VAG TDI low fuel pressure fault
High-pressure diesel fuel injection faults can be expensive to rectify; Rob Marshall shows how extra time spent on diagnosing the fault before replacing parts can save a small fortune.
We investigate low fuel pressure on the high-pressure system of a modern Audi TDI system.
While the precision offered by high-pressure commonrail diesel fuel injection has made engines far more powerful and efficient, downsides include extra complication and sensitivity to fuel pressure deviations. Naturally, the whole system is monitored by the engine management software, including the fuel pressure at various points. The low-pressure side of the system draws fuel to the high-pressure pump via the diesel filter. From here, the high-pressure pump forces the fuel into the common rail, which acts as a pressure storage accumulator to which the fuel injectors are connected.
Diyers should neither be petrified, nor complacent. Treat the fuel system with respect. Cleanliness is critical; the smallest particle of grit can cause rapid wear within the expensive pump and injectors. As the high-pressure side can experience more than 1200BAR at engine cranking speeds alone, the force at which diesel can be ejected from a loosened union of a running engine can be sufficiently high to pierce skin and enter the bloodstream. You would then have a much more severe and urgent problem with which to contend…
The purpose of this procedure is to show a logical method of testing and evaluating high- and low-pressure fuel circuit performance. Aside from the health and safety implications, don’t rely solely on fault codes. Despite our demonstration car suffering severe running problems, no dashboard engine management lamp was illuminated. Therefore, you will need suitable equipment to extract live data, be it diagnostically or using the ‘old-fashioned’ method of analogue pressure gauges.
To show the full process, we had access to a professional workshop and professional diagnostic equipment made by Delphi Technologies. While these aspects are normally out of reach of typical Diyers, it should give you an idea of the information level that is required of your diagnostic equipment to analyse these kinds of faults.
The Volkswagen Audi Group 2.0-litre TDI unit, code named EA189, was part of the ‘Dieselgate’ emissions exposure in 2015. On our Audi A5 case-study, we focus on the poor running, which required detailed diagnostics to identify the cause.
Turning detective
The demonstration car was a 2012 Audi A5 TDI, fitted with Volkswagen Group’s venerable ‘Dieselgate-spec’ 2.0-litre Euro V engine. Note that emphasis is given not just on the make and model but also details of its fuel injection system. While mechanically-minded drivers are more likely to establish patterns that may give a clue to where the cause lies, should you look after a friend or relative’s car, quiz them for as much information as possible. Has the tank been brimmed with petrol accidentally? This damages the high-pressure fuel pump through lack of lubrication. Is it overdue for a service? If so, knowing that a blocked diesel filter alone can cause havoc with fuel pressure, it is worth checking.
In this case, the owner revealed that his Audi had become either difficult to start, regardless of coolant and external temperatures, or ran roughly. The fuel consumption had increased, performance had dropped off and the idle had become erratic. The fuel tank was also above half full – a relevant point, because some engine management systems can adopt different running characteristics to save fuel, when the diesel falls below a preset low level.