Here’s why you should always get data to diagnose faults properly

Experienced vehicle technician and GW reader, Carl Ford explains why you should 'test, don't guess'

Here’s why you should always get data to diagnose faults properly
Carl Ford recently took delivery of a MINI Clubman which cut out under load.

A vehicle was presented to me with a complaint of the EML illuminating and that the engine would cut out under load, cycling the ignition and restarting the car would get it running again – if you drive steady the fault would not occur.

I questioned the customer about the service history of the vehicle and he informed me that it had not long had a service.

The vehicle was a 2009 BMW MINI Clubman 1.6 diesel (EDC16).

It turned out to be relatively easy to diagnose and repair but it’s a fault I’ve seen on lots of other vehicles where expensive parts have been replaced without correct diagnosis.

I started out my diagnosis in the same manner as most people with a simple scan of the fault codes.

Fault code readings:
  • 4570: Rail pressure plausibility quantity controlled
  • 4590: Rail pressure plausibility quantity controlled
  • 3F30: Rail pressure sensor signal

Attempting to replicate the fault, I carried out a road test and, in second gear up a slight incline, I put the throttle pedal to the floor.

At about 4000rpm the EML came on and the car cut out, I immediately hit the save button on my scan tool.

To me, this is very important because if you can’t get the vehicle to fail then diagnosis and proving the fix becomes very difficult.

On any CRD system with a fuel pressure issue, you need to test the low/supply pressure.

Even though the fault code indicates low rail pressure, the ECU only monitors the high-pressure system meaning any fault within the low-pressure system will result in a high-pressure fault code.

With this in mind, I attached my low pressure gauge between the fuel filter and the high-pressure pump…

This article first featured in Garage Wire Views, click here to carry on reading the complete article.

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