Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Why Long Term Fuel Trims Remains in Negative?

Dear Experts here im attaching log of 1SZ-FE, 1.0L engine for your kind consideration, please advice why my LTFT remains in -ive though MAF, Intake Air Temp sensor, ECT sensor and O2 sensor are working fine, also i had a compression test which is 170psi for each cylinder.

Request for your valuable comments and suggestions please.



Any one please…

Is the check engine light on?
If not I would just drive on. Otherwise, first clean the MAP sensor.
Typically fuel trims aren’t a problem until they reach 25%.
Long term + short trim is near zero, and that’s a good thing.


Thank you for the reply.
No check light, and also there is no MAP sensor only MAF. Sum remains close to 0 but i feel it effecting fuel consumption due to rich condition.

My bad. I meant MAF. I used to have a Matrix (tall Corolla) with MAF.

There is no rich condition if the front O2 sensor is working correctly.
The fuel trim #s just mean there’s a discrepancy between what the sensors indicate and what the fuel map expects.

Are you getting higher than expected fuel consumption?
Engine temp seems to rule out a failing thermostat.
However, with the car standing still the temp could look okay even with a sticky 'stat.
That leaves tire pressure, dragging brake or wheel alignment as possible causes.


The long term fuel trim is comparing the actual fuel trim (fuel pressure x duration of the pulse) to a table of theoretical values stored on a table in the computer that was derived from the engineers at the manufacturer. The actual fuel delivery during closed loop (engine warm) is determined by the O2 sensors, MAF, and other sensors.

Your air/fuel ratio is still ideal and the engine is not running lean or rich. As the engine ages, this LTFT will change as the sensors and the engine ages. It may even slowly change to near zero and then go positive or it may continue to go further negative. When it gets to about 20% either way, you will get a check engine light. Most likely at that time, the MAF sensor will be dirty, but it could be a marginal O2 sensor too.


Coolant temp is after driving for 10KM. Will check wheel alignment and brake as milage droped from 14.7 Km/L to 13.5KM/L

Thank you, you cleared my doubts about fuel trims.

With an intake air temperature of 52 C it seems that you are in a warm climate, are you using air conditioning in your vehicle?

Im from Pakistan, in my place now a days lowest temp is around 8 degree Celsius, this mileage is without AC.

So why is your intake air temp so high ?
Did you modify the factory air intake system?

No sir intake is not modified, and this reading is recorded after 10KM of city driving, and also it increases gradually from 50 to 52 with increase in rpms on idle with respect to coolant temp.

While the car is standing still it could be drawing air from the hot engine bay.
It’s probably lower while the car is moving at 30kph or faster.
Where is the air inlet located?

Reference pic of air inlet location.

The air filter box sits right on top of the hot engine.
The intake hose to the box runs through the engine compartment.
The rate of air flow is slow with no engine load (revving in neutral).
When the car is standing still all this stuff gets pretty hot.
No surprise to me that it reaches 50C standing still.

1 Like