Sanity Check for P0171

Riot_Polizei

Comp Engineering Student
V
2002 Mazda Protege5
Yo PCB, don't digress yet!

After driving around today, believe it or not, P0171 came back on (after I drove it hard for a somewhat extended period of time--was in a cruise). Only thing is, my STFT stayed where it should the whole time and never jumped above 10% or so. It appeared that my LTFT was bouncing from 2.3 to 25%, so clearly some issue is still afoot. When I was done driving hard and gently drove the thing home, no codes came on, and I had nominal levels.

Looks like my fueling system is indeed part of the problem!
 

pcb

The Diagram Dude
V
2002 mazda protege 5
I got some readings regarding my fuel trim, and it's not looking good. I only collected data while driving around, and not stationary.

STFT was bouncing all around, but only rose above 8% at very low engine speeds (around 1.5krpms and lower, I believe). LTFT stayed at 25% (overwhelmingly lean trend). STFT had "normal" readings above 1.5-2kRPM readings, which ultimately points at a vacuum leak of some kind, and not necessarily a fueling issue.

At least, it's been narrowed down, but damn I msy habe to rip open my IM again...

(Note: will analyze this at stamdstill later)
I was wondering what kind of readings would be expected if it was a fueling issue ??
 

Riot_Polizei

Comp Engineering Student
V
2002 Mazda Protege5
A fueling issue from P0171 is a little less cut-and-dry. If your STFT is overall lean, and your STFT doesn't drop with the throttle being held at 1.5kRPM and 2kRPM (while LTFT remains high), then it could be one of several fueling components. You're then looking at your FPR, injectors, and fuel sending unit.

Even then, it could even be a misfire that didn't trip the misfire code yet, or spark plugs on their way out.

Vacuum leaks are somewhat simpler to diagnose given the noise they make and their impact on STFT and LTFT.
 

pcb

The Diagram Dude
V
2002 mazda protege 5
OK thanks.

Now all I have to do is remember that.

Maybe I'll cut and paste it to my thread. lol
 

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