Low fuel pressure

[mmisch]

Hello,

I'm still having lean issues on my car and recently bought a fuel pressure tester and I think it was 100% worth it because here are some key notes:

Key on engine off = 0 bar pressure
Key on engine on = sits around 1.5 bar
Engine off after running = drops to 0 bar

Also checked on return line, showed 2.5/3 bar with ignition on/engine running.

Injectors are NOT leaking on key on. Its a brand new 4 bar fuel pump. Fuel filter is also new.

Anyone that can help?
I was planning to check pressure directly at the fuel pump and it should be 4 bars.

Couldn't do a normal upload

 

[mty12345]

Fuel pressure specs/test for AUQ

 

[mmisch]

My bad, my first test was not accurate because i mixed up the return line. (thought the fpr regulates the fuel to the rail instead of regulating fuel INSIDE of the fuel rail)
Tested it today (without starting the car because currently the car can not be started) and with KOEO the fuel pressure sits around 2.8 bar/40 psi for as long as I had the gauge hooked up (10 minutes)
Have to see if fuel pressure increases with fpr vacuum hose disconnected when the car is running again

 

[mty12345]

My bad, my first test was not accurate because i mixed up the return line. (thought the fpr regulates the fuel to the rail instead of regulating fuel INSIDE of the fuel rail)
Tested it today (without starting the car because currently the car can not be started) and with KOEO the fuel pressure sits around 2.8 bar/40 psi for as long as I had the gauge hooked up (10 minutes)
Have to see if fuel pressure increases with fpr vacuum hose disconnected when the car is running again

Yeah FPR is doing the rail pressure.

In my experience 99% of lean issues will be some sort of split hose in the PCV system, that is allowing air in that hasn't been through the MAF. Unbolt the metal bracket that bolts to the inlet manifold, and check around that area underneath it for split hoses. Give the hoses a gentle squeeze as some splits aren't always obvious. Any oily residue on the outside of a hose is often a sign that you have a split in that area.

Pardon our interruption...

All of the above hoses are likely suspects, not just the hoses themselves, but where they push together, especially that T-piece. They aren't OEM quality, but I have had a set on mine for 2 years now without issues. With the battery and that metal bracket removed, you can fit the whole set in under an hour.

 

[mmisch]

Yeah FPR is doing the rail pressure.

In my experience 99% of lean issues will be some sort of split hose in the PCV system, that is allowing air in that hasn't been through the MAF. Unbolt the metal bracket that bolts to the inlet manifold, and check around that area underneath it for split hoses. Give the hoses a gentle squeeze as some splits aren't always obvious. Any oily residue on the outside of a hose is often a sign that you have a split in that area.

Ive done a smoke test trough the whole system. I've got no leaks. Pcv was replaced last year.
Maf is good (hits around 170 g/s, car is stage 1 tuned)
New O2 sensor.
Injectors are not leaking.
Fuel pump/filter new.

Quite a fight to find the issue. Could have missed something

Pardon our interruption...

All of the above hoses are likely suspects, not just the hoses themselves, but where they push together, especially that T-piece

Yeah replaced all that BESIDES this one.

I could try to check it visually again, but if it was split it wouldve shown smoke coming out of there right?

 

[mty12345]

Usually a smoke test will show it, but it depends how much pressure you have. You can have a split that won't allow low pressure smoke out, but will allow it to open up and to suck air in during high vaccum conditions at idle.

 

[mmisch]

Usually a smoke test will show it, but it depends how much pressure you have. You can have a split that won't allow low pressure smoke out, but will allow it open up and to suck air in during high vaccum conditions at idle.

i'm using a air pump and a diy smoke machine, the air pump is about 0.55 psi (0.04 bar)
Quite a big stream of smoke if u ask me.
There was a bit of smoke coming trough the dipstick (and oil fil cap when removed), but i've heard that is normal?

 

[mty12345]

That is enough pressure that you will see smoke coming out of an open hole or open crack, but i very much doubt that is enough pressure to force smoke out. It's not about the amount of smoke, it's about the pressure of it.

When the engine is idling, the sucking force of the vaccum would be much more than 0.55psi. It would be around 22in-hg which is close to negative 10psi, and that could be enough to suck air in to a crack, that may be closed against the low pressure of the smoke machine pushing out. Visual inspection and checking all connections are tight is the only way to be sure imo.

 

[Alexis27]

You've got a pump and a valve in the PCV system as well. It's not just about hose leaks.

 

[mmisch]

You've got a pump and a valve in the PCV system as well. It's not just about hose leaks.

Which pump?
I know about the valve, ive replaced that one already + the hockey puck, because it was cheap enough

 

[mmisch]

That is enough pressure that you will see smoke coming out of an open hole or open crack, but i very much doubt that is enough pressure to force smoke out. It's not about the amount of smoke, it's about the pressure of it.

When the engine is idling, the sucking force of the vaccum would be much more than 0.55psi. It would be around 22in-hg which is close to negative 10psi, and that could be enough to suck air in to a crack, that may be closed against the low pressure of the smoke machine pushing out. Visual inspection and checking all connections are tight is the only way to be sure imo.

Yeah my vac sits about 25in-hg (which measurement that even is)
I did inspect most of the hoses like the boost hoses from n75, the TIP, and some other boost hoses. Will check the hoses around that metal bracket too (again)

 

[mmisch]

Got the car running again, and tested fuel pressure correctly now.. i think (unfortunately) my fuel system is working correctly
Disconnecting fpr vacuum hose = 3 bar
Normal conditions around 2.8 bar

It was my mistake that I've tested it via the wrong hoses (feed/return)

i've already orded a intake manifold gasket, planning to remove the intake to inspect some other vac hoses too

 

[mty12345]

Got the car running again, and tested fuel pressure correctly now.. i think (unfortunately) my fuel system is working correctly
Disconnecting fpr vacuum hose = 3 bar
Normal conditions around 2.8 bar

It was my mistake that I've tested it via the wrong hoses (feed/return)

i've already orded a intake manifold gasket, planning to remove the intake to inspect some other vac hoses too

That sounds like a good idea, the manifold is fairly easy to remove so shouldnt be too much of a pain. If the fuel system is ok and you have no intake/pcv leaks, then maybe the O2 sensor is giving you a bad reading and the car is not really lean? Can't see what else it could be other than an injector issue.

 

[mmisch]

That sounds like a good idea, the manifold is fairly easy to remove so shouldnt be too much of a pain. If the fuel system is ok and you have no intake/pcv leaks, then maybe the O2 sensor is giving you a bad reading and the car is not really lean? Can't see what else it could be other than an injector issue.

Already replaced the o2 sensor. Even though the injectors are not leaking with KOEO could the injectors still be bad then? I did DIY clean them (with a fuel injector cleaning kit)

 

[mmisch]

Update: no vacuum leaks at all (suprised me for the age of this car)
I did however do a smoke test in my exhaust and it was leaking at the downpipe gasket. (Got a aftermarket downpipe), apparently the gasket is already dead.
I've bought a oem gasket from my vw dealer and hopefully that also fixes the lean issue.
(This can cause false o2 readings right?
At WOT my fueltrims actually shoot right down to stft -24%. I'm currently still +13% ltft)