I must admit, I don’t know much about the Yamaha vst di system, but a couple of assumptions or questions perhaps, from what little I do know about basic Di systems of other makes and perhaps a little from here and there.
As far as I know there is usually a fuel pressure regulator that is worth checking for function.
I’d also try and prove if one or more injector is leaking or sticking open, although this being the case, you’d imagine that the fuel pressure would drop to below 18 psi or to zero even. (More on that in a minute)
To check the injectors you may be able to simply take the plugs out and look for signs of fuel being thrown in there when the pressure drops after key on, when it shouldn’t obviously. You could also check if they are getting a signal to open when they shouldn’t. Usually this is a check for about 5v to them with a meter.
Back to the fuel pressure….if the fuel in the vst is going somewhere from the chamber…there is usually a float that will drop with the level and eventually tell the pump to pressure up again. Someone more knowledgeable than I will no doubt be along with the info, but I’d imagine this should be the case.
I’m also aware that higher ethanol content in fuel these days has been known to have an effect on the grommets and any other easily perishable items within the vst or anywhere else in the fuel system. Grommets within the vst on the mercs have been known to do just this for certain.
There will also be an air sensor or o2 sensor it may be called. This has an influence on the fuelling and running state of the engine. Worth a check too.
Possibly also off on a tangent, but I’m aware that it is sometimes quite crucial to have the plugs gap set properly, on engines like this. Their whole operation and design purpose is to run very efficiently and frugally as they can. Things like plug gap are likely considered crucial and may even be stated as much in the manual.
Are you taking the fuel pressure reading from a fuel rail ? Perhaps this particular vst system is able to bleed off pressure from it, when not required? It may not be such a worry, if so. Early merc vst weren’t sealed system and did just this. Newer don’t I’m aware.
How’s the oil feed for function ? Any signs of over fuelling in the water from the exhaust?
Anyway. Bored you enough for know when assumptions and guesses. Hope it might help a little
