I have a 1999 ranger 4.0L (OHV)
I bought the truck back in August and it ran great, and relatively speaking, got fairly decent fuel mileage (~20-21 highway @ 65mph)

A couple months later, I noticed the mileage drop significantly (~15 mpg), and eventually a check engine light- scanned it and it was what I suspected, the dreaded P-0171, P-0174 lean condition on both banks (as well as P-0455 EVAP major leak which I attributed to the same condition)

I pressure tested/soap tested the intake manifold/vacuum system and found a pretty good leak out of my upper intake manifold. I tightened down the upper mani bolts a bit till the leak subsided, knowing this was only temporary to buy me time while I started putting together an order for new gaskets.

In the meantime, the engine started to develop a coolant leak from the back of the engine.. after troubleshooting it became obvious it was the lower intake gasket that had gone bad as well.

Fast forward to last week, I replaced the lower intake gasket, valve cover gaskets, fuel rail gaskets, injector o-rings, and upper plenum gasket, and while I was down there, put new plugs and wires. I torqued everything down to spec and everything seemed fine and back to normal, check engine light was gone.

I did a live data scan to check fuel trims etc, and everything seemed okay, however, the fuel mileage had not improved.

Then, yesterday out of nowhere, with the live data scan running, I noticed my long term fuel trims rising from a stable acceptable range, higher and eventually settling around 16-18%- At idle (all within a short time, I happened to catch it as it happened) As soon as I applied throttle, they would come back down, let off the gas, and back to the 16-18%
soon after that, I got a check engine light- P0455. I cleared it and it has yet to return for now, but the long term fuel trims are still 18.

here is a livedata snapshot I got yesterday:

Numbers of DTCs 0

Fuel system 1 status CL

Fuel System 2 status --

Calculated Load Value 30.6 %

Engine Coolant Temperature 163 °F

Short Term Fuel Trim -Bank 1 -1.6 %

Long Term Fuel Trim - Bank 1 18.0 %

Short Term Fuel Trim -Bank 2 0.8 %

Long Term Fuel Trim - Bank 2 18.0 %

Engine RPM 911 rpm

Vehicle Speed Sensor 0 mph

Ignition Timing Advanece for #1 22.0 °
Cylinder

Intake Air Temperature 68 °F

Air Flow Rate from Mass Air Flow 0.01 lb/s
Sensor

Absolute Throttle Position 17.6 %

Location of Oxygen Sensors B1S12--B2S1---

Oxygen Sensor Output Voltage Bank 0.625 V
1-Sensor 1

Short Term Fuel Trim Bank -0.8 %
1-Sensor 1

Oxygen Sensor Output Voltage Bank 0.050 V
1-Sensor 2

Short Term Fuel Trim Bank 99.2 %
1-Sensor 2

Oxygen Sensor Output Voltage Bank 0.800 V
2-Sensor 1

Short Term Fuel Trim Bank -1.6 %
2-Sensor 1

OBD requirements to which vehicle OBDII
is designed





My questions are-

1. With the short term fuel trims really not getting out of acceptable range (they tend to stay around +/- 4%) How can my long term trims be so far off? I watched the long term numbers go from acceptable range to the +18 last night pretty much as it happened, meanwhile the short term numbers never really got out of whack..

2. It appears my Bank 1 sensor 2 (downstream O2 sensor) is averaging at around or less than 1mV. I did see it bounce back and forth within range a few times (between 0.1-0.8) but its short term trim has not budged from 99.2%. Does the PCM also look at the downstream sensor values for fuel trim adjustments (which by the 99.2% trim adjustment mean it's dumping lots of fuel to try to mitigate a lean condition and would explain my mileage) or does the downstream sensor only serve to gauge efficiency of the cat?

3. Is the random P0455 EVAP code related to any of this? and to clarify, I did verify to make sure my gas cap had a tight seal (new gas cap, closed tight @ 3 clicks). I also checked the fuel filler neck, it looked good and is fairly new.

Now when I pressure/soap tested, I found minor leaks on the fuel vapor hose that connects the throttle body to the EVAP purge valve ( the valve on the driver side fender, behind the ABS pump, I believe that's the purge valve?) . I fixed the leaks anyway even though they were very small. Since the purge valve is normally closed, I could not soap test anything downstream of it where the charcoal canister is, which brings me to my last question,

4. Is there a way to jumper the purge solenoid to force it open so I can pressure/soap test the components downstream of it?. Otherwise I have No Idea why I am getting a P0455 code..

Thank you in advance,
Dan

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1999 Ford Ranger XLT- bone stock... for now..
2004 vw .:R32

Couple of ideas one is you might need to change the 1 O2 sensor even though it is for cat eff. Secondly you are now running winter blend gas which will make your mpg's drop. As far as the evap leak you are probably going to have to smoke test the system.

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1999 Ranger 4.0 and extreme Snap-on tool fetish

Thank you for the reply. I hadn't thought winter blend would affect mileage this much but that's a very plausible reason.
As far as the smoke test, I will give it a shot (Ill have to fab up a working smoke machine first, the pressure test and leak detecting soap is a good alternative as long as you are able to see and get soap on every vacuum component
But the problem remains that with the purge valve closed, I cannot check the components downstream of the valve.
Does the purge valve require a vacuum source and 12V signal on the solenoid to open? Im not familiar with its operation but did see a vacuum line connected to it as well as electrical harness..

__________________
1999 Ford Ranger XLT- bone stock... for now..
2004 vw .:R32

The smoke test will hook up to the evap line with the green cap and that is suppose to allow you to smoke the system. If you ever get bored and are down in the Charlotte area I will smoke test it for you.

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1999 Ranger 4.0 and extreme Snap-on tool fetish

I would take you up on it if I were a bit closer!

anyway today i noticed the EVAP monitor test thing shows as incomplete. Im starting to think the canister vent valve is not closing when it should, possibly creating the lean condition I'm seeing through the fuel trims, and the evap code and failure to complete the evap test..

__________________
1999 Ford Ranger XLT- bone stock... for now..
2004 vw .:R32