I've been having this problem since I've gotten the truck about year ago. it has very high long-term fuel trims and it seems no matter what I do I cannot get them to go down my fuel trims at idle are 39 Bank1 and 33 bank2 they go down to around 12>7 while driving what could this mean. And even weirder occasionally I get Bank 1 or bank 2 too lean codes at idle when cold starting????
1997 Ford F150 4.6

 

Here is what your missing.
The meaning of the codes.
Long Term Fuel Trims are two tables in program software.
One for each bank.
They determine the injector pulse width for fuel amounts.
Ideally they should be at the zero point.
Not shifted positive or negative.
What shifts them?
The OX sensors feed back to the computer the amount of Oxygen in the exhaust gas.
When the Ox content is too high it tells the computer to shift the tables toward the + rich side because the intake is too lean with excess air or to little fuel.
The shift range is +/- 25%. When this shift is reached, the codes 171 and or 174 will caused to be set and match the reading of the tables by the use of a Scanner to look at them.
There are other less often versions of troubles that can cause the tables to shift so you have to recognize that possibility.
Basically keep in mind why they shaft no matter what the cause is.
Good luck.

 

Yes I understand how fuel trims work not exactly but I understand the basic gist of them I'm just wondering what is causing them to be so high at idle and give me bad gas mileage. Air leak??

 

Lee, the LTFT don't react very fast like the short term Trims do.
Being they are full RICH your fuel mileage will be affected.
May be the cats are melted and blocking the exhaust.
And yes a large intake air leak will chase them rich.
If the cats are not plugging the exhaust you should have idle and low rpm stalling from the richness.
Everything is out of limits at this point.
Look at the back of the vertical intake manifold where the PCV hose attaches and see if the rubber has gone soft and has a hole in it.
Good luck.

 

One of the first things I did was to replace the cats I've also replaced the oxygen sensors and the Pcv hosing I suspect it's an Intake manifold leak but I can never seem to find it

Mil status off
ABSLT TPS% 19.2
ENG SPEED RPM 699
CALC LODE 24.3
MAF (LB/M) 0.50
COOLANT (F) 181
IAT (F) 55
ING ADV 13.0
ST FTRM1 1.9
ST FTRM2 -2.3
LT FTRM1 37.4
LT FTRM2 33.9
VEH SPEED 0
FUELSYS 1 CLSD
FUEL SYS 2 N/A
O2S11 0.830
ST FTRM11 0.8
02S12 0.475
02S21 0.025
ST FTRM21 0.8
02S22 0.480
OBD2 STAT CA

 

Spray carb cleaner around the intake manifold gaskets while you watch the short term trims. If they are leaking the trims will go negative from the extra fuel. Also the O2 voltage will go high.

 

Assuming the motor was up to operating temperature for the readout, two things about them.
One the temperature is considerably too cold. If it's operating in that area you will suffer poor mileage.
Two, with trims like that the motor should be stalling at idle and near idle speeds. The fuel tabls shifted that far causes to much fuel to be injected killing idle.
No CEL/MIL light needs to be looked into because it should be on for the LTFT shown.
Your looking the issues, fix them.
Good luck.

 

It means you have a small vacuum leak. A small vacuum leak has a big effect on the LTFTs when at idle. (Your display shows that you are looking at the idle fuel trims.)
Now rev it up to ~2500 rpm and it will display the off idle fuel trim cell. Since your off idle LTFTs are lower, the small vacuum leak does not have a very big effect at the larger throttle opening. (It also indicates that your fuel pump is able to keep up with the higher fuel requirements of the increased engine speed.)

Try plugging the vacuum line to your EVAP purge valve and see if that has an effect on your fuel trims. Your EVAP purge valve or canister vent valve may be leaking a small amount of air into the intake.

If all else fails, buy/rent/steal a smoke machine.