I bought an '03 Expedition 5.4 AWD with 188K miles in mid November after my wife's '00 Explorer was totaled (no injuries and the Exploder was driven home while the other two vehicles were flat bedded away!).

My OBDII reader shows that I'm running 172°-178° after 5-7 miles at 55 mph. The highest temperature I've seen was 185° during a climb from 2000 ft to 5500 ft at 55-60 mph. I'm going out on a limb here and saying my thermostat is bad or missing. It's too cold right now to work on it with no garage.

The last few days haven't gotten above freezing, yesterday morning it was 9° when I got home.

I suspect that I'm running in "open loop" due to the low operating temperature. The computer just keeps on pushing fuel to compensate, then translates it to a PO171 code. I'm sure I'll get better mileage in the Spring/Summer when I replace the thermostat.

Do you experts agree or disagree?

Dan

 

You'll need to dig deeper into the OBDII data before reaching that end conclusion. Probably not, would be my estimate. You need to also look at the bank 2 fuel trims to see if they're also skewed or if they're normal. They might be just on the edge of failure so there isn't yet a P0174 fault code.

A PO might have installed a lower temp thermostat. Until you open it up and look, you'll never know for certain. The correct thermostat is in the 190-195 range.

My advice is to always fix the obvious first, then retest. Some of these elements have a cascade effect through the system and it's necessary to get to the root cause without getting lost on side trails.

 

Why wait? Put on an extra sweater and go out and change the t-stat, it's not that hard. Maybe wait until the spring to do things like a coolant flush.

 

They're heated O2 sensors, if they're heating up properly it only takes about 3 minutes for them to get to their operating temp and send the system into "closed loop". I would be looking for a vacuum leak on the passenger side of the motor. The PCV area is very common area. Take a can of carb cleaner and spray it around that area and the front of the throttle body where the other vacuum feed source is. I think you'll find a vacuum hose cracked or disconnected. With the age of the Expy it's very common to start finding bad vacuum tubing.
The P0171 code is a lean burn condition (vacuum leak), not a over rich condition.


-Mike



Inspect this vacuum elbow at the front of the throttle body.....




And check the PCV tubing closely .....

 

I'm by no means an expert,but I don't think you are running open loop. .(What does your scan tool say?)

I would suspect the fuel system goes closed loop as soon as those "heated" O2 sensors come on line.

Additionally, I would suspect the computer is pushing fuel to try and compensate for a lean condition (P0171). What do the fuel trims indicate?

 

I've read several places including explorerforum that Fords don't go into to closed loop until the operating temperature reaches 180°. This is why I was thinking the thermostat might be the root in this case.

Driving home this morning at 4:00 AM I was watching the O2 sensors, STFT and LTFT. I don't know what readings are normal. When I got home and parked after a 26 mile drive, the last 6 miles at 55 mph (coolant 172°, outside temp 12°) at idle the readings were as follows:

O2
Bank 1 Sensor 1: 0.66 V
Bank 1 Sensor 2: 0.63 V
Bank 2 Sensor 1: 0.79 V
Bank 2 Sensor 2: 0.66 V

STFT Bank 1: 3.12%
STFT Bank 2: -.078%
LTFT Bank 1: 28.91%
LTFT Bank 2: 28.91%

When I increased the engine speed (1000-1200 rpm) the LTFT dropped to 10-14%.

Dan

 

Waste of spray unless done properly (observing either STFTs or upstream O2 sensor voltages).

Actually, it's an over-rich condition. The PCM has reached its limit for ENRICHMENT but the sensors are still indicating "lean".

The problem is in BOTH banks, one just tripped sooner than the other as I previously suggested.

Go inspect the PCV system from end to end, it accounts for over 90% of these problems. Usually it's a failed elbow 'round back or one of the rubber hoses near the PCV valve.

Normally both the long and short term trims should be around zero. It's when their sum (Bank 1 or 2) exceeds 35% that you'll get a DTC for the fueling system.