Homemade is not always best option

24 February 2012  

ONE?OF

One recent job, still ongoing at the time of writing, highlights the complexity of the diagnostic process. It’s a 2007 Vauxhall Astra Common Rail (Figure 1) which has an intermittent non-start/difficult to start symptom. The garage that brought it to me was unable to get the vehicle to misbehave during the time they had it so the customer had the vehicle back and it broke down again. When the vehicle was dropped off, the battery was ‘click the solenoid’ flat. I charged the battery back to 12.6V (in doing so, perhaps changed the fault condition and cured the symptom). After this, the car displayed the extended cranking symptom but always started. The cranking speed and current draw (relative compression) appeared to be normal.

Check the fluids

Petrol is a less dense fluid (more refined) so the hydrometer (a calibrated weighted float) sinks much lower into the petrol sample than it does in the diesel.

I drained off a sample of the diesel from the problem Astra and used the hydrometer to check the SG. It was heavier than expected. My reference samples of diesel have all come in at 0.83 but the cloudy sample (Figure 3) measured 0.85 – denser than diesel. I keep a sample of clean diesel on hand as a reference to compare, as the SG can vary with temperature. Both samples were at 15 ºC and Figure 4 shows a comparison of fresh diesel fuel. What is obvious is that there is a ‘problem’, less obvious is why. The fuel had an aroma of ammonia which made me think perhaps the diesel was perhaps contaminated with Adblue however, the SG of Adblue (typically 30%) would drop the SG of the sample, not increase it.

Fuel hooch

I suppose the vehicle owner read on t’internet that ‘cheap fuel is cool’ and won’t cause a problem. Well Iím sure you’ll agree, based on the vehicle’s starting performance that weíll beg to differ. The customer will want to know if the odd fuel oil is definitely the reason for his starting issues. The best answer I could come up with, based on the tests I had done, was probably.

Want to learn more?

He also has some Saturday training courses coming up: Engine Management March 10, Common Rail Diesel March 24, CAN Bus April 14 amd Advanced Oscilloscope Diagnostics April 28.

James has introduced a new practical one-day course into the schedule on Direct Petrol Injection. This is designed to help you understand the technology and effectively diagnose faults and problems. All of the major components are covered, the system’s different modes of operation, fuel system analysis and system diagnostics. Technicians will learn how to apply a variety of tools including the multimeter, scan tool, fuel pressure gauges, injector flow bench and the oscilloscope, to analyse and diagnose function and performance. Available dates are March 27 and Saturday May 12.

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