the positive test lead from the DVOM (the negative sensor is connected to a known good motor ground). Use an infrared thermometer to check the actual coolant and fuel temperatures.

Check the temperature versus voltage diagram (found in the vehicle information source). With it, you can determine if each sensor is working correctly by comparing the actual voltage to the desired voltage. If any of the sensors in question does not display the correct voltage value (based on actual coolant temperature and fuel temperature), suspect it is faulty.

If the individual signal circuits of the sensor reflect the correct voltage level, 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.

Suspicion of PCM failure or PCM programming error only if all ECT and fuel temperature sensors and circuits are within specification.

You can find help with the diagnosis by matching the vehicle, symptoms and codes stored in the respective Technical Service Bulletins (TSB).

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