I feel like I'm running leaner than I should be if my trims are this high (see the attached graph). Any comments?

 

I don't see a scale for the STFT but both bank 1 and bank 2 are the same, they track the same, so O2s should be OK

If an engine is actually running Lean it will ping/knock, especially when accelerating, engine under a load

If STFT is at or above +15% then check fuel pressure, pre-1998 should be 35psi, 1998 and up 55psi

+10% or -10% STFT is fairly normal

"Fuel trims" are the fuel injectors "open time" as calculated by the computer
If O2 on one bank shows too much oxygen(lean) then computer increases injector open time on that bank by 2%, then 4%, then 6%, ect...............until O2 shows correct Oxygen content in exhaust, this takes less that 2 seconds

So if both banks are effected it needs to be something common to both banks
MAF sensor
Upper intake vacuum leak
Fuel pressure
Computer itself

 

Thanks RonD. The scale is on the left - it's typically pretty close to zero at idle but then during driving it's up around 25 (I've combined short & long to make it easier to read). I've checked the pressure a number of times and I'm consistently around 40psi. Injectors are clean, fuel filter is clean, I have no vacuum issues, pressure regulator is good. The engine it out of a '98 mountaineer but the stamp on the engine says it's a '96 engine.

 

40psi should be OK, I don't think Explorers/Mountaineers got Returnless until 1999, 55psi

What's the year of the computer?
Computer is whats programmed with expected fuel pressure

If it starts with F8 its a 1998, F6 will be 1996, XL2 would be 1999

Yes, 0 STFT to +25 STFT is a big swing(25%), with a vacuum leak I would expect idle to also be +10 STFT
Its possible the MAF is dirty so not increasing air weight enough at higher RPMs
And when you tested fuel pressure did you hold RPMs at say 2,500 for 30seconds or so to make sure pressure wasn't slowly dropping, i.e. your STFT goes up when cruising under a load(high fuel demand), but is OK at idle(low fuel demand), computer expects same pressure regardless of fuel demand

 

Looks like my ECM is F67F so ‘96 and 45 should be plenty. It sounds like you’re leaning in the same direction - fuel pump. I have checked everything else in the line and once went driving with a fuel pressure gauge attached and visible. I didn’t see any pressure drops but I also didn’t experience any hesitation or spikes in the trims.

I’m going to attach the gauge and go for a much longer drive. I actually have a spare fuel pump ready - just have to drop the damn tank to put it in.

 

1996 was 1st year of the V8 Explorer so not sure if they ran Returnless system

Are there 2 fuel lines hooked up to intake(fuel rail) or just 1
2 would mean 30-40psi
1 would mean 50-60psi

But 45psi on the 1 line shouldn't be +25% fuel trims, maybe +15%

 

It’s a return system so definitely 30-40, which is what I saw on the gauge. The trims are close to zero at idle and then climb as revs go up (very obvious on the graph). Every now and then (haven’t been able to isolate the cause) the engine hesitates and the trims spike. So I’m pretty sure the hesitation / trim spike is my faulty pump - it’s brand new but when I called the manufacturer they didn’t even argue or question, they just sent a new one, which tells me they must fail out of the box a lot.


The more subtle issue is that during normal driving, when there’s not obvious hesitation and I’m able to pull through the gears freely I’m still feeling like those trims that the graphs show are a bit high (25-30%). Shouldn't the trims be pretty stable through the rev range? Or is it normal for them to go up with load?

 

Yes, thats does read like falling fuel pressure at the engine end