LS Fuel Pressure

[custodian]

Do you have a return style system? Assuming yes, you should have a feed line no smaller than 3/8ths from the sending unit pickup to an inline filter and then into your pump. Between your pump and fuel rail should be a finer mesh, high pressure filter. On the outlet side of the fuel rail should be the pressure regulator and a minimum 5/16th return line back to the tank. Any bigger difference between the feed & return lines is a restriction and will cause your pressure to be higher.

This is the style of the fuel sending unit I have in the truck right now. Is the return inside the take, the knob looking thing, a vent?

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[custodian]

This is the style of the fuel sending unit I have in the truck right now. Is the return inside the take, the knob looking thing, more off a vent?

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This is the one I think I need.

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[Bextreme04]

Yeah, that is a non-return line sending unit. How is it being vented? You can use that if you have a vented filler cap, except that the wobble valve on the bottom of the vent tube will be a bit of a restriction. Did you run 5/16" hose all the way back from the fuel rail to that vent nipple on the sending unit?

 

[SquareRoot]

This is the style of the fuel sending unit I have in the truck right now. Is the return inside the take, the knob looking thing, a vent?

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If your using this one, then you must be using the vent (5/16) as the return, which is OK except how are you venting the tank?

LOL, Eric and I must have hit send at the same time.

 

[custodian]

If your using this one, then you must be using the vent (5/16) as the return, which is OK except how are you venting the tank?

LOL, Eric and I must have hit send at the same time.

I guess I'm not.

 

[custodian]

Yeah, that is a non-return line sending unit. How is it being vented? You can use that if you have a vented filler cap, except that the wobble valve on the bottom of the vent tube will be a bit of a restriction. Did you run 5/16" hose all the way back from the fuel rail to that vent nipple on the sending unit?

Yes, 5/16" line as return all the way back. Vented, I guess not. Will have to get this fixed. That might be the cause of my fuel pressure.

 

[SquareRoot]

I discovered that a vented cap only means it's vented one way. It allows air in to compensate for the fuel being used but it does NOT let air out. That means if you don't vent the tank via a charcoal cannister or other means and the fuel expands it will create pressure inside the tank. This will add pressure to the fuel system and may contribute to the high pressure your seeing. However, I would do the following if I were you.
1. Verify fuel pressure at idle with the vacuum hose off. It should be higher than 58 psi but NOT 100.
2. Reattach hose an it should drop to 58 (or whatever the setpoint is for that application/fuel table).
3. Unhook the return hose and run it into a container and check psi at idle. If unchanged, you have a bad regulator. If it does change you have a restriction in the return line.

If you're going to replace or change anything, now is the time to do it right. Get a better, adjustable regulator and make sure the tank is vented correctly.

 

[CalSgt]

IIRC I was having a high pressure issue from wiring the tank switch valve wrong. It was causing the system to try to return to the wrong tank but would not flow at the volume as it did when wired correctly.

That's the same regulator that is on the 7.4L L29 motors. Its a piece of junk. I would suggest getting a blockoff plate from ICTbillet and run an adjustable aftermarket one.

I went through 4 used L31 manifolds (and regulators) half of them were no good driving my pressure over 88 PSI.

 

[custodian]

I discovered that a vented cap only means it's vented one way. It allows air in to compensate for the fuel being used but it does NOT let air out. That means if you don't vent the tank via a charcoal cannister or other means and the fuel expands it will create pressure inside the tank. This will add pressure to the fuel system and may contribute to the high pressure your seeing. However, I would do the following if I were you.
1. Verify fuel pressure at idle with the vacuum hose off. It should be higher than 58 psi but NOT 100.
2. Reattach hose an it should drop to 58 (or whatever the setpoint is for that application/fuel table).
3. Unhook the return hose and run it into a container and check psi at idle. If unchanged, you have a bad regulator. If it does change you have a restriction in the return line.

If you're going to replace or change anything, now is the time to do it right. Get a better, adjustable regulator and make sure the tank is vented correctly.

Your #3 advice was what came in mind, will do that first. As for the venting, an oversite on my part. I still have the charcoal canister that was on the truck. I have ordered the 3 outlet sending unit. I'll be using a different fuel pressure tester. Had just put a cheap gauge on the fuel test port.

Thanks for mentioning the return line in your earlier post, that got me thinking on how I set it up. Just an oversite on my part.

The truck has been driven around 1100 miles since the swap.

 

[Camar068]

guessing you'll get better gas mileage with the same if not better performance

 

[custodian]

I hooked up my fuel pressure tester, the big gauge, the fuel pressure showed 53psi before starting and after I started it it showed 110psi. I unhooked the return line and run it in a gas can. The pressure gauge showed me 53psi before starting and 53psi after starting, and while it was idling the pressure stayed at 53psi.

Would 53psi be around normal at idle?

Will be changing the fuel sender unit from a two outlet to a three outlet today.

 

[Hunter79764]

53 is pretty close, that's about 10 inches of vacuum, so might be a little high but not outrageous if you have a cam etc. I think stock goes a little under 50 at idle and up to 58 at WOT and/or vacumm line unplugged, but the ECM should be able to adjust +/- 20% ok.

Sounds like you have a return restriction, probably the sender based on the pictures, but possibly the line. Take the fuel cap off (both tanks, just to rule out) and see if it drops. If so, you have a venting problem. If not, I'd start with blowing out the return line. It could still be a tank venting issue even if removing the caps doesn't work, try capping tanks separately and see when it changes, but I think you're on the right track.

 

[SquareRoot]

Great. You obviously found most of the problem. I'm not sure what the pressure is supposed to be for your application. Is this a factory tune, factory injectors? If so, what year, make is the engine from? This will determine what the pressure is supposed to be. Typically, the pressure at idle will be set to *** with the vacuum regulator disconnected. When you hook the vacuum up, it should drop some amount. For my EFI, I have #29/hr injectors, and a fuel map (tune) based on those injectors. At idle my pressure is 30 psi, at WOT it's close to 50 psi.

 

[custodian]

Great. You obviously found most of the problem. I'm not sure what the pressure is supposed to be for your application. Is this a factory tune, factory injectors? If so, what year, make is the engine from? This will determine what the pressure is supposed to be. Typically, the pressure at idle will be set to *** with the vacuum regulator disconnected. When you hook the vacuum up, it should drop some amount. For my EFI, I have #29/hr injectors, and a fuel map (tune) based on those injectors. At idle my pressure is 30 psi, at WOT it's close to 50 psi.

2002 Silverado 5.3 with stock injectors, LS swapped tune for the stock injectors with the fuel return line. i didn't check the fuel pressure with the vacuum hose disconnected from the regulator.

 

[Hunter79764]

Basically you should see 58 psi with atmospheric pressure on the vacuum line (key on engine off, unplugged vacuum line while running, or WOT operation should all get you about the same "full" pressure), then with the vacuum line connected, your fuel pressure drops 1:1 with engine vacuum, which is about 2"-Hg per PSI. So if you have 14" of vacuum, you should see a ~7 psi drop in fuel pressure at idle. If you have a vacuum pump, full vacuum should get you down close to 43 psi, although I wouldn't bother trying to go that low. Make sure you are getting it trending down until you get to about 50 psi and you should be good to prove the regulator is working.