Quote:
Originally Posted by ShadowReaper
Good information. For the future, what would most likely be the cause of really low fuel trims (like -20s or less)? I have read that if they're really high it can be an exhaust leak.
Also, how should my STs and LTs fluctuate when I'm have a bigger percent throttle. I haven't looked at them while doing a pull.
|
Large trims in one direction is likely to be due to incorrect maf or other sensor scaling.
Explanation: Under low loads, the engine runs in Closed Loop. This means that the maf sensor sees an amount of air going by, and tells the ECU the amount of air. The ECU gets input from other sensors, and uses that and airflow to determine the amount of fuel to put into the engine. The fuel and air combust to create other gasses. Then, the ECU CLOSES the loop by checking it's work through reading the lambda sensor in the exhaust and comparing to what it expected. It uses the fuel trims to adjust until it gets it right.
Ltft is stored in RPM bands. If your engine is running at 3500rpm, the 3000-4500 ltft value will be applied. It will be applied in both closed loop and open loop situations.
Stft does not get stored, so there's no point in having categories for it. Stft is only applied in closed loop operation.
Meaning that when you do a pull, ltft at any rpm will be what the stored value for that RPM range is. Stft will be 0 after a short delay.
More explanation: If you increase load or rpm past a preset point, the engine will enter open loop operation. Open loop is the same as my closed loop explanation above, except the ECU DOES NOT CLOSE the loop. It simply doesn't care how accurate it is, and ignores the signals from the lambda sensor. It will still apply the ltft because it has it available, but it does not change it. The reason for this is that AFRs are generally more rich at WOT than traditional lambda sensors can accurately measure, so the ECU expects the feedback to be junk anyway.
Hope that helps.