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VW Golf Diagnostic

10th October 2013

This post is information on how we traced the fault on a Volkswagen Golf 1.4 Petrol. We carry out real time tests on the system to identify the fault. It`s unfortunate that there seems to be a misconception within the motor industry that we can now fix cars using just a computer alone, this is not the case in any situation, the fault codes that we see on the screen are an indication only so we then have to carry accurate tests and analyse data to accuratly diagnose your vehicles fault.

So, what did the computer say? The fault code read ‘P0172 Fuel Trim System Rich’. So we assume that the engine is running rich (Using too much fuel). Or maybe a sensor is faulty making the management system think that the engine is rich.

I will try and keep these test steps as simple as possible! We started by checking EOBD data, mainly looking at the fuel trim, which was reading -100% !! This means that the ECU is taking fuel away for a rich condition to compensate for the fault. I checked the lambda sensor reading using data list – didn’t seem too bad. I used an oscilloscope to test the output directly at the lambda sensor, from the pattern being displayed I confirmed that the vehicle was running rich. Now to find out why!

The vehicle doesn’t have an air mass meter, instead uses a MAP sensor. I checked the engine vacuum using a gauge and compared with the sensor readings – all ok. The following checks were;

  • Test fuel pressure – too high will cause to run rich – this was ok
  • I checked for an air leak, I wouldn’t have expected one for this fault, but still a good test to eliminate any leaks, we did this using a smoke machine – all ok.
  • I then checked the spark plugs and coil packs, using the oscilloscope and checking the distance the spark would jump… The vehicle had already had all four coils replaced to try and fix this fault! But we still need to test them ourselves before ruling them out – all was ok. 
  • Tested the Lambda sensor, after all it’s this sensor that is telling me and the ECU that the engine is rich, maybe the sensor is faulty? So, I created a lean condition by removing the brake vacuum pipe whilst checking the oscilloscope, I could see the sensor responding instantly to this lean condition. We also sprayed a small amount of easy start into the intake, to make sure the lambda also responded to this – which it did, so we now know that this sensor is doing it’s job properly. 
  • So, scratching my head, double checking myself at this stage as we’d covered most normal issues. I carried out oscilloscope tests on all four injectors, the patterns all seemed fine. But this doesn’t rule out an injector fault. I removed all four injectors complete with the fuel rail, placed some blotting paper underneath and crank the engine over. I found that injector 4 was leaving a larger patch of fuel on the paper! 
  • So, it looks like number 4 injector is the fault. Just one more test, back to scoping the injectors. I checked the ‘on time’, so the amount of time that the ECU is requesting the injector to be ON, also known as Pulse Width Modulation signal. The on-time was the same on all four injectors. This confirmed fault as the injector.
We replaced the injector, rebuilt and cleared the learn values (Fuel trims). Carried out road test, now running much smoother!

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