Remember the golden rules of diagnostics

Annoyed at your noid light? Perhaps a more methodical approach is needed

Published:  14 June, 2013

By James Dillon

Faults which strike fear into the heart of the average technician are those which have a symptom, in this case a non-start, but don't set a trouble code. Our test vehicle (a Peugeot 307 fitted with a Siemens system) had failed to start one morning and appeared at TTHQ on the back of a breakdown truck with an initial diagnosis of a high pressure fuel pump fault. I have a test routine for non-start common rail diesels and this confirmed that there was a lack of fuel pressure as measured at the rail pressure sensor, figure 1 shows engine RPM and fuel pressure but there were no external leaks and the low pressure supply (in this case a vacuum system) was performing to specification.

By using a hydrometer and by looking at it we confirmed that the fuel was of the right spec and quality. A lack of high pressure fuel will mean that the ECU will trigger the injectors, so no point measuring this (just yet). To all intent and purposes, we've got all we need, apart from fuel pressure - it would be easy to assume that the pump is a 'gonner'. More testing is required. The answer to this diagnostic problem lay in the finer details.

The system uses two point fuel control; inlet metering to adjust the 'load' on the pump and pressure regulation to control the pressure within the rail. The two functions are achieved through ECU control over two individual 12V solenoids. My next step was to prove that the fuel pump had the capability to generate pressure (the capability test) which tests the pumps output on the vehicle. Instructions of how to perform this test are detailed in the Diagnostic Assistance software (which I co-authored) referred to at the end of this article. With the pump manipulated into capability mode, the pump can be seen to generate pressure, see figure 4, sufficient to prompt injection and sustain combustion. So, the pump didn't appear to be at fault, and the pressure control solenoids also appeared to be capable of working.

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