Fuel Pressure Doesnt Change with/without vac line

[EvoInside]

Ok so here is what I have

99 Mitsu Galant
Built 4G64 w/EVO Head

Stock fuel feed line from tank to firewall, adaptor from stock hardline to -8 into an AMS fuel rail, Denso 720cc injectors, -8 out of the rail and into an Aeromotive A1000 FPR to a stock return line.

The pump is a Walbro 255 re-wired with 10 gauge, being fed from its own 30amp relay and fuse wired from the battery, controlled by AEM EMS.

So here is the problem....

The fuel pressure is a solid 48psi at idle on my mechanical gauge which is on the regulator itseld and also on my electronic gauge which is the sender is on the rail itself so I know the gauge is reading correctly. When I connect the vacuum line there is no change in pressure, boost gauge is showing me pulling -22 in/hg at idle. I thought I had a bad FPR so I bought a brand new one, no change. I thought the return line to the tank was clogged, blew high pressure air thru it, air made it to the tank ok (I knowthat air is much more penetrating than fuel, but it was my only option) So since my car has no emissions, I next took the stock vapor vent hardline that runs fromt he tank and I disconnected the rubber vapor line from the tank and swapped the vapor line to my stock return hose at the tank and firewall, essentially making the vapor line my return line since its slightly larger in diameter than the stock return, this also had no bearing on the pressure. I am out of solutions at this point. Is it possible that the -8 feed from the hardline and the -8 back to the regulator is too much volume for the stock return to handle?

My sending unit isnt like a DSM, or else I would just run -6 feed and return from the tank to the rail and call it a day.

Any help is much appreciated.

 

[redeclipse7782]

I had the same problem, as have many others, you need to replace the factory return line, either with a 3/8 hard line or with a 6AN line. The pressure is too high going into the return line and its backing up causing the fuel pressure not to drop, rather then flowing smootly back into the tank.

The A1000 should have a 6AN or 8AN inlet and outlet, just run a 6AN or 8AN outlet (return) line from the regulator all the way back to the tank. The 6AN line will fit over the stock return nipple on the tank, just use a hose clamp on it.

 

[EvoInside]

I wish it were that simple.

our fuel pump sending units are not like that of a 1g/2g, the return line fitting on the sending unit is a cast plastic nipple thta is part of the top cover itself and its smaller than a -6.

 

[oldman]

Are you able to lower fuel pressure via the adjustment screw?

 

[Talesin]

Huh... a '99 Galant. Does that still count as a DSM? Never was really clear on that.
In any case, if you have a fuel pressure solenoid, and it works the same way as the FPS on a 1G, try running a straight vacuum line from the intake manifold nipple to the boost/vac reference port on the Aeromotive. The FPS blocks all vacuum reference for a while on first starting the car to raise the fuel pressure and help prevent hot-starting problems. You can also test to see if this is the problem by pulling the vac line off the Aeromotive and putting your finger over the end. If you feel vacuum, put it back on and the pressure still doesn't drop then it may be the return line thing (and good luck).