HELP!!!! I am trying to solve a 2003 4.6L that has a Service Engine Light that's coming from a P0175 code. The truck has 200k miles. I have replaced the spark plugs. All intake gaskets. Swapped the COPs and Injectors to see if I could move the problem from one bank to the other. No luck. Also replaced the pre-cat O2 sensor on both banks. I did a compression check and the 6 and 7 cylinders on Bank 2 are a little under the rest (less than 10%) and they are all within spec. I am not getting any misfire codes. I have done a visual inspection of the exhaust manifold and checked it with a thermal gun and haven't found anything to indicate that the manifold has a crack. I have also done a vacuum check on the EGR vale and it holds spec vacuum. I have checked all hoses and haven't found any leaks. I did have a smoke test done and they couldn't find any either. The O2 sensors and reporting the correct range voltages. The Short Term Fuel Trim values are constantly moving at + or - 4% which I think is normal. The Long Term Fuel Trim indicates that everything is normal on Bank 1, but Bank 2 is consistently at greater than -10% and goes as high as -23%. However, I have also done an old school test. With over 200 miles on the plugs I pulled them out and checked the color of the center electrodes. They appear to be normal to maybe even a little lean. Any ideas would be greatly appreciated. John

 

@ShadeTree55 your detailed diagnostics and description is very helpful - and completely consistent with your symptoms. The DTC P0175 (System too rich Bank2) indicates the (NEW) Bank2 O2 sensor is telling the ECU that there is too much (unburned) fuel in the combustion process on that bank. That condition will result in the ECU driving fuel trims negative on that bank, and it is. Short term trim being near normal confirms the ECU has effectively "compensated" for whatever the problem is causing too much fuel to get into the intake air for Bank2 or it is not ALL getting burned in the combustion process.


So -- Long Term Fuel Trim of 10% to 23% indicates the ECU has cut the base injector pulse width to the four injectors on bank 2 by that much, and can generally get the fuel/air mixture corrected. This is confirmed by your visual on the plugs on that bank appearing they are running lean.


These conditions eliminate the possibility of a vacuum leak, and an exhaust leak, as the fuel trims would go the opposite direction under those scenarios. Also, the fuel trim adjustments occur fairly quickly, but not instantly. So, after swapping injectors & COPs between Bank2 & Bank1 - I would suggest (if you haven't) to do a battery disconnect ECU reset (and thus all the fuel trim and adjustment tables). This will give a clearer picture of the effect of the changes.


The big question to me seems to be, how can excessively fuel laden air get to the O2 sensor on Bank2? I do not think it relates to MAF - or un-metered air as that is common to both banks. It smacks of intermittent misfires (NOT ENOUGH TO SET A 300 Series DTC) that are few enough for the ECU to compensate for. I would suggest an OBDII Mode $6 test - which gives pre DTC misfire counts for each cylinder during the current drive cycle and the last ten drive cycles.

OR: possibly look for 'flakey' electrical connection on a COP plug, or at the ECU. This age vehicle frequently experiences broken plastic electrical connectors and/or corroded connections. You might clean them with a good spray electrical cleaner and apply electrical dielectric grease to the rubber seals. Are the spark plug boots new / clean / and did you use dielectric grease on them?


OR: If your truck has a separate Fuel Rail Pressure sensor / regulator on each bank (I don't know - mine does not), the Bank 2 fuel rail pressure might be too high. But if there is only ONE, it would be reflected in both banks, so that wouldn't be the problem.


But you are on the right track. For some reason excess fuel is getting into the combustion process on Bank2 that isn't all getting burned up. A few misfires will very quickly lower Short Term Fuel Trim - and it relatively quickly filters into Long Term Fuel Trim. You can verify this by monitoring those signals on your scanner while you momentarily unplug a COP. But after doing something like that, it is a good idea to do a battery reset so the system isn't running on BUM data in the tables.


Hope this helps. Good Luck

 

F150Torqued - I can't thank you enough for your insight. This has been driving me nuts. I haven't seen any misfire codes, but if they can occur without triggering a code than that gives me a path to check. I thought that by swapping the COPs from one side to the other that it might change the problem to the other side but that didn't happen. I am working on this truck for a friend that has almost no budget. I know that I have seen some other broken electrical connectors in the cab. I have an Actron 9580A scanner. I'll have to figure out if I can do a misfire count test with it. But that certainly makes a lot of sense. If I can see misfires occurring, then I can follow the path. My daughter lives in Cibolo, the next time that I am down that way I'd like to buy you a cold one.

 

Accepted!

I had a COP connector knocked loose by A/C guy changing the A/C dryer and recharging the system. Same symptoms. It's hard to get a grip on how many of the codes and DTC's seem opposite of reality. But, for some reason, the 'few' misfires - occurred ONLY on hard load / acceleration, like trying to pass. Bank1 Short Term fuel trim was -2% to +2%, LTFT was -25%, while Bank2 was all perfect. I would probably still be looking for the problem if I had not noticed the Mode 6 data showed half a dozen misfires on Cyl No 4 during current drive cycle - but about a thousand in last ten drive cycles. All others were ZERO. That clued me to look at #4 where I found it.

It is a MIND BENDER grasping how a relatively small number of misfires results in negative LTFT on the effected bank without and appear not to effect STFT. But when I monitored live data on STFT and unplug a COP, I see STFT go negative real quick, but goes + for a moment when reconnected. But it into LTFT and takes a little time to work its self out. Keeps us on our toes!

 

Yes it does. I'm from the old school with carbs, etc. But logic still says that what is shared by all the cylinders (MAF, Air intake, throttle body, vacuum hoses, etc.) will affect both sides. But understanding how all the pieces go together to make the fuel system work correctly is challenging. One little inconsistent thing and it produces all of these symptoms that have to be figured out. I've been working on cars for over 40 years and I still learn something new everyday. Your posts put all of my thoughts back in line and helped me make sense of what my brain was already telling me, I just couldn't see it. Thanks again.