Fuel Pressure Oddity??
 

First the backround info...
1987 GXL
*Newly rebuilt motor... good compression (95f, 100r, all even bounces)
*No vacuum leaks
*No code returns on the ECU diag box
*All injectors rebuilt and tested fine by injector shop
*idles perfectly at 750 rpm's
*good grounds all the way around

I have been chasing down a general lack of power and no power above about 3200 rpm's. Gradual power build to 3200 is fine... mashing the accelerator gives me no power right away and no rpm's... like it is starved for gas or air. Figured it was a problem with secondary injectors activating... fuel or electrical. It is not electrical.

Anyway, in checking the fuel system I have returned some odd results. The system fuel pressure is 39 psi... (top end of spec) when measured with a t-coupler out of the fuel filter (new). When I measure the pressure after the FPR with a t-coupler and the car running I have no pressure what so ever. I changed out the FPR and it made no difference. So, is all of the fuel being consumed by the primaries? I don't think the fuel system is pressurizing properly as I never get a vacuum release when I open the fuel filler cap. What could be causing no pressure after the FPR????? The secondaries not getting fuel would seem to support my general lack of power problem.

There should be no virtually pressure after the FPR because that line is pumping back into the tank. It's basically open-ended inside the tank so any pressure you measure is simply overcoming the resistance of the fuel lines.

At low load (i.e. not much fuel being injectors), nearly all of the fuel flow is being pumped back into the tank via the return line. But you won't measure pressure in this line because there's very little resistance to flow.

but the FSM indicates you should have 28.4psi with the vacuum hose attached after the FPR and i have none????

Hey there Adzam.

Well I thought of something.....But then I thought it was stupid....So i'll let you judge,

What about that valve on the secondary fuel rail? It looks like a smaller version of typical vacuum valves. The one that's right above the fuel hose that goes to the emmisions rack.

Could this thing be troubled?

um, stupid I know.....I got one I could send you.

not sure what you're referring to temple??? theres a check valve on the input side of the fuel rail and a fuel pressure regulator and thats the last thing on the fuel line before return. Good to see you buddy... check your pm's. lol

most if not all cars have no pressure after the FPR. the other line is just a "drain" line.

That's the pressure measured straight after the fuel filter, which is the only place you should be measuring fuel pressure. The FSM doesn't say to measure the pressure after the FPR, because there is none. You have misread the instructions.

Well the FSM has a whole section on the FPR check and pressure readings. It says to T a connection (with a guage on the t) after the FPR and evaluate the PSI with the factory specs... which is 28.4psi with the vacuum hose attached. Dont think I read it wrong. You would have to have some pressure in the secondary rail and therefore some after the FPR in order for it to make the secondary injectors function.

No, you've got it wrong. There is no meaningful pressure in the return line. It's open-vented into the fuel tank. The pressure in the system (lines, filter, both rails) is caused by the resistance of the FPR. That's it's job. But varying the resistance it causes, the pressure is changed.

The only place you measure fuel pressure is straight after the fuel filter. I've read the section on checking fuel pressure several times before and it doesn't say anything about teeing into the return line.