Glenn Cutter, Technical Training Manager, Mill Autoquip
We have recently hosted a number of Technical Training Evenings at our training centre in Cornwall, to give the average technician more of an insight into Common Rail diesel technology. Why Common Rail? Well it has been around for ages and most people understand it don’t they? The short answer is, not really. I overheard a conversation recently where a technician said he didn’t need to understand Common Rail because it was all so complicated that it can’t be repaired in the aftermarket anyway! He couldn’t be further from the truth as with all things, training aids understanding and practice aids proficiency which leads to competency.
In the past, diesel and petrol systems bore no resemblance to each other what so ever and technicians used to specialise in one or the other. Recently though, the systems are increasingly similar. When I first saw a common rail system it reminded me of electronic petrol injection and indeed the systems share many of the same components and operating principles.
Simple tests
When pump-operated diesel vehicles broke down or wouldn’t start, we used to crack an injector pipe off and see if there was diesel present. Do that now and you will never get the vehicle to start plus you will need a new pipe to boot. So how do we see if there is useable diesel, at pressure, in the rail without cracking an injector pipe?
If we consider how the system works, this is quite an easy question to answer. The ECU looks for fuel pressure in the rail that must be above a certain value, (approx 150 bar for BOSCH systems and 200 bar for Delphi), before it will switch the injectors. This pressure is measured using the fuel rail pressure sensor and is expressed as a voltage, by using a multimeter we can see if useable pressure is available. As a general rule of thumb, we need to see above 1.1 – 1.2 volts before a vehicle will start. This is an easy non-intrusive test that any technician should be able to complete easily.
In a previous life I trained apprentices – some must have liked me because they still talk to me. One ex-apprentice recently phoned me to pick my brains about a BMW 120d. The car had been recovered as a non-start and on initial inspection, there seemed nothing obvious to stop the car from starting and running.
Injector problem
Their first port of call was plugging in the Autologic kit to check the stored fault codes but in this instance, none of the stored codes helped. Their next step was a ‘leak off’ test which showed there was absolutely no fuel leaking from any of the injectors. After more investigations, their conclusion was that although fuel was being delivered, the fuel supply was backwards. When I questioned this, I was told that all of the fuel was returning to the fuel tank and that the pump was somehow pumping around the wrong way. The technician then explained that they had changed the pump for a known good one with a good metering valve but the problem was still the same. I asked the technician if they’d checked to see if the injectors were switching and injecting fuel – using a two channel oscilloscope set to volts and current, with an amp clamp. You can then see the switch signal to the injector and the current draw to check that the injector pintle is actually moving. The response was that they didn’t have an oscilloscope but there were no codes stored for injectors, only for fuel rail pressure. I asked if they had checked the voltage at the fuel rail pressure sensor, the response was no. The technician checked the rail pressure sensor through the Autologic, which reported over 200bar pressure.
At this point, I was certain it was an injector problem. The reason there was no leak off was that the injectors were not doing any work but this would need to be checked. A couple of days later I called the technician to see if talking through the problem had helped and it turns out that I was right, it was the injectors at fault. They had been receiving a switching signal from the ECU but were unable to inject fuel as the ends had been smashed. It turns out that the customer had been less than honest with the description of what had happened. The car had indeed stopped, due to terminal failure of the turbo. Parts of the turbo had been ingested into the engine causing damage to the injectors. The customer had changed the turbo but when the vehicle would not start, they decided to get the vehicle booked in but not say anything about the turbo failure.
There are two main lessons to be learned from this scenario. The first is to always gather a full and accurate history of what has happened from the actual person involved, even if a third party seems to know what has happened. Secondly, make sure you know how the system works and what it does before you try and diagnose a fault. In this instance, the technician was on the right lines with the diagnosis but questioned their diagnosis because they did not fully understand the system.
I have mentioned in previous articles that we need to know what we expect to see before we go looking for a fault and this incident proves how important this is.
