Fmu Question?

Discussion in 'Fox 5.0 Mustang Tech' started by MY 85 GT, Nov 16, 2013.

  1. Ok this is a weird one...I have been installing a supercharger on my wifes car...COUGH a 91 z28 350 tpi car I know I know...its a chevy But FMUs are all the same no matter what car...anyways I cant get this FMU to work...its an ATI adjustable from 12-1 to 4-1 but it boost the fuel pressure does not SPIKE so I thought the FMU was bad so I ordered a rebuild kit for it did this yesterday and nothing doesn't spike the fuel up to were it should lost I know I have the lines hooked up right from the regulator back to the return line to the gas tank...I have no idea what to do...this should work what am I missing???????
  2. if you hook it up backwards it will spike the fuel pressure through the roof constantly, at least if its like the old vortech, Paxton and cartech fmu's. so you probably have the fuel lines hooked up correctly in terms inlet and outlet of the FMU. it has to have boost signal from the vacuum system to blow the return shut and raise FP. is your vac line hooked up directly, or closely to a port off the plenum? pics would help.
  3. Thanks for the reply yes its just like a vortech FMU...exactly like I have the boost reference line going to its own source off the Plenum..just like they say to does not rasie fuel pressure..I bought the FMU used it was sitting for awhile so I bought a rebuild kit just rebuilt it last nite installed it took a test run no spike in fuel pressure in boost I have an Air fuel gauge,a fuel pressure gauge so I know it isn't raising fuel pressure...I just don't get it?..its a simple thing boost closes restricts return up fuel pressure but the damn thing just doesn't work can it be a faulty FMU I mean its so simple to rebuild..thats why im confussed just doesn't see why its not working...
  4. when you say rebuild did you just replace the diaphragm or did you take apart the piston too?
  5. I took apart everything I took the base off also... to replace the little metal shim, the o-ring down there and the gasket...also had the piston out looked clean no burrs I replaced the Diaphram on top...but I just don't see movement in that Piston I pressed down on it with my fingers and it didn't seem to move a hell of a lot..If that piston is supposed to close down the return port I really don't think it is....
    #5 MY 85 GT, Nov 16, 2013
    Last edited: Nov 16, 2013
  6. My question would be about the fuel system on the car itself. Is it a return style system?
  7. did you check the input line for proper vacuum? You have it hooked up from a vacuum port to the FPR?

  8. The car does have a return style system..I just installed a walbro high volume/high pressure 255 lph pump...

  9. ATI says it wants its own vac source so I have it to its own vac source on the Plenum...
  10. IIRC (because I haven't used an FMU since about 98) yes boost will push on the calibration plates which push on the piston. I have a couple of those early vortech and Paxton fmu's as well as an old cartech adjustable. they are just taking up space in my tool box, for a long time.

  11. Would you be willing to sell one ?...Im just wondering if something is wrong with the one I have something isn't right with it.......
  12. I've got the same ATI one & had the same problem as you. The little piston & fuel diaphram & metal shim (not really a shim) have to be perfect. I even sent my old one to a "so called expert" for rebuilding. It came back worse than I sent out. I bought a new Vortech & it works. I found out, just because it's blue, it might not be a Vortech. There a lot of copies. Vortech does not sell rebuild kits, only the vacuum/pressure side parts.

    I was able to test all these with a hand held Mity-Vac by using the pressure port on the Mity-Vac. Fuel pressure is read from a gauge on the fuel rail. Connect to FMU & a few lbs air pressure will raise fuel pressure to 100 psi instantly. I also have the same 255 HP pump. Unable to adjust FPR below 40psi & almost no vacuum compensation. This was done with engine off.

    BTW, I have ATI P600B, S/N 6 ( so they told me once). It's the 1st production run & has no S/N on it. Didn't know they were using buyers as beta testers. And then all the mods required to make it work for the next couple years. Actually glad I never raced it, who knows what would have happened? But it sure made a lot of power for 1993 on pump gas only. Pretty much the strongest street car around.

    Kinda feel scammed by ATI, called them a couple years ago & didn't even have a record of the sale, although I bought it directly from them. The only thing they actually designed or made themselves was the pump itself. Everything else was sourced from others, some good, some crap. Intercooler was Cartech, several extra pipes in the kit that didn't go anywhere.

    Who knows where they got the FMU's? Certainly not from Vortech!
    #12 Blown88GT, Nov 16, 2013
    Last edited: Nov 16, 2013
  13. So you think I should just buy a new one or one that I know that works?? You mean you have one you want to sell???
  14. Either. You'll never get that ATI POS to work. 2 engineers here, couldn't do it.
    Thought about selling the new Vortech, might not need it anymore since am going to new ECU. It currently has 30lb pressure kit but have others. What's it worth including the lines for Ford? Actually have 2 sets of braided fuel lines, 1 still in the box.

    FYI, the original problem was the crappy fuel lines from the FMU, they swelled internally & spiked the fuel pressure to 80 psi at idle. Took me a long time to find that one.

    If interested, PM me. What were the chances of someone else here having the same ATI FPR as I do? Are we in the Twilight Zone?
  15. PM sent...
  16. Only because of the air bleed valve. When under vacuum, it's just a leak. When under pressure, it bleeds some off. Better to use add a check valve so you don't have a vacuum leak but it will still bleed pressure.
  17. If you think you can make a vortech fmu for a fox work I have one brand new in the box same thing, taking up space in the garage. As much as I am for tuning rather than the fmu it's yours if you want it.