I have a 2000 Mercury Grand Marquis.
At idle:
STFT bank 1 is normal +0->+4%
LTFT B1 is +18%.
When I increase RPM's STFT goes up to +18 and LTFT goes to 0. Bank 2 is fine.
I checked for vacuum leaks with smoke machine all ok.
No codes.
Idle rough
Could it be clogged injector?
Clarification: What is the mileage? Is the car experiencing any symptoms that made you want to scan the vehicle?
120000 miles. Rough idle is reason I checked codes.
FIXED. Front injector passenger side cylinder #1 wasn't making good contact.
Now all fuel trims within a couple % of zero.
Bank 1 on passenger side slowed me down a bit. Assumed Bank1 was driver side.
So it's correcting 18% on bank 1 at all times. Weird that it isn't throwing a misfire code if one cylinder is running that lean but maybe start with checking the injector connections on each injector on bank 1.
You may easily be able to determine which cylinder is the problem by simply unplugging one injector at a time on bank 1 (at idle) and seeing if has any affect on the way the car idles. (If you unplug an existing non-functioning injector the engine won't idle any worse. If you unplug a functioning injector the idle will be negatively affected)
It's far more likely if an injector isn't working the problem is with the injector connector on that old 4.6L. So test the connector if you find a non-functioning injector. (Plenty of youtubes on how to do that).
If that checks out OK do a resistance test on the "suspect" injector using a multimeter. (Plenty of youtubes on how to do that).
I will unplug each injector and see if I can identify a faulty one.
FIXED. Front injector passenger side cylinder #1 wasn't making good contact.
Now all fuel trims within a couple % of zero.
Bank 1 on passenger side slowed me down a bit. Assumed Bank1 was driver side.
Thanks for letting us know how it worked out.