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By Des Davies AAE MIMI, Top Gear Motor Services
Vehicle: Renault Clio 11, 1.2 petrol 2001 Mileage: 82,708
Fault codes: DF002
Customer complaint: Running rough at idle and loss of power when cruising on the motorway. No fault codes were displayed on the instrument panel with the customer using his own fault code reader.
Where do we start the diagnostic process? Well, I find visual checks are a good starting point. I carried these out underneath the bonnet in the engine compartment to see if there was anything that looked out of place and for any new parts previously fitted. I normally find new components fitted as I conduct these checks, especially when the customer states that no previous work has been carried out on the vehicle, but not this time.
I started up the vehicle and it clearly had a bad misfire at idle, there was no need to road test this vehicle. I connected my scan tool to the OBD connector to extract any fault codes stored in the ECU and with the ignition on and then ran the vehicle to get it up to working temperature. Figure 1 shows the codes displayed on my scanner.
There were no codes present relating to the constant misfiring engine but there was a code for the throttle potentiometer and engine immobiliser. The problem when extracting these fault codes is has there been other people checking for these faults before or, have disconnecting components and then running the engine created false fault codes? I have seen this many times before, don't go erasing fault codes until you have to, as it takes away the freeze frame data and other important information that can help you with your diagnostic plan and process.
My next step was to check the live PIDs on my scanner to see if I could find anything out of place with the DF002 fault code for the throttle potentiometer. You can see the results in Figure 3 at idle speed and then at full throttle in Figure 4 – all were displaying the correct information of what the ECM was reporting.
Don't go chasing fault codes or rely on the scanner results or you will go around in circles – always test first and don't guess. Only replace components after conducting relevant tests to confirm the components are not within the manufacturer’s specifications or tolerances.
We do have to check the fault codes to eliminate them from our diagnostic plan. I decided to ignore this fault code, but I always document and keep them in the back of my mind, in case I need to revisit the fault code later in my diagnostic process.
Remember, back track if you are not sure of your testing measurements and results, it is better to be safe than sorry and it’s no good going down a rabbit hole. I have been there before by rushing and not taking time to properly assess test results. Do I move forwards, or retrace my diagnostic steps again? It's better to be sure of your test results as this decides your next diagnostic testing methods and approach.
There’s no shame in making mistakes, as long as we learn from them!
Could Mode 6 and fuel trim analysis provide some clues?
For my next test, I decided to go global and check to see if I could get any help from Mode 6 to try to identify the offending misfiring cylinder or cylinders but as you can see in Figure 5, it was not supported, so was of no help.
Bearing in mind this vehicle is quite old, I did not expect Mode 6 to help but it was worth a try, as I have had a lot of luck in the past and it only takes a few minutes to use. It can be a great tool to help with your diagnostics to enhance your skills and knowledge.
I decided to check the fuel trims next, see Figure 6, to see if there were any problems or issues, and to see what tests I could do next and, sure enough, they were all over the place. Long term fuel trims were +50.78 and short term fuel trims were + 39.84. The ECM was trying to add a total of 91.62% to compensate when I did not expect to see more than 10% total fuel trims!
I wanted to use my 4-gas analyser to see what was happening at the tail pipe and you can clearly see the HC Hydrocarbons were quite high and the engine clearly was not combusting and burning efficiently, there was plenty of unburned fuel emanating at the exhaust tailpipe, see Figures 7 and 8.
You can see from the diagrams that my O2 sensor had just died on my emissions analyser, typical when you're trying to diagnose faults, the O2 content in the exhaust is valuable information, but these readings gave me enough information that the air/fuel mixture burning in the combustion chambers were incorrect.
I then used my PicoScope to see if there were any ignition or injector related faults causing the problem, see Figure 9. On this vehicle, the injector wire was very easy to access to check all the injector voltages and current using my current clamp, so I decided to test these before the ignition system, simply because of the ease of access.
All the injector connectors, wires, voltage and current waveforms were good, so my next step was to check the ignition system. The primary and secondary ignition wires were connected to my PicoScope check and again, they all looked good.
I wanted to do a quick non-intrusive diagnostic check, the relative compression test, to check the engine for any mechanical problems as the fuel and ignition systems were displaying good results with no visible issues. I would normally carry this out earlier in my process, but I was sidetracked in the earlier stages by the fault codes.
With the firing order 1342 from the flywheel end, cylinders 2 and 3 have a major problem. The relative compression test is not foolproof, but provides us with a quick analysis of the mechanical condition of the engine. I like to doublecheck my testing with a vacuum gauge, which is a quick and easy test to do, to confirm my previous test results, by attaching it to the inlet manifold or vacuum hose connected to the inlet manifold.
When I was an apprentice, many years ago now, I was taught how to use the vacuum gauge, it is a great tool – very quick and accurate when trying to diagnose engine mechanical problems. I also used this tool to retime engines when back in the days when we had very little diagnostic tools –no scanners then!
If possible, try to find a central point on the inlet system for more accurate results – this is not possible on some systems, as access to these areas on can be very difficult, therefore common sense needs to be used. The gauge was reading 14ins/hg and floating backwards and forwards, indicating a mechanical fault. I would expect around 18to 22ins/hg on a fairly good condition engine, see Figure 13.
These tests confirmed that the faults were engine mechanical issues and further dismantling of the engine was required to confirm the customer’s complaint of poor engine performance. The customer did not wish to pursue any further due to the age of the vehicle and the cost of stripping down components to attempt a repair of this engine.
In summary...
You need to understand the fundamentals and basics of how these systems work in order to interpret your test results for correct diagnosis and analysis. It does not matter where you start testing, as long as you use the correct testing methods and equipment in order that you can interpret your results against manufacturer’s specifications.
Remember you can always go back and retrace your diagnostic checks if you need to, there are no set rules where we start diagnosing faults – I like to start my diagnosing at a mid-way point or to locate the easiest access point for testing, depending on the vehicle and system being tested. Remember the best tool we have available to us is between our ears!
We all make mistakes but as long as we learn from these mistakes we will have gained and developed new skills and experiences. I am learning every day and enjoy all these challenges, well, most of them!
