Finding the cause of low fuel pressure
When tuning a diesel engine to increase performance, one of the most important prerequisites is ensuring that the engine and fuel system are in perfect working order. While a remap can offer excellent gains in torque and power, it also places additional demand on components such as the injectors, fuel pump, and pressure regulators.
If any underlying faults exist, tuning can expose them very quickly – and this can lead to frustrating issues like poor performance, fault codes, and even engine stalling.
Here’s a real-world example we encountered recently on a 2.0 HDI / TDCI with a Delphi DCM3.5 ECU. We’ve also seen the same symptoms on 1.6 8v HDI engines with the Bosch EDC17C10 ECU, so it’s worth being aware of this if you’re running or working on one of these platforms.
In this particular case, the customer complained of poor performance and intermittent engine shut-off. After some investigation, the live data and fault memory revealed the following:
Common rail diesel injectors operate at extremely high pressures – often above 1,000 bar. They’re incredibly precise, but they also need to return a small amount of unused fuel through an internal path. This is known as leak-off or return flow.
Leak-off is completely normal in small, balanced amounts across all injectors. However, when one injector starts to leak excessively, it acts like a pressure bleed in the system. Fuel rail pressure drops below target, and the ECU struggles to maintain pressure – especially under load.
In most cases, the ECU will try to compensate by increasing duty on the fuel pressure regulator or pump, but eventually it will trigger a fault code or shut down the engine to prevent damage.
In our test, the faulty injector’s return line filled its collection bottle before the others had even started dripping. This is a textbook example of an internal injector leak. Often, a worn internal seal or solenoid fault is to blame, and unfortunately, this usually means replacement is the only fix.
In this case, replacing the single faulty injector restored proper rail pressure immediately. The engine ran perfectly, and no further faults were logged. We then successfully completed the remap and handed the vehicle back in top running condition.
While injector leak-off is a common issue, it’s not the only thing that can lead to low rail pressure. Here are a few other culprits we often see:
Interestingly, we’ve recently seen an uptick in blocked or partially restricted new fuel filters – even ones fitted within a few hundred miles. This raises some concerns:
Both are plausible. We recommend using OEM or high-quality branded filters where possible, and if a car is regularly used on short journeys, consider checking the filter more often than the standard service interval.
This case is a good reminder that tuning is the final step in a chain – not the first. A healthy engine, clean fuel system, and well-maintained components are essential for getting the best results from a remap.
If you’re seeing fuel pressure issues before or after tuning, a leak-off test is a simple but powerful diagnostic tool. And always be wary of parts quality – it’s not just the map that affects reliability and performance.
Although we are very knowledgeable with fixing problems, we are also very busy. If you have problems, please take your car to a good mechanic before contacting us for help. If your mechanic is stuck and you really can’t get to the bottom of it we can try to help, but remote diagnostics advice is never going to compare to a good mechanic running tests and examining real data.