I'm searching for the cause of a PO171 code, and find my downstream 02 sensor (just replaced) is reading in the .075-.1v range with the engine warmed up. According to a couple of articles on the web, the downstream reading should mimic the upstream sensor readings.

Upstream sensor (also new) makes the normal amount of cuts, and the fuel trims are both + or - about 5. Freeze-frame data tells me the PO171 code seems to set at higher RPMs, like winding up in the lower gears to get on the freeway. With the engine idling, rapidly revving the engine to above 2500 moves STFT to 25.

Any ideas?

 

O2 sensor voltage range is .10-.90volts, O2 sensors only read Oxygen levels not fuel levels
.10 means high oxygen levels, which is Lean for fuel
.90 means low oxygen levels, Rich with fuel

Upstream O2 sensor(s) get raw exhaust, so if fuel mix is correct they will be .30-.70v, sweet spot is .45v

Downstream O2 sensor is after the Catalytic converter so it "sees" cleaned exhaust, it should always show lower voltage, higher oxygen levels, than upstream O2 sensor

So your reading looks OK, under .2v

Lean/Rich codes come from fuel injector "dwell time", open time, "pulse width".

Computer gets the MAF(mass air flow) sensor data for the "weight" of the air coming into the engine.

Computer then calculates the 14.7:1/air:fuel ratio based on the weight of the air(this is a weight ratio, 14.7lbs of air to 1lb of fuel)

Computer then opens the fuel injectors(based on expected fuel pressure) for a length of time that will allow the calculated amount of fuel to flow into the intake to achieve the 14.7:1 ratio.

Computer then reads upstream O2 sensor data to see if the calculation is correct(downstream O2 is used to see if Cat is working, not fuel trims)

If O2 sensors shows voltage under .45v "lean" then computer opens fuel injectors longer.
If O2 sensor shows above .45v "rich" then computer reduces the open time for fuel injectors.

Engine never actually runs lean or rich.

Fuel Trims = open time for fuel injectors
0 fuel trim is the computers calculation based on MAF sensor, air temp, throttle position, and fuel pressure(this is a fixed number, computer has no way to "know" the fuel pressure)

+1 fuel trim means computer is opening fuel injectors longer that calculated based on O2 sensor voltage
-1 fuel trim means computer is reducing the open time for fuel injectors based on O2 sensor voltage.

If fuel trims get over +20 for any length of time the computer will set a Lean code, like P0171

If fuel trims get under -20 for any length of time then Rich code is set, P0172.

Again, the engine is never running Lean or Rich, computer is adding or reducing fuel to the mix.

The code is to notify the driver that there is a problem with the calculations.


Air leaks, like a vacuum leak, means all the air going into the engine is not passing thru the MAF sensor, so computer is basing its calculations on bad data, so it needs to add more fuel because of the "extra" air coming in, and you get a Lean code.

Low fuel pressure is the same, computer expects 35psi at the fuel injectors on a 1997 engine, if fuel pressure is lower then less fuel flows in when injector is open, so Lean code is set when computer has to open injectors longer to get correct amount of fuel.


Since yours occurs at higher RPM/load I would first change the fuel filter, it could be causing fuel pressure to drop when fuel demand is high.


To test for air leaks, after engine is warmed up and idling, unplug the IAC(idle air control) Valve, idle should drop down to 500rpm, barely running, or engine may even stall, either is good it means no vacuum leak.
If idle stays high then you do have a leak.

MAF sensor uses a heated wire to calculate air flow, over the years it can get a coating on the wire(dirty air) which acts like an insulation, so air flow data is incorrect, MAF is easy to clean, and should be cleaned every 5 years or so.

 

As before, Ron, thanks for your prompt reply.

That said, I should have mentioned (I was pressed for time) what I've done so far.

1. I changed out the fuel filter. My fuel pressure was 30 psi at idle before, 40 with the vacuum tube pulled off the regulator at the rail. After the swap, I picked up about 2 psi more at each reading. The filter was deinitely clogged; could barely blow thru it.

2. Changed the PCV valve. It was definitely bad. It barely rattled.

3. Checked out the brake hose for leaks. None.

4. Replaced several lengths of hose along the way to the purge valve. Definitely soft; if they weren't leaking the certainly were going to be soon.

5. Ran a vacuum test. Engine had 22' at idle, steady needle.

6. Cleaned the throttle body and replaced the hose that runs underneath the upper intake. Checked all bolts for tightness.

7. Did the spray test with carb cleaner after the above work. No detectable changes at any vacuum fitting.

8. Cleaned the MAF sensor with CRC sensor cleaner.

9. Dumped a bottle of mechanic-in-a-can fuel injector cleaner in the tank last week.

I will do the IAC test after lunch and see where we go from there. Hope it's not a fuel pump, but it's beginning to sound like it.

ETA: forgot to mention one item in the fuel pressure test. I noted from several other posts here that the Ranger fuel pump has an internal check valve that's supposed to hold pressure for a long time. I left the fuel gauge hooked up and noted that the fuel pressure drops to nearly zero ( ~2 psi) within two minutes. However, there are no hard-starting problems as other mentioned in those posts. Significant?

 

IAC test: No difference at all in idle. So, the hunt for the vacuum leak continues? I can't figure out where it might be. Guess I'll have to order intake gaskets next.

 

So engine is idling higher than it should.

With engine warmed up and IAC valve unplugged I would disconnect vacuum hoses and plug that port with your finger, see if you can track down a leak that way.

PCV system expects a semi-sealed engine, this is why an engine can rev up or stall if you remove the oil filler cap while engine is idling, straight vacuum leak with cap off.

It shouldn't do that because PCV Valve is suppose to be almost closed at idle, high vacuum pulls the valve closed, as vacuum drops a spring pushes valve open to suck oil vapors/blow-by out of engine.

 

From watching my torque app (take it for what it is at $5) my upstream tends to vary (generally in a spiked wave) while my downstream remains pretty constant.

My truck also only drops to about 680rpm when I disconnect my IAC, though a few days ago when I tested and before I fixed a bad plug wire it would drop to 500 and almost turn off...