I have a 2002 F150 with a 4.2 V6. I had cleared the check engine light previously and then drove for about 5 days and the light came back on. the codes I get are P0171 System too Lean (Bank 1) and P0174 System too Lean (Bank 2). The truck doesnt idle rough or run with a miss, although the fuel mileage seems to suffer a bit. My question is what is most likely causing this to trip? I could understand it happening after 1 or 2 restarts, but after several days of turning off and on, I am lost. Any help or ideas is greatly appreciated.
These codes would give you better than expected fuel milage, not worse than expected. Since the code is that the mixture is lean, this means that there is less fuel being delivered than the sensors want to see. If too much fuel was being delivered, then the code would notate a rich condition, and mileage would suffer as a result.
Lean is not a good thing, by the way.
My first guess would be an O2 Sensor on the affected bank, BUT you have a code generated by the O2 sensor in both banks. Odds do not favor both sensors failing at the same time, but therer is certainly a possibility of that happening.
Are you sure that the repeat is both of these codes? You said the light comes back on, but I'm asking if the SAME codes are coming up, or just one of them. Or, a different one.
I may have given a response that sends you to the store to buy some O2 Sensors. Sorry. The sensors can play a role because they falsely detect a lean condition, and the computer is supposed to send in more gas to make the mixture rich -- which would harm fuel consumption rates, sorry, I got that wrong too -- and the sensor would send another report of a lean condition, and the computer would send in more gas, and the sensor would send another report of lean, and so on. Eventually, the computer will have made the mixture as rich as it can, and the sensor is still reporting a lean condition, so the code comes up.
Now, this would certainly lead to a drop in fuel mileage, as you have noted. Having said that, I still have trouble with both O2 Sensors having the same failure at the same time. Either one reporting a lean condition would cause me to replace the O2 Sensor (there are two of them in your V6), but having two with the same trouble at the same time makes me look upstream of the sensor to find something that feeds the fuel. There is a test you can do with a volt meter that will tell you if the O2 sensors are bad.
The first thing I would do is pull codes again to verify that the light is actually on for the same reason as you had before.
By the way, Dave's suggestion is a good one. A vac leak in the right place may not cause the engine to stall, but could upset the fuel mixture by making it too lean.
Every hose has two ends, and either end can leak. Many hoses go to t-fittings, and this creates at least 4 more ends to check. You have to trace all of the hoses and check them for cracks -- typically at the ends, but a hose can lay on a hot part and get burned in the middle too. There is no quick and easy way, you just have to look at everything. Having said that, if the motor runs okay, then your vac leak is probably coming from a small hose as opposed to a large one. (If the leak is too big, then the engine will not run well.)
Dave, in another thread on this discussion, suggests a vacuum leak. I like his reasoning. There are diagnostic tests that can be done on an O2 Sensor that only require you to use a volt meter. I've not done the test, so I do not recall how to do it. Before you start going after the vac hoses, maybe it would be prudent to see if the sensors are actually working right.
Leaking Intake Manifold Other Vacuum leak Contaminated MAF Bad O2 Sensor
My guess - since both banks are showing up as too lean - a contaminated MAF sensor. Second guess - you have been using a lot of cheap gas and your injectors are partially clogged.
If it was by truck, I'd try cleaning the MAF sensor (See
Intake Gasket? Yes, I suppose that could be the source of the leak.
The O2 Sensors see the lean condition and set about to alter the mixture by adding more gas -- this happens by extending the injector timing. The injectors open X (where X is a distance) every time they fire. The variable is the time period that they remain open. If open for 500 milliseconds, then you get a certain amount of gas, if open for 750 milliseconds then you get more gas at the same throttle setting, and this makes the mixture richer. (I have no clue what the actual timing is, I selected these numbers only to illustrate the idea.) If the O2 Sensor tells the computer that the mixture is lean, then the injector timing is lengthened. If rich, then the timing is shortened. If the sensor(s) continue to report a condition that the injector timing can not correct, you will eventually get a code. Keep in mind that the O2 Sensor doesn't directly control the injector timing, it only reports the fuel content of the exhaust stream immediately as practical after the exhaust valves. The combustion process requires there be a trace amount of unburnt fuel relative the amount of air. The sensors merely report to the computer what the content is, and the computer looks at other inputs and makes changes to bring the O2 sensor reading back to what is expected.
If you had a problem with only one of the sensors, then I would not hesitate to think the sensor itself was the fault. But, yoiu are telling us that both sensors are reporting a fault, so I would be looking to the fuel delivery system -- low fuel pressure, that sort of thing -- or too much air -- leaky hose or gasket.
On the fuel delivery side, there is a vacuum operated fuel pressure regulator. The fuel pump delivers (for example) 60psi constantly. At idle, you might only need 30psi to get the fuel needed, but at 70mph, you would need every bit of that 60psi. If the pressure was low because the regulator was letting fuel return to the tank that is actually needed at the injectors, then you would have a lean condition. If you look at the injector rail(s), you should be able to locate the fuel lines (yes, there are two.) One line is bringing fuel to the rail, the other line is after the pressure regulator and is sending fuel back to the tank so it can be used later.Pressure regulators seldom fail where they remain closed which results in high pressure at idle, they generally fail in a way that lets fuel go past them when the pressure should be held as high as possible.
If you drove around town for several days and there was no code reported, then went out onto the highway for a high speed trip to the next city and the code popped up, then I think I'd dial in on the pressure regulator. Of course, the fuel pump itself can deliver low pressure too ...
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