I recently replaced my Fuel Pump in my 86. Upon first test, the pressure went to 43 psi and fell back to 36. I then had to change the gasket, but had to hand make one due to no material. Some of the hold down bolts broke, so I have about half holding it in place, The pressure starts out at 43 for a second or 2, then drops to 20, even when running. Is that due to gasket and bolts. Is this system pressurized and then loses psi due to leaks?

 

NO...you are leaking in the tank. GUARANTEED!!! The pressure should hold for a long time....and NOT drop that fast to 20 psi. The bolts that hold your sending unit to your gas tank are just that. They hold it in place and seal it up and they have NOTHING to do with your fuel pressure.

DUB

 

you not losing pressure from the tank.

... Here are the sure fail tests for the fuel system to save you time and money.. Take a fuel pressure gauge attach it to the Schrader valve.. Turn the key on and off a few times to build up the pressure in the system... If you are having starting trouble the pressure will probably drop as soon as it hits 38 or 40 and the pump turns off it will probably leak back to zero within a few minutes. Ok now right after you prime the system pinch off the return line near the tank if the pressure drop stops it is a bad fuel pressure regulator.... Next pinch off the feed line if the pressure holds then the leak is before the point where you pinched the line IE fuel pump check valve..hole in the metal line on the sending unit caused by it rubbing on the inside of the tank you should visibly be able to see this if you pull the pump and the sending unit...(this was my problem) or the pulse dampener which is the piece between the pump and metal line on the sending unit.. the diaphragm tears.. and last if you pinch both the feed and return line near the tank and the leak down of the pressure continues then a fuel injector is your problem.. This is the GM test.. saves time and money..

 

Thanks, you were exactly correct. I corrected issue and is now correct

 

I have a similar problem . I have an aftermarket pump , goes to 50 after the key is ON ,then drops rapidly . 30 seconds its at 10 pounds .
I thought it may be the Dampner between the pump an supply line so I replaced it with HP fuel line . Still have a pressure drop . Is there a check ball in the system somewhere ?

 

What I did was remove the assembly, look at it really well, reseated the pump, made sure the strainer was new, reinserted the assembly in tank and connected my pressure gauge at the outlet and checked it with my pressure gauge, first to check pressure and then to see if it held the correct pressure. It should read about 43-46 psi and hold. If that is good, I then went to the Fuel rail checked the regulator for fuel, then turned on ignition to see what the pressure was. Note, your Pressure should hold for a few mins anyways. By checking the pressure at the Tank, it help to relieve where to look next or go back in the tank

 

There is a check valve in the pump itself. Between the check valve, and the fuel pressure regulator, pressure should not be lost when off, between the pump and the regulator. Follow John A Marker's test procedure; it is spot on and conclusive....