Recently changed the COPs and plugs on my 05 5.4L 4x4 and afterwards it started idling rough and low. I also forgot to reconnect the fuel pressure sensor on the fuel rail before I reconnected the battery and fired it up, keep that in mind.

So after changing the upstream O2 sensors and checking the vacuum lines on the back of the intake (they're connected), I pulled all the plugs and COPs to see if there were issues, all of them work fine. I moved to the injectors and checked the injector connections with a noid light, all of them blinked off and on. I ran an ohm check on the injectors and all of them are between 12.8 to 13.3 in resistance, so no joy.

I then did a cylinder balance test. When I pulled the injector connectors on 4, 7, and 8, the engine RPM and vacuum gauge didn't change. Is it possible that they will ohm test good and still be out?

I had the vacuum gauge connected to the vacuum line on the fuel pressure sensor. Out of curiosity, I disconnected the gauge and ran the engine and the rpm's picked up and the engine smoothed out at idle. Might the FPS be bad also?

 

I would try the relearn process.

 

What is the relearn process?

 

From page 264 of your owners manual.


Because your vehicle’s engine is also electronically controlled by a
computer, some control conditions are maintained by power from the
battery. When the battery is disconnected or a new battery is installed,
the engine must relearn its idle and fuel trim strategy for optimum
driveability and performance. To begin this process:


1. With the vehicle at a complete stop, set the parking brake.


2. Put the gearshift in P (Park) (automatic transmission) or the neutral
position (manual transmission), turn off all accessories and start the
engine.


3. Run the engine until it reaches normal operating temperature.


4. Allow the engine to idle for at least one minute.


5. Turn the A/C on and allow the engine to idle for at least one minute.


6. Drive the vehicle to complete the relearning process.




• The vehicle may need to be driven 10 miles (16 km) or more to
relearn the idle and fuel trim strategy.

 

This is no guarantee but sometimes when changing out plugs and coils (even if you didn't disconnect the battery, which you hopefully did) the computer still needs to do the relearn as the new parts should be running better than the old ones. I would start it all over by disconnecting the battery and pumping the brakes a few times then letting it sit for a while to make sure all the remaining charge is drained then do the relearn first thing. Its free to do and might just fix your problem.

 

Much appreciated and I'll give it a shot. Good to know that I need to pay more attention to the owner's manual for other than checking the fuse block

 

Ha! I think most of us are guilty of that one. In fact I think the last 2 times Ive dug mine out it was to check fuses LOL

 

That why you alway disconnect the Battery everytime you work on the motor. First step in any manual is to disconnect negative battery cable.

 

Always do. Anyway, I did the relearn and although I could tell the PCM relearned all the sensors and new equipment, I am still coming up with a P0174 code, Bank 2 System too lean, so I hooked up my scanner, posted are the results.

I did change the upstream O2 sensors and it looks as though the downstream sensors are in need of the same service, so I'll replace them and do it all again.

Any other possibilities would be welcomed.