Small Block & Big Block Chevy Specific - Fuel Pressure problems




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regal85
03-23-2011, 12:42 PM
I have a 85 buick regal that i recently rebuilt the motor in (358 cu in). complete rebuild
got the car running and i have fuel pressure upon start up then after about ten minutes of running it drops from 7.5 pounds to 5pnds to 3 then to abt a half pound the car still runs with this pressure it doesnt die.

It has new Holley 70Gph Mechanical pump
new fuel pressure guage
new holley 670 street avenger
new fuel lines and
recently cleaned gas tank with new sock on pick up

can anyone help me figure out why im losing pressure


1 FMF
03-23-2011, 01:54 PM
after 10 min of running assuming you mean just idling, losing pressure makes me think of fuel lines routed too close to heat and the fuel inside the lines is turning to vapor and causing a pressure loss, just an idea to look for. but being a mechanical pump i'd bet you have a leak on the vacuum side of the pump... from the pump back to the pickup in the tank. other than that, it may be the check valves in the body of the fuel pump but i would be more inclined to think if there was a problem with those you would have no pressure on startup and not declining pressure after a few minutes, not sure how you could check this, maybe fab up some fuel line to fun from fuel pump inlet into a gallon gas can so you know there's no vacuum leak.
other thing it could be is fuel line from the tank to pump is rubber and is collapsing under vacuum, in which case you'd want to use hard line or reinforced rubber fuel line that won't collapse.

regal85
03-23-2011, 06:23 PM
I did have it to a gas can and it did the same thing also. All of the fuel lines are away from heat and the fuel lines are new and isnt collapsing


Oldsmobility85
03-24-2011, 01:43 AM
Make sure your fuel pump pushrod is moving with ease when warm and not binding.

I put a bolt into the front motormount hole when I was a teenager that was a hair too long and it locked the pushrod up in its bore, and the pushrod went down but never returned up.

If you were having a vaccume problem you should have a real hard time getting it started after sitting a bit.

cambirdracing
03-24-2011, 10:42 AM
Make sure your fuel pump pushrod is moving with ease when warm and not binding.

I put a bolt into the front motormount hole when I was a teenager that was a hair too long and it locked the pushrod up in its bore, and the pushrod went down but never returned up.

If you were having a vaccume problem you should have a real hard time getting it started after sitting a bit.

Could have wiped out the fuel pump lobe on the cam also. I have seen this done before.

Oldsmobility85
03-24-2011, 07:32 PM
^ Absolutely correct.

regal85
04-13-2011, 10:12 AM
update:

Put a third fuel pump on the car and it is holding pressure.

Adjusted the rocker arms because some of them had a chatter to them and discovered that about six or seven of them wasnt moving a whole lot which would mean the cam is whiped. Pulled the intake off and the lifters out and seven lifters was bad completely mangled ,and the cam is now junk.