Chances are, if you are reading this then you may be considering tuning your diesel vehicle and have no doubt seen the huge variety of ‘plug in’ boxes that are being marketed as a tuning solution. In this article we will explain the ins and outs of these boxes and shed some light on what they do, and why you should take care when fitting them and consider other tuning options.

Back in the early days of TDi (turbo diesel injection) tuning things were simple, most engines used a simple high pressure mechanical fuel pump and these could be tampered with to increase the fuel pressure, often by a simple turn of a screw. You could also adjust the boost just as easily thanks to adjustable actuator arms that controlled the turbo pressure. Fortunately these engines are now almost obsolete and modern day TDi engines are equipped with advanced engine management systems in the form of Electronic Control Units (ECU’s).

The ECU is nothing more than a small, but highly complex computer that controls inputs and outputs of the engine, it gathers data from hundreds of sensors and inputs and from this information it generates output signals to tell the engine what to do and when to do it. The ECU knows how to do this as it contains ‘maps’ which are developed by the vehicles manufacturer. These inputs and outputs are processed by the ECU maps thousands of times a second, this results in an engine that is refined, smooth, powerful and economical.

So the engine now has no way of being adjusted or ‘tuned’ by a simple turn of a screw as it is controlled by a complex computer system that is only really understood by someone that understands software programming, electronics and an in-depth knowledge of how modern diesel engines work, these individuals are few and far between and to become one of these individuals it would take years of research and learning.

This is how the tuning box came into existence, people knew that you can make a diesel engine produce more power and torque by simply adding fuel, nothing more complex than the old fashioned method of turning a screw, but now that this wasn’t possible, so a new method was developed. This involved fooling the ECU into adding this extra fuel by altering the signal that controls fuel pressure. If you cut the input signal to the ECU from the fuel pressure sensor and use a simple resistor to bridge the break, the ECU will think fuel pressure is low, low fuel pressure can damage an engine and the ECU counteracts this by opening the fuel injectors for longer and/or adds more pressure, this adds more fuel than the ECU thinks and the engine makes more torque. ALL tuning boxes work this way whether it’s £30 from ebay or bought for £300 from a glossy advert in a car magazine, they all use the exact same basic principle, creating an imaginary fault to trick the ECU.

So where’s the problem? I hear you ask. So here it is, using such a primitive method to trick such a highly complex ECU doesn’t even scratch the surface of what is possible. We bought a few of the most expensive boxes available and tested them, we also monitored the vehicle for power output, fuel input, fuel pressure, injector duration, exhaust gas emissions, turbo boost pressure and exhaust gas temperatures.

Our initial findings were shocking, on all of the boxes tested we found that extra fuel was only added in higher load conditions, meaning that although the box did trick the engine into making extra power this would only happen under full throttle. The power gain was also very ‘peaky’ meaning that the extra torque was not generated for very long during the RPM range. Fuel pressure was increased by over 22% (adding stress to the fuel pump and injectors) and we didn’t even need to look at the exhaust gas emissions to know that things were not right, this was obvious by the huge clouds of smoke emitted from the exhaust, a problem that is associated with tuning boxes. Luckily the test vehicle did not have a DPF filter, if it did then it would undoubtably become clogged very quickly. Please visit our website for more info on DPF filters.

Let’s look at why this smoke is here, due to the extra fuel pressure, more fuel is being forced into the engine, however no extra air is being requested in the form of turbo boost pressure meaning a very rich mixture and wasted fuel. But the manufacturers of these boxes have added a small amount of adjustment in the form of a screw for you to turn to alter the amount of extra fuel, one had a series of switches or ‘jumpers’ to move around. So effectively your modern diesel engine is being controlled by an adjustable resistor in a box. Some claim to use microprocessors, however none of these microprocessors match the speed and ability of the ones in your ECU that they are trying to fool, this creates conflicts.

These conflicts can result in fault lights on your dashboard, your engine running in ‘limp home mode’ and we have even seen a Mercedes which had the wiring loom catch fire after having a tuning box installed. Other problems are not noticed immediately and occur over longer periods of time, these include failed injectors, failed fuel pumps, Blocked DPF filters, increased fuel consumption and clogged EGR valves, none of these are easy or cheap to put right.

So why do people sell these boxes? It is simply profiteering. A dedicated and professional tuner will always look for the best way to tune a vehicle and not base their final product on development time, cost or profit, neither will they take the easiest route and sacrifice the final products reliability and quality. Tuning boxes are the exact opposite of what the professional tuner strives to create, many contain nothing more than a few cheap components inside a flashy box, often manufactured in China and offered to resellers in the UK for just a few Pounds per unit.

So in summary, a tuning box is a compromise and also a risk to the reliability and longevity of your vehicle. We certainly do not sell or install them, and this is the case with most professional tuners, however we do remove a huge amount of them from vehicles when diagnosing faults, in most cases this resolves the symptoms of the fault and the customer then looks into having their vehicle professionally remapped.