Reading between the lines

A lacklustre Freelander is sorted by delving into the not-so-obvious data

Published:  04 October, 2013

If you've been reading this magazine for any amount of time, you'll be well aware of the obvious benefits of using an oscilloscope when diagnosing faults, but it is the less obvious benefits that with practice can lead to the correct diagnosis.

One such case was a Land Rover Freelander presented with low performance from its 2.0 diesel engine. A test drive confirmed the fault but there were no codes stored and live data looked normal. It felt like a lack of fuel as the performance was almost there but not quite right, especially at high speed and loads. The first test was 'low pressure supply'. It can be a time-consuming procedure to break into the low pressure supply and measure flow rates and supply pressure but it is possible to quickly test the pump electrically using an amps clamp. The current drawn and speed of the pump can prove that it is spinning satisfactorily. This doesn't rule a low pressure supply problem out of course, but will identify a faulty motor in the pump. In this case, the supply trace looked good.

So with good low pressure and good high pressure generation, the next step is to check the injectors. Using the amps clamp again it is possible to observe the current drawn and 'on' time of the injectors. This can prove the condition of the energy supply circuits without any interference from the ECU.

It could be a faulty injector or it could be a mechanical problem with that cylinder. Relative compression testing on diesel engines is quick and convenient and you don't need to remove glow plugs or injectors. The test showed one cylinder to be down on compression, see figure 2. So that's the diagnosis, quick simple and conclusive.

What about the test drive? The vehicle felt like it was lacking fuel, not misfiring. Would a 22% lack of compression result in these symptoms?

This is where you must challenge what you see. 'Scopes can be used to compare and contrast data... But what can you compare it against? In this case, the faulty cylinder has been compared with another from the same engine; this information has allowed us to determine that the cylinder is slightly down on compression. What if we compare the injectors with a known good injector? This is where having a library of known good waveforms proves invaluable.

We know the current draw is good but could it still be an injector failing to deliver enough fuel? Typical injector 'on' time (at idle) values for this engine are in the region of 600-700µs. The injectors on this engine are all in excess of 900 µs.

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