p0171 heading south in a hurry

[americanclassic16]

Hello all,
This is my first time posting in a ranger forum, though it certainly isn't my first time reading them!
So, some backstory on my problem:
Initially, I got a p0171 (on a four cylinder) code and the performance rapidly deteriorated. A couple of days later, it idled really rough and felt like it wanted to stall. I took some fuel line and listened for a vacuum leak which I found on the lower intake manifold gasket. I swapped in some new Intake manifold gaskets and some new PCV hose since the hose was in pretty rough shape. Now it runs even worse, stalling only a couple of seconds after starting. I checked the lower intake manifold gasket again and I don't hear the vacuum leak anymore. Unplugging the MAF makes the idle smooth out, but it still dies a couple of seconds later.

Before I swapped the Intake manifold gasket, the STFT was about 43% at idle and 30% at higher RPM's. The MAF was at 6 g/s at idle as well. Now I can't really get it to run long enough to get any useful data, but in the couple seconds I have seen, Fuel trims are lower (10-20%) and the MAF is at 10 g/s. When it stalls out you can smell fuel, and the spark plugs look a little darker than they should be, but dry.

I'm really not sure where to go from here and I could really use some help!

 

[tomw]

I would check fuel pressure. Sympton is the same as if fuel pressure dropped to zero.
tom

 

[RonD]

Welcome to TRS

MAF sensor set for grams per second, g/s, can tell you alot
Under 1,000rpms, say 700-800rpm the g/s should show very close to engine displacement
i.e. 4.0l V6 would be about 4 g/s, 3.0l 3 g/s, so your 2.5l 4cyl should be 2.5-3.0 g/s

Actual rule of thumb is at 500rpm you would expect 1 g/s per Liter of displacement.

Yours is too high assuming idle RPMs at or under 1,000rpms

And that could cause a P0171 Lean code, but I would expect 0172 not 0171

Lean or Rich codes are about computers "on the fly" calculations of open time for the fuel injectors.
Computer software will be setup for 2.5 liter engine with 50-60psi fuel pressure using 14lbs injectors(for example)
So computer knows how much air will be sucked into a 2.5l engine at any give throttle position or RPM.
It can then calculate how long to open each fuel injector to get the required gasoline to make the 14.7:1 air:fuel ratio.
And that calculation is 0 STFT(short term fuel trim)
After the fuel is burned the O2 sensor gives the computer feedback for Oxygen content in exhaust, .1-.9 volts, .1 is lean, .9 is rich
If Lean then computer opens injectors a little longer, and thats STFT, +5% stft means computer is opening injectors 5% longer than calculated to get .4volts from O2 sensor.
-5% stft means computer is opening injectors 5% LESS time than calculated.
So the +/- symbol is important to include for STFT or LTFT

MAF sensor fine tunes air WEIGHT for air temp and altitude above sea level
O2 sensors provide feedback for the calculations, this feedback sets STFT because thats all the computer has to know if its calculations are correct

Since your MAF seems to be reporting MORE air than there should be, I would expect Rich Code(P0172), not a Lean code
And the fact you are smelling gasoline smell means engine is running Rich, plus browner spark plug tips

Unplugging the MAF helped, so I would retest for g/s and if its still high then replace MAF
When you unplugged the MAF the Check Engine Light(CEL) should have come on fairly quick, MAF is a main sensor.
If it didn't then computer may have the problem, but longshot.

With MAF unplugged computer use tables in memory to do the air flow calculations, BUT.........they are biased using LTFT(long term fuel trims).
As an engine system gets older there will be smaller vacuum leaks, lower fuel pressure, more oil getting into cylinders, ect.............just normal wear and tear.
LTFT is there to compensate for that.
When you cold start the engine the O2 sensors can not be used, they need to be above 650degF to work, so computer must run engine from the air:fuel mix in memory, same as when you unplugged the MAF.
So even though MAF is unplugged your high or low LTFT would still be effecting engine operation.
You can't "clear" LTFT, wouldn't be much good if you could, lol, it has to return to "normal" after you repair the problem effecting STFT
LTFT is an average of STFT and a few other systems.

If engine has over 150k miles and you don't know when upstream O2 sensor was last changed then change it.
Also change fuel filter, every 5 years, so if you don't know when it was last changed then spend the $10 and change it so you know when it was last changed,

 

[americanclassic16]

Thanks RonD for the info!
I checked the MAF just now and it was fluctuating between 9-12 g/s as the rpm's fluctuated between 700-1600. It still only ran for a few seconds before it stalled, but it is definitely too high. I'll check to see if a new one works and report back.

 

[RonD]

You can also pick one up from wrecking yard, just clean it first, these don't fail that often

1997 to 2001 3.0l or 2.3l or 2.5l MAF are the same

 

[americanclassic16]

Fuel pressure at 4psi.........
Guess I'll be changing the fuel pump.

 

[americanclassic16]

Well, it turns out the problem was indeed a fuel delivery issue. It appeared to be a clamp connecting the fuel pump to the pressure regulator in the sending unit being too loose, which explained why it wasn't holding any pressure. As soon as I took off the bracket holding the fuel pump to the sending unit, the hose slid right off along with the clamp (it appears Airtex lacks any quality control). I already had a new pump so I threw it in anyway with the new clamps and the truck fired right up and idled like a dream. Already a couple of drive cycles and no codes! Hope this helps someone in the future, and also thanks to all the great folks on this forum for all their insights!

-americanclassic16

 

[RonD]

Good work

Quality Control requires Labor, and labor is expensive, parts are cheap

Years ago the opposite was true, parts were expensive and labor was cheap


Motorcraft fuel pumps are tested, Ford still requires quality control, and they cost $400-$600(on sale), and have a 90 day warranty.

3rd party don't test and they are $90-$180, and have a 1 year to 2 year warranty.