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Case Study: Misleading fault codes on a SEAT Ibiza

By autotech-nath on April 20, 2022

By Des Davies AAE MIMI,

Top Gear Motor Services

Vehicle: Seat Ibiza 1.4 16V 2008, mileage 75,546 Symptoms: Fault code PO322, stalling & non-start

A customer brought their 2008 Seat Ibiza into the workshop, complaining that the vehicle had started to play up recently, it would stall then crank over but not restart. After allowing it to cool down for 30 minutes or more, the engine would start but not rev up, there was no power and it would stall again. When, eventually, it did start up once more it would run normally. This had happened on two occasions and the customer was now frightened to go very far but needed to use it for longer journeys as he was off to university.

This vehicle had been ‘diagnosed’ and checked over at different garages who suggested that the fuel pump and ignition coil needed replacing and they had fitted new spark plugs. No other previous work was disclosed by the customer.

I first checked inside the vehicle to see if the check engine light was displayed on the instrument panel when the engine was running, but there was no warning light present, and the vehicle was actually running well.

My next job was to conduct a road test and attempt to recreate the faults, but the vehicle performed really well and displayed none of the symptoms the customer was complaining about.

I then connected my scanner to the OBD connector to check for any fault codes to help me with my diagnosis. There was a fault code PO322, which I was interested in, and a fault code for the ABS system and central locking codes.

Common symptoms for code PO322:

• Engine misfire
• Engine stumble
• Lack of power in the engine
• Engine hesitation
• Engine cranks but will not start.

Sounds familiar to me, so let us have a look at some of the possible causes for this code:

• Open in control circuit (ground circuit) between the distributor/ignition/engine speed sensor and PCM

• Open in the power supply circuit between distributor/ ignition/engine speed sensor and PCM

• Short to ground in power supply circuit to the ignition distributor/ignition/engine speed sensor

• Faulty ignition, distributor, or engine speed sensor.

Other possible causes:

• Defective crankshaft position sensor(s)
• Defective camshaft position sensor(s)
• Corroded or faulty crankshaft position sensor(s) connections or wirings
• Misfiring engine • Rough running • PCM fault.

Checks for anomalies

I then used my scanner to check the live data Parameter
IDs, PIDs, to see if anything stood out, but all looked OK. The fuel trims were good and there was no evidence of ignition misfires. The 4-gas analyser was then inserted into the exhaust tail pipe – this is a quick and easy test I like to do to check for any anomalies and to see the burning of the air/fuel mixture at idle and cruise in the combustion chambers. For a 14-year-old vehicle, it was running well.

Idle – CO 0.8%, HC 170ppm, CO2 1410%, O2 0.8%, Lambda 1.03λ

Cruise – CO 0.3%, HC 90ppm, CO2 1420%, O2 0.3%, Lambda 1.01λ

It was now time now to work smarter and not harder and plan a diagnostic testing method to help me identify the fault, or faults, within this system. I decided to get some technical data to familiarise myself with the system and circuits involved and to set up a plan of attack.

I was looking for the easiest and best system component to do a non-intrusive test – a good starting point of my diagnostics. I decided to check the crankshaft sensor output using my PicoScope as I thought that this would be an easy sensor to locate and check. However, I discovered that this sensor was at the back of the engine and the access to the connections was awkward.

I could not physically or visually see the crank sensor, so I decided to check the sensor at the ECU, which was located at the bulkhead near the battery. It was fairly easy to connect my test leads. I referred to my technical data to find the pin no’s 82, 87 and 108 for the crankshaft sensor, which were yellow and grey wires and brown/blue at the engine ECU. See Figure 1.

I tested the crankshaft sensor, see Figure 2, then tested the camshaft sensor. This was located on top of the engine and was easy to access, Figure 3 shows this waveform. Both these sensor outputs were good. I then ran the engine for a while to check for dropouts with the two sensors on a longer time base, but all was well, so it was time to have a think about the next step.

Figure 1: I referred to my technical data to find the pin no’s 82, 87 and 108 for the crankshaft sensor, which were yellow and grey wires and brown/blue at the engine ECU

I could check the sync of the cam and crank sensors to try to get a known good sensor reading, but I did not have one in my own library. I could research online for a known good sensor synchronisation or perform an in-cylinder test, but as the cambelt cover on this vehicle is easy to remove, with only two clips on the top cover, it made sense to check this first (thinking smarter not harder).

Figure 2: The crankshaft sensor waveform was good

On removing the top cover, I shone my hand held light down towards the timing belt and could see the guide pulley and its plastic surround had disintegrated and was probably lying at the bottom of the engine near the crankshaft sprocket waiting to do some damage! I could get my hand down onto the timing belt and move the belt, I felt a lot of play and slackness in the belt, indicating that the valve timing must be out.

At last, I had found the problem! The camshaft timing was allowed to move out of sync with the crankshaft, moving the valve timing away from manufacturers specifications, and therefore causing the intermittent issues – where was this in the fault code description? Do not always rely on fault codes as they will bite you on the arse and I have previously had big teeth marks there, painful, and embarrassing! This vehicle needed a new timing belt kit complete with guide pulleys, tensioner, water pump and auxiliary belt.

Figure 3: The camshaft sensor waveform also proved to be good

I have had a few problems with previous vehicles that have either a cam or crank sensor fault codes and whilst testing these systems they have displayed good waveforms, but when you check both together on the oscilloscope against a known good sensor waveform, they have had cam/crank sync problems, indicating valve timing issues. One example was a vehicle that had been fitted with three, yes three, new cam sensors by a local garage as the check engine light kept being displayed after replacing the camshaft sensor. They assumed that they were faulty sensors! The previous garage had been road testing this vehicle after fitting each camshaft sensor with a broken tensioner and flapping belt, how lucky were they! This had been going on for a few weeks.

In summary
Fault codes can be misleading. You can end up chasing your tail and you may start playing parts darts, which is no good for the customer or your reputation.

I have some customers asking me to check their vehicle for fault codes when the check engine light appears on the dashboard, they then want me to give them the codes so that they can repair the vehicle themselves. This does not work.

Some customers believe that I can tell them what the problem is with their vehicle just by reading the fault codes. I have to be very diplomatic with them and explain that I need to test the vehicle systems and components to help diagnose the faults, you just cannot fix vehicles going by what fault codes have been retrieved.

Trust your knowledge of basic systems, your testing methods, tools, equipment, and always think outside the box. If you are not sure of your testing results, test again using a different method and test equipment, go back again and revisit your last previous test to see if you have missed something.

We are all under pressure with time and workload, the customer always asks how long it will take to fix the car before you start diagnosing the problem. It is very easy to rush and make mistakes, I am guilty of this myself.

Training and learning are essential! As is having the right information and vehicle data to help you fix the vehicle. Knowledge is power!



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