My Ford Taurus lost power going down the interstate highway. I mean, it
lost a lot of power. I had trouble doing 40 mph on level ground, and 20
mph going up hill. My trip computer showed Open Loop. Computer Codes
P0171 and P0174 indicated Lean conditions on Bank 1 and Bank 2 exhausts.
I have 103,000 miles on the Oxygen Sensors, so I thought that was the
problem. I replaced the 2 up-stream sensors. That helped for a few days,
then the problem happened again. I changed the 2 down-stream Oxygen
Sensors; that did not help. On cold starts, the system goes into Closed
Loop after about 10 seconds. That means the O2 Senor heaters are
working. After a few minutes of driving it goes into Open Loop and
throws the 171 and 174 codes again. No power at all.
I searched the Internet for suggestions. What I discovered caused me
to post the following videos. As it turns out, Open Loop is not about
Oxygen Sensors. It is about Air / Fuel delivery. The Emissions Computer
maintains the system by monitoring the Oxygen levels in the exhaust.
The O2 Sensors do the measuring, the ECM does the mixing of the Air and
Fuel. So the Mass Air Flow Sensor (MAF) plays apart, as well as the fuel
pump, fuel filter, fuel injectors, air filter, air pump, MAP Sensor, and
many other sensors and parts.
I fixed my problem; it was the Fuel Filter; restricted flow rate. The
Fuel Bleeder Valve on the Fuel Injection Rail would only squirt fuel
about 6 inches when I pushed the valve. After replacing the Filter, it
squirted 6 feet.