Hi Martin

I am by no means an authority on this topic, but here is my understanding.

The inlet metering valve controls the low pressure flow into the high pressure pump. It is a frequency modulated (modulated is a posh word for controlled) device that when fully energised will strangle the fuel flow into the pump. Conversely when it is unpowered (or you unplug it) the valve opens fully to allow maximum fuel throughput.

If you look at the third image down in my first post, you can see a sharp rise in the red signal, briefly after a small peak in the blue signal. This red signal, is the inlet metering valve current & what you are seeing is the effect of the management system fully energising the valve, so as to strangle the fuel flow. This is done as a response to the peak on the blue signal, which is fuel rail pressure.

As for what is actually wrong with the inlet metering valve, I am not sure, but I suspect it may have been sticking or maybe operating at the extreme end of its working tolerance.

Another common type of pressure control solenoid seen on various systems, is a unit that will regulate the high pressure outlet from the pump & these work in the opposite way to the inlet metering valve. By that I mean that when they are fully energised by the management system, the fuel pressure is increased to maximum. A bit like sticking your thumb over the end of a garden hosepipe. The harder your press, the higher the pressure developed. When this type of solenoid is unplugged, instead of developing maximum pressure, it will default back to approximately 50 – 60 bar. This default pressure is created purely by a small spring that presses a ball onto an orifice, to stem the flow. (Remember the inlet metering valve works the opposite way & strangles the fuel flow when fully energised).

There are other types too, but I don’t want to bore anyone to death (any more than normal anyway )

I find the oscilloscope very useful for helping me understand how each system operates. By looking at the controlling current, duty cycle or even frequency I can tell how the fuel pressure is regulated on the various setups. This doesn’t make me an expert, but it certainly helps me understand what is going on & makes things a little less mysterious. The one thing that does leed to much confusion I think, is the varied terminology & abreviations used to describe these control valves. MPROP, VCV, IMV, FPS etc etc.

HTH

Dave

PS If I have any inaccuracies in my comments, please feel free to take the pi**