Low fuel pressure when starting

[msr18]

I’ve been experiencing hard starts on my lq4 swap. The first attempt the motor will crank for as long as I hold it and not start. Second attempt the motor will start with a small sputter and see 59psi. My fuel setup is an aeromotive stealth 340 pump with an inline Holley regulator with return. During prime, the fuel pressure gauge on the rail will rise to 20psi and drop immediately. If I clamp the return line off the regulator I see 40psi at the rail and it will hold for 10 minutes. I pulled the rails and placed paper towels under the injectors and primed the system. I did not see any gas on the paper towels. Appreciate any help.

[rodstored-72]

tagging along, as i have a '13 5.3 in a '86 & and also have low fuel pressure (55-58psi) , good at low RPM but as i rev higher it drops to the 53-ish range. hopefully get some help along with you....

[biketopia]

Originally Posted by msr18

I’ve been experiencing hard starts on my lq4 swap. The first attempt the motor will crank for as long as I hold it and not start. Second attempt the motor will start with a small sputter and see 59psi. My fuel setup is an aeromotive stealth 340 pump with an inline Holley regulator with return. During prime, the fuel pressure gauge on the rail will rise to 20psi and drop immediately. If I clamp the return line off the regulator I see 40psi at the rail and it will hold for 10 minutes. I pulled the rails and placed paper towels under the injectors and primed the system. I did not see any gas on the paper towels. Appreciate any help.

What ECU are you running, what injectors, and what base fuel pressure is the tune set up to run at? 59psi is plenty for that to run on. The factory pump and regulator set up has an anti drain back valve to keep the engine mostly primed when it's off. I also had a 340 and I don't believe it did. I was running a Holley ECU so I was also able to set how long I wanted the pump to prime for. Always let it do it's prime and then crank. Don't just hop in and crank it right away.

[msr18]

Originally Posted by biketopia

What ECU are you running, what injectors, and what base fuel pressure is the tune set up to run at? 59psi is plenty for that to run on. The factory pump and regulator set up has an anti drain back valve to keep the engine mostly primed when it's off. I also had a 340 and I don't believe it did. I was running a Holley ECU so I was also able to set how long I wanted the pump to prime for. Always let it do it's prime and then crank. Don't just hop in and crank it right away.

Stock ecu, injectors, and startup tune. I let the fuel pump complete it’s prime before attempting to start. It idles and runs beautifully after starting, no drop in pressure. For the time being, I have installed a check at the rail and it seems to help but not the fix I was looking for. Digging a little deeper it looks like pump isn’t recommended for pulse width modulation. Is a different pump the better route to take? I honestly ordered under assumption since it was the pump available through Boyd when purchasing the tank.