Fuel System Loses pressure

[stanleyscreamer]

Hello There, I recently aquired a really beatup 85 GN that was sitting in someone's yard for the last 12 years. I am in the initial stages of putting it back together. Of course after 12 years of sitting the fuel pump was toast. So i reconditioned the tank and the fuel sending unit and installed a new holley 190 lph fuel pump. This got the car to fire and run. During this I hooked a fuel gauge to the fuel rails and noticed that while running i do have a good 40 PSI on the rail. However after shutoff the pressure falls quickly to 0. I know this means that more is wrong with the fuel system. but i'm looking for a little advice on which direction to go next, fuel regulator or injectors. I'm leaning tward injectors being the leak but i have no way of knowing. Any test procedures or hints would be appreciated.

Stephen Edmonds

[Keller]

Best test would be to pull them and have them cleaned and flowed. However, given their lack of care, it might be best to just replace them while they are out.

If the car is going to be just stock, another set of used stock injectors (28 lb/hr) would be cheap, and you could have them cleaned/flowed in advance. Plus, you could put an adjustable regulator on the system while the fuel rail was off just to cover the bases. If any upgrades are planned, a minor upgrade in injector size (36 lb/hr perhaps) would be easy. Just be sure to get the properly calibrated chip to go along with them. These injectors would be OK with the stock turbo, and be ready for an upgraded one should it come about.

Be sure to use NEW O-rings and lube them when you put them in. See http://www.gnttype.org/techarea/fuelsystem/injector_swap.html

[tommy231]

It is either the injectors leaking, the fuel pump leaking (internally or one of the hose connections), or the regulator leaking. You could try "pinching" the return line to see if it is the regulator. A pair of vice grips and a couple of small pieces of wood on the rubber section of the return hose should work. Even if you can't pinch it off completely, if it changes the rate of bleed-down then it probably is the regulator.

A simple key-on engine-off test should suffice. First the pressure will be much higher than usual as the regulator is not being allowed to return the excess fuel. The pump will only run for a few seconds. Above about 75-80 psi will probably leak-down through the check valve in the pump (this might be normal, depending on the pump), but once it gets down near the normal key-on engine-off pressure you should be able to see if fuel is leaking back through the return line by how long it takes to bleed down as compared to not having the line pinched. If pinching the return line has an effect on the leak-down rate, replace the regulator.

A similar test can be performed on the feed line to see if the feed side is leaking. The hose connection to the pump in the tank could have a minor leak or the internal check valve in the pump could be leaking. However, for that test you'll need to pinch the feed line while the pressure is still high (and the pump is off) and see if it changes the bleed-down rate.

Personally, I suspect leaky injectors.