[JL421]

So I've had this car for about 2 months now, since the problems with the other car started. I finally bought a poor man's scan tool (Bluetooth OBD II adapter and Torque) and was playing around with it tonight.

Most things look normal:

Coolant Temp : 186F
O2 Sensor Temp : 950F
MAF : 95 CFM
Intake Vacuum : 35 kPa
Intake Temperature : 5F
STFT : 0

Except...

LTFT : -7 when idle and -13.3 to 13.7 when on the highway... This is constant, I've never seen it get near 0.

Is this something I should be worried about, or just leave it alone?

I also don't have horrible mileage, normally I average between 24 and 28 on the highway.

 

[cardoc]

Negative means the computer is pulling fuel by reducing the injector flow. It bases fuel trim on the feedback of the AF sensor(s) and the calculated load, as well as where your foot is on the accelerator. The trim value is equivalent to the percentage in relation to the combustion cycle. In other words, the computer is pulling 13+% of its base fuel map for each combustion cycle. That's a lot.

Usually, when the fuel trim is negative, it means the fuel isn't getting burned efficiently within the combustion chamber and the oxygen content is low by the time the gases reach the AF. Remedy is usually a set of plugs, clean the fuel system and check your air filter and PCV valve. Poor quality fuel will also cause it since it doesn't burn effectively.

Are there any codes stored in the ECM?

 

[JL421]

I did get the car used at 72k miles on 12/19/12 and it's at 79k now. (**** I drive a lot.)

No codes, just changed the spark plugs 1k miles ago, and the old ones were decent looking NGK Iridiums, replaced with the same.

I use 87 Octane 0% Ethanol Simonson gas.
Replaced a horribly dirty air filter at the same time as the spark plugs.
Finally, ran a can of seafoam through the tank right after I replaced the plugs.

Basically at 75k I took the assumption that no maintenance had been done except oil changes, and changed everything that was due. (Except transmission, that's coming with the next engine oil at 80k.)

 

[cardoc]

How cold was it outside when you took the data? Is the IAT reading correct at 5F? With heat generation of the engine and transmission at the coolant temperature shown, the IAT should be higher than 5F.

 

[JL421]

I believe the IAT is correct, it's -1F outside now, and at 70 MPH I'd assume that it'd remain fairly cold at the intake.

 

[cardoc]

Can you look at the barometer measurement the sensor is showing and compare it to the current pressure where you are?

Can you post a data stream of the AF sensor and O2 sensor output? If you can't, the AF sensor should be around 3.8V fluctuating minimally and the rear sensor should be around .6-.7mv when cruising.

If its good, let it go.

The cold temperature could be causing it. The cold temperature makes the air more dense and would cause the alteration in trims.