Lean On Me, Or Not

I had this '97 Ford F150 in the shop with a P0174 code stored. The customer had the problem "diagnosed" by one of the parts stores as a bad pcv valve and hose. I imagine that was an employee armed with a code reader pulling a lean code and printing a list of suggested possible causes. A pcv vacuum hose leaking is a very common problem on a Ford but just a wild guess at this point in time as to the cause of the P0174. There is a lot of difference between diagnosing a problem and guessing what it might be..... but I digress. This engine has a fuel monitor O2 sensor on each engine bank. Passenger side is bank 1 (with cylinder 1,2,3,4) and driver side is bank 2 (cyls 5,6,7,8). P0174 means the trucks computer has determined bank 2 to be running a lean fuel mixture. Too much air or too little fuel, in other words. If I had a lean code for both banks, P0171 and P0174 then I might be thinking vacuum leak too. A pcv hose pulling extra air will usually put both banks lean at idle and it will be stored in freeze frame as a problem at idle or low rpm. I decided to not immediately look at freeze frame but to first look at the live data from both upstream O2 sensors. Now, most of the time a lean, or even a rich fuel condition code sets because the oxygen sensor is working fine. I mean the job of the sensor is to report the amount of oxygen in the fuel mixture. Don't kill the messenger, right? I even saw more than one tech site that said a lean fuel code won't be a bad sensor. Well..... never say never.

I captured the data from both upstream sensors. On the top half of the first picture is the sensor that did not set a lean code. The lower half of the pic is the O2 that did set a lean code. This is a live datastream with engine running at about 2000 rpm. An oxygen sensor range is from 1 volt to 0 volt. 1000mv to 0mv. Anything below 500mv is lean, anything above is rich. A good O2 sensor with a good fuel control should switch well between the two conditions. Now since I didn't find an obvious problem with the truck, such as a bad pcv hose and since I see no particular problem with the way it is running or with anything that would affect only bank 2 I think it would be worth replacing the bank 2 sensor and running another data capture. 

After replacing the bank 2 sensor the next capture looks like this:

Notice how the sweeps are in a much wider range now even on the "good" bank 1 sensor. The bank 2 difference is very substantial. I haven't done anything to change fuel mixture other than replacing the bank 2 sensor. Now we have full fuel control and fixed a lean code without finding a lean condition. 

Hope the pics are viewable. 

Thanks for reading,

Kenneth Hayes

G&G Auto Repair