The function of the fuel pump check valve is to prevent fuel from siphoning back to the tank from the supply line, correct? Does the check valve serve any other purpose? I understand the hold pressure should be maintained for 5 minutes. What allows the pressure to release on an engine that has no faulty parts? Is it released back to the tank via the return valve that I believe to be the FPR? It must be designed to function a certain way, I am just trying to understand the internal workings of the injector system. Thanks
That the pressure bleeds off indicates the presence of "faulty" parts...... if the pressure bleeds off yet remains within spec, we deduce that the parts aren't "faulty enough" to replace since we are likely seeing no running problems created by this. No fuel is supposed to leak from the system - but there is an allowable amount of pressure drop before it becomes a running concern.
The whole purpose of the system (when it is shut off) is to maintain pressure in the fuel rail, this keeps the fuel from vapourizing and also keeps air from infiltrating the fuel rail. When we first turn the key to the run position, we hear the fuel pump run for about 1.5 seconds to ensure that the fuel rail is at the proper "static" pressure. By "static" pressure, I mean the regulated fuel pressure when the FPR "sees" atmospheric pressure at the diaphragm (a bit more on this later).
Fuel can drain from the pressure side of the fuel system at any number of literally dozens of places... the fuel pump, the regulator (both past the control valve or past the diaphragm), the injectors themselves or any one of the myriad external connections - some leaks slow enough that any tattletales might evaporate before they ever become evident. Naturally, any external fuel leaks should be serviced since they will be a safety concern before they are a running concern.
Back to that pesky fuel pressure regulator...... the orofice at the tip of the fuel injector gives its optimum spray pattern at a particular pressure drop across the orofice (got to be careful since we could enter the zone where we need to reconcile manifold vacuum and manifold absolute pressure - two totally different ways of expressing the same thing). Anyway, when manifold vacuum is high, the FPR "sees" this high vacuum at the diaphragm and reduces the rail pressure to maintain the desired pressure drop across the injector tip - resulting in the optimum injector spray pattern. When driving conditions drop the manifold vacuum (i.e., manifold pressure approaches atmospheric pressure), the FPR diaphragm responds by increasing the fuel pressure, again maintaining the ideal pressure drop across the injector tip
Thanks Jim, you really know your stuff I appreciate your help. We have been working on a friends 96x It will not start up right. I put a pressure gauge on it and the pressure goes to 38psi and then drops to 0 in 5 seconds. We have changed the FPR and have eliminated the injectors, plugs are dry after attempts to start, No black smoke, no gas smells etc.
What I have noticed is that while under the truck at the fuel filter, if I turn key on and when pressure gets to 38psi I clamp off the supply line, the pressure will hold for a long time. If I then try to start and remove the clamp it will fire up like new.
With engine running if I briefly clamp off the return line the pressure will go up to like 65-70 so the FPR seems fine. I believe the problem is in the check valve at the pump, would you agree? Other than the hard starting the truck runs great plenty of power. Thanks
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