Strange goings on in fuel system
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Teching In
Joined: Mar 2024
Posts: 24
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Strange goings on in fuel system Hey fellas I spent a bit of time trying to find some answers in the forum but i'm still unsure.. 5.3 boosted , AF external pump , im having a few weird things going on & not sure if it just set up issues {previous owner} or a certain part fault, when i turn the ign on, pump primes but no pressure show on the reg gauge, start the truck starts fine gauge sits on 50psi on idle with vaccum line connected, the AFR sits around 12.5 @ idle, while driving under boost AFR is @10.5 & off throttle 12.5/13.5 , only driving issue I have is open road cruising 100kph {60mph for you } it runs super rich AFR as low as 8 add a slight bit more throttle jumps back to 12/13 again but hard to keep a crusing {legal} speed. Then soon as I switch the truck off fuel pressure guage drops to 0 , it has a check valve between the pump & reg so i'm a little lost with it. So after all that I guess my questions are FP being 50psi at idle & if throttles blipped it jumps up to 60/65 does that sound right? Then the fact no gauge pressure with key on only when engines started, then instant drop to 0 with key off , what should I look at ?? Othewise it goes good dynoed 540 at the wheels on 8psi. I'm just concearned the hi &low AFR levels are causing me damage .
cheers
cheers
Last edited by Twsdid; Mar 13, 2025 at 01:15 PM.
Almost no aftermarket regulators will hold pressure once the pump is off. Dropping almost instantly back to zero is unfortunately pretty normal. Otherwise, it sounds like you just have a truly **** excuse for a tune. Anybody can get a WOT dyno number in just a few minutes. The other stuff like idle and drivability are what takes time and skill.
TECH Fanatic
Joined: Jan 2009
Posts: 1,067
Likes: 477
From: Bostwick, GA.
"start the truck starts fine gauge sits on 50psi on idle with vaccum line connected, the AFR sits around 12.5 @ idle, while driving under boost AFR is @10.5 & off throttle 12.5/13.5 , only driving issue I have is open road cruising 100kph {60mph for you } it runs super rich AFR as low as 8 add a slight bit more throttle jumps back to 12/13 again but hard to keep a crusing {legal} speed."
1. Is this a blower or a turbo app?
2. What's the pressure set to with vac/boost line off and plugged?
3. AFR at 8 cruising at what fuel pressure?
4. What ecm/control system are you using?
Post a data log and the tune.
A sketch of how the system is plumbed would help.
1. Is this a blower or a turbo app?
2. What's the pressure set to with vac/boost line off and plugged?
3. AFR at 8 cruising at what fuel pressure?
4. What ecm/control system are you using?
Post a data log and the tune.
A sketch of how the system is plumbed would help.
Joined: Apr 2006
Posts: 3,909
Likes: 1,158
From: Grand Rapids, Michigan
Your fuel pressure regulator is behaving correctly. Under vacuum, it will be LOWER than the set point ( set with vacuum unplugged ). As you reach 100kPa ( 0 vacuum ), your pressure should rise to the setpoint on the regulator. Under boost, it should rise 1:1 with boost pressure. So, if it's set to 60PSI unplugged, under boost and plugged back in, if you have 10PSI, your fuel pressure should be 70PSI. When you blip the throttle, you decrease vacuum which makes it jump up.
