It's summertime, people are on holidays, same as most of us, so we don't expect too many cases in our workshop at this time of the year. One customer did turn up and the description of his problem attracted my attention:
"The issue is present for 2 years. The car has even been at authorised Citroen dealers and nobody has diagnosed the problem so far! When I bought the car 4 years ago there was no problem with it. Now I can drive the car normally only during the winter months. In summer, when it becomes hot the car will drive only for about 80 km and then the engine stops responding to the accelerator pedal and starts to choke." (I concluded that this problem sounds interesting and it's worthwhile looking at.)... The customer continued, "Every workshop always said that they found no DTC stored in memory. The problem is that I cannot drive the car since I don't know when the car will let me down when I have my family on board...."
10 workshop visits (in the end also an authorised Citroen workshop), where gradually all the following parts were replaced or tested:
After every attempt to repair the vehicle the customer drives about 80 km and the problem just keeps showing up again and again. The Citroen dealership returned the car to the owner quoting that the car is perfectly OK and has no issues. The customer asked if they made a test drive with the car and they replied that they drove the car for 30 km and that there is nothing wrong with the car and that it drives as it should. He reminded the workshop that when he was describing the problem to them, he said that the issue only shows up after driving 80 km. No response to this comment.
The customer wants to find the cause of his complaint "No response to the accelerator pedal after driving about 80 km during summer and the engine feels like its choking". "Seeing a desperate expression on the customer's face and the challenge in front of me I decided to accept this car for repair.
I was listening very carefully to the customer's words when writing up the repair order. Listening to the customer's words allowed me to determine which sensors I need to LOG during the test drive. I have selected a very basic EOBD scan tool for this task. The original Citroen scan tool software is not very suitable because it groups the "live data" values into specific packages so it is impossible to LOG record your own combination of values, e.g. Fuel, Ignition, Emissions, El. Components, Driving, Start, etc. This software setup is not suitable for our diagnostic methods.
Before going out for a test drive I am going to install a pressure transducer to the fuel injector rail and connect the upstream and downstream Lambda sensors and monitor these values with an oscilloscope. The test drive conditions are correct today and may not be OK tomorrow so I need to capture all the required values today.
For the scan tool LOG, I selected the following live data values: ACCELERATOR PEDAL, ENGINE RPM, VEHICLE SPEED, INTAKE MANIFOLD PRESSURE, BOTH LAMBDA SENSOR VOLTAGES, SHORT TERM AND LONG TERM FUEL ADDAPTIVES. With the signal recorder that has a 50 times higher sampling frequency than the scan tool we are recording the FUEL PRESSURE and with a differential connection we are recording both the LAMBDA SENSOR signals. It turned out that this selection of sensors was correct and enabled to determine the source of the problem.
Details of this case can be viewed through this link Air bubbles similar to champagne.
The first step was a conversation with the owner and his description of the fault. Good description and our attention to what he says is an important starting point for diagnosis and will determine if we can record the correct data first time. The second step is the connection of the OBD module to the vehicles diagnostic connector.
The Citroen workshop installed using a trial and error method the BSI unit, Lambda sensor and the intake manifold pressure sensor. This kind of repair/diagnosis makes no sense. The cars issue is still present. The car drives OK in the wintertime but in summer it stops after 80 km.
When the ambient temperature is low the fault doesn't show. This is an indication that the fault is not of an electric nature and for this reason we shouldn't miss any opportunity to capture many signals as possible. The scan tool software doesn't have fuel pressure data available so we have to monitor the fuel pressure with an independent pressure transducer connected to an oscilloscope directly to the engines fuel rail. If the engine is going to have ignition misfires we want to see the Lambda sensor signals about 50 times quicker than through the OBD diagnostic connector.
Pressure transducer on the fuel rail and 4 voltage probes on the signal and ground wires of the Lambda sensors. The vehicles OBD connector cannot provide the high-quality signals that we require.
The test drive into Prague didn't reveal anything unusual. On the way back the first signal changes were recorded. As you can see from the red trace representing LONG TERM FUEL ADAPTIVES that the trace started to respond to something. Many of you will most probably already recognize where the problem is. After a short while when the vehicle stops and restarts to accelerate again the adaptive value is about +13%. When the car enters the motorway the adaptive value drops to 2% but very quickly rises to a value around**20%. A DTC fault is triggered only when the adaptive reaches 25% or higher. This is why there is no DTC stored in memory and the vehicle doesn't respond to the accelerator pedal. The engine hesitates and chokes. When the accelerator pedal is released the car begins to accelerate again. The accelerator pedal must be applied only lightly otherwise the car will slow down again.
The pressure sensor is only telling us that the fuel pressure is dropping. The fuel pumps el. current will tell us "why" the fuel pressure is dropping. The el. current value is clearly a localization measurement that is telling us why the pressure is dropping. Description of the fuel pumps problem can be found in the detailed article related to this case in the FCD.eu website archive (paid section).
Anything that is measured under static conditions (including fuel pressure) is incorrect. When a car is driven "nothing is static". More importantly, the technician will tick off this value in his head as being OK and within the limits as specified by the manufacturer's service manuals. A very common problem is that the values specified in the service manuals are very often incorrect.
The first step was a conversation with the owner and his description of the fault. Good description and our attention to what he says is an important starting point for diagnosis and will determine if we can record the correct data first time. The second step is the connection of the OBD module to the vehicles diagnostic connector.
The Citroen workshop installed using a trial and error method the BSI unit, Lambda sensor and the intake manifold pressure sensor. This kind of repair/diagnosis makes no sense. The cars issue is still present. The car drives OK in the wintertime but in summer it stops after 80 km.
When the ambient temperature is low the fault doesn't show. This is an indication that the fault is not of an electric nature and for this reason we shouldn't miss any opportunity to capture many signals as possible. The scan tool software doesn't have fuel pressure data available so we have to monitor the fuel pressure with an independent pressure transducer connected to an oscilloscope directly to the engines fuel rail. If the engine is going to have ignition misfires we want to see the Lambda sensor signals about 50 times quicker than through the OBD diagnostic connector.
Pressure transducer on the fuel rail and 4 voltage probes on the signal and ground wires of the Lambda sensors. The vehicles OBD connector cannot provide the high-quality signals that we require.
The test drive into Prague didn't reveal anything unusual. On the way back the first signal changes were recorded. As you can see from the red trace representing LONG TERM FUEL ADAPTIVES that the trace started to respond to something. Many of you will most probably already recognize where the problem is. After a short while when the vehicle stops and restarts to accelerate again the adaptive value is about +13%. When the car enters the motorway the adaptive value drops to 2% but very quickly rises to a value around**20%. A DTC fault is triggered only when the adaptive reaches 25% or higher. This is why there is no DTC stored in memory and the vehicle doesn't respond to the accelerator pedal. The engine hesitates and chokes. When the accelerator pedal is released the car begins to accelerate again. The accelerator pedal must be applied only lightly otherwise the car will slow down again.
The pressure sensor is only telling us that the fuel pressure is dropping. The fuel pumps el. current will tell us "why" the fuel pressure is dropping. The el. current value is clearly a localization measurement that is telling us why the pressure is dropping. Description of the fuel pumps problem can be found in the detailed article related to this case in the FCD.eu website archive (paid section).
Anything that is measured under static conditions (including fuel pressure) is incorrect. When a car is driven "nothing is static". More importantly, the technician will tick off this value in his head as being OK and within the limits as specified by the manufacturer's service manuals. A very common problem is that the values specified in the service manuals are very often incorrect.
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