4.0L SOHC Explorer w/OBD-II Codes P0171 and P0174

[Jon Wilder]

Hi all. My '99 Explorer has the Check Engine light on and codes stored are -

P0171 - System Adaptive Fuel Too Lean (Bank 1)
P0174 - System Adaptive Fuel Too Lean (Bank 2)

It had these codes when I acquired the vehicle (was my sister's old vehicle). My sister had the intake gaskets replaced about 40K miles ago when she owned it. I noticed that the engine was running a bit cold, took a long time to warm up and didn't hold constant temperature. I replaced the thermostat to rule out the possibility of the engine temperature causing false codes (turns out it still had the factory thermostat). Now it warms up quick, runs about where it should on the gauge and holds constant temperature.

After doing this, I disconnected the battery for a minute to reset the computer.

After 50 miles of driving, the Check Engine light is back on. Repeated code scan and it's the same two codes.

I just cleaned the MAF sensor with CRC Mass Air Flow Sensor cleaner tonight. Also, I filled the tank and put STP Fuel Injector Cleaner in the tank. Last tank averaged 14.6mpg. Disconnected the battery to reset the computer as well. Still have yet to put 50 miles worth of driving on it.

The vehicle drives fine. Quick off the line, has plenty of power and no surging or other driving abnormalities.

If cleaning the MAF does not fix this issue, where should I go next?

 

[2000StreetRod]

upper & lower gaskets?

Were the upper and lower intake gaskets replaced or just the upper gaskets? My lower gaskets were also leaking. See Lean condition procedure

 

[Jon Wilder]

Were the upper and lower intake gaskets replaced or just the upper gaskets? My lower gaskets were also leaking. See Lean condition procedure

Yes, both the upper and lower intake gaskets were replaced.

After 40 miles of driving today, the light is back on. Have not yet had a chance to pull the codes to make sure I'm still seeing the same codes. I don't yet have a code scanner so have to take it to a local Pep Boys to do it (I am always present at the time they pull the codes).

Could a faulty gas cap possibly cause a large enough vacuum leak to set those codes?

 

[96eb96]

PCV Elbow, Vac lines?

 

[2000StreetRod]

fuel cap monitor

. . . Could a faulty gas cap possibly cause a large enough vacuum leak to set those codes?

I think the 1999 and later models monitor the fuel tank pressure and illuminate the "CHECK FUEL CAP" warning when abnormal.

 

[imp]

If the vehicle runs well, as you say, performs acceptably, it is unlikely that it will be harmed by continuing driving it as-is. (may burn out the CEL, or drive the observer to distraction), the codes displayed are likely aimed at emissions-concerns more than anything else.

This is simply my opinion, be aware! The lean condition is likely not harmful, as the PCM will make mixture adjustments as appropriate. If outside air is being drawn into the engine undetected by the system, that air is not part of the "controlled combustion" aimed at maintaining exhaust emissions at or below dictated levels.

imp

 

[Jon Wilder]

I decided to check the PCV valve...it's a Motorcraft and I'm assuming it's probably the factory PCV valve.

I removed it and shook it and there is a rattle present when shaken. I also blew air through it, and noticed that air seems to flow both ways through it. Is that supposed to do that? I thought the PCV valve was a one way type valve.

In any event I do plan to replace it since there's no evidence that it's ever been replaced.

 

[Jon Wilder]

OK nix the PCV and the fuel cap. Just had the codes re-scanned. This is the first time I've had a chance to scan the codes since I cleaned the MAF sensor.

After cleaning the MAF sensor and running the fuel injector cleaner through the tank, it turns out that the bank 2 lean code disappeared.

So now I'm only dealing with a bank 1 lean code. Plan to pull a spark plug on that side and take a look see to determine if it's really running lean or is the O2 sensor faulty.

 

[Jon Wilder]

OK...haven't had a chance to work on the Explorer for a bit until recently.

Due to my last code scan revealing that the bank 2 code disappeared, I decided to change the passenger side upstream O2 sensor. After 40 miles of driving, the CEL came back on. This time I decided to purchase a pocket scanner so that I can run the codes myself.

Wouldn't you know it...I'm back to having both codes again. Same two codes...P0171 and P0174.

Now...even though those codes are present, my fuel mileage is within 5% of what it should be. There are also no noticeable drivability issues present. This leads me to think that my issue is fuel related. I don't have a fuel pressure gauge to check it, but my thesis is that due to reduced fuel flow, the computer has to run at the limits of its fuel trims in order to supplement the engine with the correct amount of fuel. Since there are no noticeable drivability issues, it is probably running "borderline" lean. This coupled with the maxed out fuel trims is what is triggering the CEL light, yet there's no noticeable decrease in fuel mileage.

Am I on the right track here?

 

[koda2000]

FYI: You shouldn't be able to blow air both ways through a PCV valve. Replace it. Shaking a PCV and hearing a rattle does not mean it's good.

 

[2000StreetRod]

perform the steps

. . . Wouldn't you know it...I'm back to having both codes again. Same two codes...P0171 and P0174.

Now...even though those codes are present, my fuel mileage is within 5% of what it should be. There are also no noticeable drivability issues present. This leads me to think that my issue is fuel related. I don't have a fuel pressure gauge to check it, but my thesis is that due to reduced fuel flow, the computer has to run at the limits of its fuel trims in order to supplement the engine with the correct amount of fuel. Since there are no noticeable drivability issues, it is probably running "borderline" lean. This coupled with the maxed out fuel trims is what is triggering the CEL light, yet there's no noticeable decrease in fuel mileage.

Am I on the right track here?

I suggest that you perform the steps in the lean condition procedure.

 

[Jon Wilder]

Can someone link me to the lean condition procedure?

I replaced the fuel filter last weekend and took a 400 mile round trip to see family. It did great on the way there, then on the way back the MIL came back on.

At first, only code P0171 was present. Then just a couple of days after that, my scanner showed code P0174 added to the mix but in "pending" status. Then today it was there permanently.

So while I have both codes present, they are not appearing at the same time.

Tomorrow I will hopefully be able to get my hands on a fuel pressure gauge just to see where my fuel pressure is at.

 

[2000StreetRod]

see post #2

Can someone link me to the lean condition procedure? . . .

The link is in post #2 of this thread.