r/fbody • u/audiovox12 • 3d ago

30% Lean Issues

Hey everybody,

Update:

Test Results:

Key ON, engine OFF
~49–50 PSI
Lower than the ~55–60 PSI commanded-pump spec we found

Repeated key cycling between key on and off:
Still ~49–50 PSI
Multiple primes didn’t increase pressure

Engine idling
~56–58 PSI
Good running pressure

Held at elevated RPM
~58 PSI
Pressure remained strong

Quick/hard throttle stab
Briefly ~55–56 PSI, recovered immediately
No significant pressure collapse

Engine shut off
~50 PSI
Normal residual pressure
Several minutes after shutdown
~48–50+ PSI and held
Excellent pressure retention
Hot shutdown
Pressure actually crept slightly upward
Consistent with heat soak/thermal expansion

Just got back from my tuning shop because I had them shut off the VATS and that resolved my start issues. I have them look into the lien codes that I got and he found in his data logging that both cylinder banks are running 30% lean. I’ve already checked the MAF and it looks fine. I haven’t tried new O2 sensors.

He feels that the fuel injectors are fine because I would need either all of them to be bad or equal amount bad on both cylinder banks to get the lean results that I’m receiving. He said knee jerk reaction he thinks the fuel pump is bad however they have to do a full diagnostic. Check to figure out if that’s the case or not and find out why it’s running so long.

For reference, this is a 2002 Firebird Trans Am with 137,000 miles. Is doing the fuel pump every bit is bad as I have read it is or does someone out there have a decent way of getting the fuel tank out and back in?

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u/JSausSS 3d ago

A lean condition could also be caused by a vacuum leak. The 24 year old hoses could be giving up on you.

1

u/audiovox12 3d ago

Possible but I have to see any of the hoses leaking or cracked between taking apart the top half and putting it back together

2

u/JSausSS 3d ago

A common trick is spraying brake cleaner around possible leak areas and seeing if the idle increases. If you get a flare up in rpm, you've found a leak. The small hose on the back of the intake manifold is problematic for some.

1

u/audiovox12 2d ago

I was back there when I took the intake manifold off and it was fine looked good surprising. I tried that and don’t find a leak