Fuel pressure confusion

T

TurboDetroit

Member
Joined
Jul 15, 2012
Getting a fuel pressure drop when braking and it's not adding up. Magnafuel pro tuner 750.

-fuel cell is topped off
-magnafuel filters cleaned out
-idles fine
-makes a surging/grinding noise when slowing down sometimes, not every time.
-pump basically only has dyno time and idle time. Maybe 5 miles on it.
-fuel pressure is somewhat inconsistent and not 100% steady. Bounces 34-36 at idle.
-ground is to the frame grinded down to bare metal.
-dual relay setup.
-tried a bigger vent
-pump isn't hot to the touch

Ideas? Help me out! Lol the dream cruise is this Saturday!

Also, a slight stutter when taking off from complete stop, not sure is this is in the xfi or a pump issue.
 
First of all 34-36 psi seems too low. Should be around 43 psi, vacuum on. What's a dual relay setup on a fuel system? What size are your supply and return lines? Also what fuel pressure regulator are you running , it should be adjustable. If you have done any vacuum line mods take a snap shot of it for visuals. Sorry last question, what size injectors and how old are they.
 
Everything is new.

Magnafuel adjustable regulator
160lb injectors(e85)
-10 to y block feeding dual 8's to rail
-6 return line
Vacuum block setup
Fuel pump has dual relays in case of failure
 
rag231 said:
First of all 34-36 psi seems too low. Should be around 43 psi, vacuum on. What's a dual relay setup on a fuel system? What size are your supply and return lines? Also what fuel pressure regulator are you running , it should be adjustable. If you have done any vacuum line mods take a snap shot of it for visuals. Sorry last question, what size injectors and how old are they.
Click to expand...

Everything is new.

Magnafuel adjustable regulator
160lb injectors(e85)
-10 to y block feeding dual 8's to rail
-6 return line
Vacuum block setup
Fuel pump has dual relays in case of failure
 
Do you notice a voltage drop when braking?

Does your RPM drop real low, maybe your IAC throttle follower/ throttle blade stop need adjusting?

Other than that, all I can think of is maybe air in the system? Is the pump mounted low enough without a high point in the suction line from the fuel cell?
 
Yes cell is raised off the floor in the trunk, pump under floor.

No voltage drop when braking And I haven't noticed a real big rpm drop ‍♂️
 
TurboDetroit said:
Getting a fuel pressure drop when braking and it's not adding up. Magnafuel pro tuner 750.

-ground is to the frame grinded down to bare metal.

Ideas? Help me out! Lol the dream cruise is this Saturday!
Also, a slight stutter when taking off from complete stop, not sure is this is in the xfi or a pump issue.
Click to expand...

Why did you run the ground to the frame? The frame is a poor ground unless you ran a ground cable from the battery to the frame. You should ground it to the body.
 
Chuck Leeper said:
Huh??
Click to expand...
The frame is a poor ground unless you run a cable from the negative battery post to the frame. The battery is typically grounded to the body. The frame is isolated from the body though rubber mounts. The only real electrical ground connection between the frame and the body is the brake lines going down to the proportioning valve. I have seen issues where customers were grounding electrical components to the frame and having issues (voltage drop). The OP did not mention he ran a ground cable from the battery to the frame until I made my comment.
 
Sorry if I am hi-jacking but....Since I have my ground from battery to engine and have a ground from engine to frame w/ poly mounts............am I good??
 
belsie said:
Sorry if I am hi-jacking but....Since I have my ground from battery to engine and have a ground from engine to frame w/ poly mounts............am I good??
Click to expand...
Need more info. Running a ground from the battery to the frame does nothing unless you have some electrical devices also grounded to the frame. Typically all electrical components are grounded to the body. That's why there is a ground wire going from the negative battery cable to the body. If you do have some electrical devices grounded to the frame then your ok. The cable from the frame to the engine block will provide a low resistance path back to the battery. Do you have anything grounded to the frame other than the engine block?
 
Thanks for the response.........actually I have to re-check the routing !!! But I do know I have battery to block (at exhaust manifold) then block to frame (at bell housing) and block to body behind coil. Along with heavy cable from alt. to battery I hoped all was well????? Any thoughts.............?
 
belsie said:
Thanks for the response.........actually I have to re-check the routing !!! But I do know I have battery to block (at exhaust manifold) then block to frame (at bell housing) and block to body behind coil. Along with heavy cable from alt. to battery I hoped all was well????? Any thoughts.............?
Click to expand...
Looks like your ok. The one from the block to the frame really isn't necessary but your good.
 
According to the electrical "experts", a DC circuit should have all critical items like fuel pumps and ECM's with a ground back to the battery using the proper size copper stranded wire, not through a crappy low carbon steel frame or a body with many bolted-on parts which are old and corroded.

On all our builds, the fuel pump wiring is using copper wires directly back to the battery.

With a performance build, we set base fuel pressure close to 50 psi with gas, and 55 psi with e-85.

My opinion is to have more pressure at the injector as I do not trust gauges, and I would rather have more fuel available at the injector than less fuel.
 
Ok guys... I grounded the pump straight to the battery and put a new body ground on. No change.... I just don't get it. Is my pump smoked?
 
mikestertwo said:
The frame is a poor ground unless you run a cable from the negative battery post to the frame. The battery is typically grounded to the body. The frame is isolated from the body though rubber mounts. The only real electrical ground connection between the frame and the body is the brake lines going down to the proportioning valve. I have seen issues where customers were grounding electrical components to the frame and having issues (voltage drop). The OP did not mention he ran a ground cable from the battery to the frame until I made my comment.
Click to expand...

I understand the concept. However, batt to frame, then using the frame, does not guarantee the entire frame is a good conductor. The poor weld quality is always suspect.
OP. Do a volt drop test on both the + and the - sides. Check all ground integrity w/ a ohm meter.
 
Where is your return line dumping back into the tank ? I bet it's dumping right next to your supply pick up and it's airating the fuel causing the pump to cavatate . Something to look into
 
dank GN said:
Where is your return line dumping back into the tank ? I bet it's dumping right next to your supply pick up and it's airating the fuel causing the pump to cavatate . Something to look into
Click to expand...
Good idea, and is easily tested by filling the tank all the way. If the problem goes away you were right, if it stays the same keep searching.
 
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