Yep, it likely isn't your problem but it's not wasted money either.

The whole fuel pressure checking is a often a problem with DIYers trying to get away with poor quality gauges and fittings not fitting properly.

+ make sure the bit of hose at the regulator is still in good shape as well, that's a bit of unmeasured air if it is leaking.

Well, I went to use the old gauge today and the adapter that attaches to the fuel rail was leaking. The o-ring was bad. Went to replace the o-ring and the center pin fell into oblivion. So I had to get a new kit as I could not find the adapter around town.

I had replaced that small hose piece last time I was under the car during this process. It was dry rotted and upon trying to get it off, it crumbled. So I'm good there. Thanks for checking.

Haha so I had a chance to check my fuel pressures and I think I am just going to add to the confusion! I have a 35 year old snap on gauge that (I am pretty sure reads low) says 43 and 47 PSI and I have a home built digital gauge (arduino based with oled screen showing min, max, current and average) that I built a couple of years ago that says 49 and 55.5 PSI. I never did check the calibration on the digital gauge and was going to check it against my Fluke pressure adapter (also about 35 years old) but I remember that something was wacky about the switch on the Fluke and never did get around to fixing that or going any further with checking the home built gauge.

Update:

Replaced the following over the past 2 days:
- fuel filter/regulator
- 2 precat oxygen sensors
- spark plugs

Tested car on scanner in between each change.

- Old fuel filter was full of the typical black crud coming out of the inlet, so it was due to be replaced anyway. Fuel pressure after filter change has dropped to around 47 psi at idle, I assume I can attribute this to the new filter having more free flow.

- Not sure oxygen sensors were ever replaced, so cross that off the list.

- Lastly, spark plugs were pretty bad as well. Replaced them.

After spark plug change, Bank 1 and Bank 2 STFT are closer together. Not much delta.

What I am seeing on the scans is somewhat the same as last time.
STFT starts out high (15+) and creeps down low over time (-15+). When driving STFT is above 0 and very high. When idling, it goes negative. And if you sit for idle long enough it will continue to creep down negatively.

So I am still getting STFT too high/low, out of normal range.

Attachment 83716

Again, the long term trim is going to tell the story. If things changed (o2 sensors started toggling when they didn't with the old MAF) after changing the MAF then it's likely the old one was no good and I wouldn't put much faith in the $20 one. "just adds to the soup" as Clavurion would say! ;)

If your pump running fuel pressure really did drop you might have a look at the syphon jet o-ring.

P0171 P0174 Check Engine Light CEL
 

https://www.motor.com/magazine-summa...agnostic-tool/

A very good article.

Suggesting a stuck open purge valve.

If the hose is soft enough you can pinch it shut with a couple pieces of wood and a locking pliers (or pick up the hose pinching pliers made for this job)

Purge valve or dirty / sticky injector.

Have you run injector cleaner? This model engine uses math to estimate the amount of gas per intake stroke. If the fuel pressure is off that's one way for the math to be wrong but when dirty the injectors will shoot less fuel thank designed so fuel trims will have to rise to compensate.

Since there's no feedback loop for How much actually comes out except O₂ sensor and the ECU assumes the injectors are perfect out blames the intake air.

One other test for stuck open valve: I wonder if there's a path to the fuel tank from the purge line a smoke test would feed back out the fuel fill. I can't remember the exact plumbing out might only go into the charcoal canister.

The best way to check the purge valve is to pull it off.

I remember your old posts showing the gas tank and all the plumbing. I have a small evap leak I cant find and your posts in the past were helpful.

https://youtu.be/jipBVzO1P7A?si=5aosIDZUQk_z6DhR

It's for e46 so the intake air box is different but speed exactly where to find the purge valve.

A stuck open purge valve should throw an error and smoke should have found a bad hose.

https://youtu.be/HiPlKLP9E6I?si=W8AZKhpLq-b6OC8q

Testing the purge valve.