Still battling P0174

I don't believe it can, I don't think there's a sensor for fuel pressure, but it can see commanded and actual, IIRC? Bubba has that scanner, maybe it has some bells and whistles I'm not aware of.

Once again, they give great advice, but just touching on one thing really quick.

Unless I'm mistaken, path of fuel delivery lines is Pump -> Filter (optional) -> Driver side fuel rail+regulator -> crossover to passenger fuel rail -> Passenger fuel rail+regulator -> Return to tank.

If accurate, the passenger fuel rail would be the farthest delivery point before fuel returns to the tank, and be at risk for the lowest pressure. That means, if @Fragman is spot on and you're showing the 1st stop (driver rail) is near-lean, and passenger rail is so lean it's throwing a code, a weak pump could point to that **OR** and the pump isn't working in high-speed mode/defaulting to low-speed mode (because fuel pump relay is stuck in low-flow, or resistance on the hi speed direct circuit is corroded or otherwise problematic).

There are ways to test the theory. You could bypass the resistor so the fuel pump can't enter low-speed mode, and measure resistance on the high-speed line. You could test the fuel pump relay to verify it's not stuck or sticking. I'm sure there are other ways.

 

Alrighty then...

In order to just post two pics, the coolant temp was ~187.

 

The ambient temperature is 120 degrees where you are? The intake air temp should be the outside air temp. If it reads higher than it really is, then that would mean more air going in than it thinks and it would need to add fuel, but I would not expect 25 percent. But still....

If your fuel pressure regulator on the passenger side was stuck open, it would reduce fuel pressure and that would get worse as engine revs increased.
If your fuel pressure regulator was stuck close, you'd prob get negative fuel trim at idle and it would improve as the throttle opened.
So, I'm probably leaning away from the FPR, at least for now.

Reason is this. The FPR has a spring loaded diaphragm in it. It's natural state is to be closed, essentially 'pinching' off the fuel return line and therefore increasing pressure in the rail.

Enter engine vacuum. This is caused by the pistons sucking in. When the throttle is nearly closed, that generates vacuum. As the throttle opens, the air comes in more easily and vacuum drops. The engine vacuum (via that hose on the FPR) sucks on the top of the diaphragm in the FPR, pulling it against a spring and opening it, allowing fuel to flow more freely out the return line. Which is what you want at idle, otherwise the fuel pump is pumping more fuel than the injectors are injecting, increasing fuel pressure. As RPM increases (or actually throttle opening), vacuum goes down, the diaphragm closes, pinching off that return hose and maintaining fuel pressure for the injectors delivering more fuel.

At least that's my understanding of how it works. I wrote it out to 'show my work' so if I am wrong about how it works, the helpful folks here will correct me!


Fuel pump issues tend to get worse as engine revs increase as the ability to keep up gets worse. Yours is improving slightly.

All that said, I'd still measure fuel pressure. You'll need a gauge and an adapter. Autozone may loan them, not sure

You either have unmetered air or a fuel delivery issue. It's just not possible for all injectors to go bad, so I'm thinking air is getting in there. Something that 'global', I'd expect it to get a little better as throttle opens more as suction goes down (as throttle plate is open further). That's what you are seeing.

You want to spray propane or something like that round the manifold, You're looking for the SHORT TERM fuel trim to go negative, not the long term to get better. That would confirm a leak.

That would be my next step.

 

It was hot as balls yesterday... hot as my balls. I drove her around my little town to warm her up, then sat out in the street with the engine running, hood closed, hooked the scanner up and did the readings. That temp doesn't surprise me, given what I see on my track Miata when she's heat soaked and not moving.

I'll try to get to checking the manifold for leaks this weekend. Not happening tonight.

 

Do the readings in the table in post #25 confirm or deny what I've quoted above?

 

Post 25 deos not contain readings with enough engine speed to make any determination. My post was based on your later #32 post.