Slea

I recently modified my '89 GT and since I've started up the engine after the mods, I've been having a loss of fuel pressure at WOT (on dyno). The car idles fine, runs ok (but will run better once it's tuned on dyno cause it's running rich), and only hesitates whenever it's walked on.

Every pass on dyno shows a dramatic loss in fuel pressure, as low as 21 lbs. Fuel pressure is set at 39 lbs and the gauge shows a rise in pressure whenever it's revved. On the dyno, we thought it might be a bad fuel pump (255 lph), but we swaped it and it's still running the same.

Anyone one have a clue? We're starting to suspect it may be the fuel pressure regulator's diaphragm that's in backwards!

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My mods are: H/C/I, 42 lb injectors, 255 lph pump, new fuel filter, new adjustable fuel pressure regulator, fuel pressure gauge, new fuel rails, ignition box, new distribributor/coil, mild Vortech supercharger (about 5 lbs of boost), 3.55 gears, speedo conversion, 352 bhp/345 lbs. torque (so far)

mygt500

wrong section and welcome to the forum! moved

marx91

see if theres fuel being sucked into the vacum line, that would be a dead give away that theres a problem with the diaphram, and cause the drop in pressure

mjr46

PINCH OF THE RETURN LINE AND SEE IF LEAK DOWN OCCURS IN koeo MODE

Slea

Thanks guys, I will try that first before yanking everything apart... and re-post in correct forum. (Sorry!)

Slea

Just finished the tear down... after pinching off the return line and removing the vacuum line from the regulator and giving it a good sniff (motor off, of course) with zero results.

I opened the regulator and the diaphragm looks fine and it's put in correctly. I don't what else to try, this is getting frustrating since, according to the dyno, I'm losing fuel pressure at high rpm, yet the car is running rich (backfiring).

AdderMk2

get a booster pump... your running the rail dry at high RPM with those injectors under boost

Slea

Thanks for the suggestion, but I've got a 255 lph pump (in-tank), I think it should do the trick. (I did swap it for another just to see if it may be the pump, but the problem remained) A local guy suggested I may not have big enough fuel lines supplying the injectors, but another guy claims that the stock fuel lines can supply any motor up to 500 HP.

Still searching for the solution....

AdderMk2

Im glad you think your 255lph pump is enough. Im telling you from first had experience. Your loss of pressure is because the rail is going dry.. get a Vortech T-rex, or Aeromotive inline A1000 and your problems will go away

Slea

That may be the case and an in-line may be the way to go. However, I did fail to mention in my original post (sorry for omission) that I have a serious miss when I walk on the gas (especially in 2nd). So it's not always at high rpms. (Once it's going, and we hit high rpms do we see a loss in fuel pressure, but no hesitation) I also have a power surge during cruising but felt only when the cruise control is on.

Is it still possible it's the rail being emptied? I apologize for the confusion as I know it's hard to offer help when you don't have all the symptoms!!