The main reason diesel tuning boxes are “safe” is because of safety, not safety of the engine but the safety of people who are working or servicing the engine.
Modern common rail diesel engines work at extreme fuel pressures, up to 2500 BAR (over 36,000 PSI).
To have a part in the fuel delivery system fail and spray fuel in a chaotic manner when someone is looking or working on the engine is unacceptable. One can imagine the fluid adding like a laser on anything it comes into contact with.
And this is why diesel tuning boxes, are safe, at least when it comes to fuel pressures.
Sensors & Fail Safe
The fuel system has a number of sensors monitoring fuel pressure and if the ECU thinks there is an issue with the fuel pressure it can trigger various mechanisms to cut fuel pressure and if for some reason this is not effective, there is usually a mechanical pressure relief valve in the fuel system which will mechanically drain excess fuel pressure.
There are some companies in the diesel tuning industry that want to rubbish diesel tuning boxes as a concept, by saying that tuning boxes which increase fuel pressure will damage components in the fuel system. But because of the measures I listed in the previous paragraph it would be almost unimaginable for the pressure in a common rail diesel fuel system to exceed the maximum pressure tolerances.
So while we can be comfortable knowing that it is next to impossible for a tuning box to raise fuel pressures to dangerous levels, increasing fuel pressures is not without downsides.
As with any other power-adders like a bigger turbo or different camshafts there is always a compromise ie service life versus power.
The fuel system will be operating under highers pressures, on average, with a diesel tuning box versus a fuel system that is not running a tuning box.
When components in the fuel system are being asked to work harder more of the time it would be logical to expect that these components might suffer from a reduced service life.
How much is service life effected?
No component on the engine has an infinite service life, everything is going to fail at some point, the question is by what percentage will that service life be reduced?
This is a question that is impossible to answer because every driver works the engine differently to others.
Fast drivers will work the engine harder than fuel economy focused drivers, so the service life of parts on the hard driven engine will be less.
If we say for the sake of argument that on a hard driven engine a diesel injector will last 100,000 miles.
Taking a complete shot in the dark, a hard driven engine with the addition of a tuning box, I would say it is possible for the life of the injectors to be reduced by 20%.
Is this noticeable?
It depends on how long you keep your car.
The average driver takes 8.3 years to drive a 100,000 miles. If you only keep your cars for a couple of years, having a tuning box fitted is not going to make any difference to you. If on the other hand you keep your cars for 300,000 miles you might notice that you replace parts sooner.
300,000 Miles as an example
With a 20% reduction in service life, over the course of your ownership, you would buy 2 sets of diesel injectors without a tuning box and you would buy 3 sets of injectors with a tuning box.
Say a set of diesel injectors on a 4 cylinder engine cost 400 Euro, over the course of 300,000 miles the running cost of the car would increase by 0.001 cents per mile with a tuning box.
(The difference in cost between 2 sets and 3 sets of injectors in 400 Euro, spreading the cost of 400 Euro over 300,000 miles equals 0.001 cent per mile)
Fuel injectors will not be the only part that will be effected by the increased fuel pressure but you can use the same calculation to work out the extra costs of other components in the fuel system, like the fuel pump, using the same method.
In my opinion, if a car is driven with some degree of sympathy I don’t think the life of the gearbox would be effected but again, this comes down to the individual driver, if you think your gearbox will be effected by a tuning box, feel free to include transmission related costs into your calculations
From a purely fuel system point of view, it is hard to imagine how fitting a tuning box will damage a common rail fuel system as soon as you put it on the car.
Service life of parts, yes, they will be reduced but does it matter to you and your specific circumstances?