Fuel pressure fluctuations

Hi,

i'm debugging my setup :

since i installed my fuel pressure sensor in the fuel rail, i always see fluctuations in the pressure.

my base pressure is set at 3.4bar, and i see between 2.9 and 4.0 at idle, but mostly around 3.4bars, and it is fluctuating very fast.

If i replace the electronic sensor with a manual gauge (glycerine filled), i see no fluctuation.

I guess that the Glycerine dampers the needle and i see a steady 3.4bar.

I had doubts about my wiring harness on the fuel pressure sensor, so i did it again, following the HPA Motorsport wiring course, it is now bulletproof : same fluctuations.

I changed the fuel regulator to a DWR1000, same fluctuations.

The setup is :

> In-tank Walbro 255L/H (flow measured with the regulator : around 255l/h)

> then, direct to the fuel rail, dash AN-6 hoses

> 4x ID1050 injectors (K20a engine with Rotrex supercharger)

> Then DWR1000 fuel pressure regulator, base pressure 3.4bars

> return to tank (AN-6 hose)

I wonder if i need to install a Fuel damper on my rail to absorb those fluctuations or if i have another problem in my setup.

What do you think ?

I think you need to filter (slow down) your pressure reading in your ECU or analysis software.

Since you likely have your fuel pressure regulator referencing the manifold pressure, could the pulsations in the intake manifold at idle cause your apparent fuel pressure fluctuations. I would expect a gauge fuel pressure to drop at idle, and rise as your supercharger adds manifold pressure.

If you can subtract the manifold pressure from the fuel pressure you will have calculated the differential pressure that the injector actually sees. This is the value we want to remain stable for consistent tuning.

Does the engine idle well or are you having issues?

I have fluctuations with and without vacuum line on the regulator, it's not about the base pressure affected by vacuum/boost.

But i just found that moving my sensor from fuel rail to regulator (so closer to reg) actually makes it almost stable (way less fluctuation (+/- 0.1b) with or without vacuum line).

I was thinking that rail pressure and reg pressure were always consistent, but looks like the 50cm of hose + rail affect the reading of the sensor...

I will reduce sampling to 2Hz and i will be fine i guess !

The engine was mapped 2 months ago without trouble (without monitoring fuel pressure).

Unstable pressure :

Almost stable pressure :

The fluctuations you're seeing are quite likely to be real. As you can imagine, when the injectors open and deliver fuel, the fuel volume in the rail will change and particularly at certain engine speeds, this can show quite a bad harmonic. Usually you don't see this unless the fuel pressure variation is very bad, or you're sampling the fuel pressure at a very high frequency. This however is one of the reasons why OE fuel systems include a pulsation damper on the fuel rail. I'd probably suggest mounting the sensor on the regulator and be done with it. Since these pulsations are real though they can affect the tuning so if you're seeing any weirdness in your mapping then this could be your problem.

Thanks Andre, so i don't really need to install a fuel pulsation damper if my car is running fine ?