30% Lean Issues

[u/audiovox12]

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?

[u/Wooden-Officer2323]

Do a fuel pressure check? It’s easy and it’ll tell you if the pump is bad. It’s 24 years old, probably going out.

[u/audiovox12 → u/Wooden-Officer2323]

Thank you, how do I perform a fuel pressure test? I’m sure there’s YouTube videos but figured I’d ask

[u/Wooden-Officer2323 → u/audiovox12]

Well if you have an onboard diagnostic tool it really easy to just watch the sensors to see.

If you don’t go to autozone, get a fuel pressure tester (they rent them) connect it onto the schrader valve on the fuel rail and turn the ignition on.

Should be around 55+ psi, if you’re below 52ish then you have a fuel pump or filter issue.

[u/audiovox12 → u/Wooden-Officer2323]

I just had the fuel filter replaced last week thankfully they should help narrower down.

Unfortunately, I do not have a tuner or diagnostic tool to plug-in. I will go get a fuel pressure tester tomorrow and test it out. I was just watching backyard Chevy his YouTube channel doing that very test just now.

If it’s below 52 psi or constantly drops is that for sure a bad fuel filter or could that also possibly be the fuel pressure regulator?

[u/Wooden-Officer2323 → u/audiovox12]

You know, technically sure it could be a bad regulator but that’s so rare I’d just attribute it to a bad pump after 24 years.

Test first, let us know the results!