What’s the difference between short-term and long-term fuel trim live data?
Quote from AutoFixerMike on June 16, 2025, 10:53 amI've seen this quite a few times while diagnosing vehicles. When LTFT (Long-Term Fuel Trim) stays at +18% but STFT (Short-Term Fuel Trim) hovers near zero, it usually means that the ECU has already adapted to a consistent lean condition. The short-term is constantly making small adjustments around the learned value.
Common causes include:
Small vacuum leaks (especially intake manifold gaskets, PCV hoses, brake booster line)
Dirty or failing MAF sensor
Weak fuel pump (low pressure but not fully failed)
Exhaust leaks ahead of upstream O2 sensor
You may want to start with a smoke test to check for vacuum leaks. Cleaning the MAF sensor is often a simple first step too.
I've seen this quite a few times while diagnosing vehicles. When LTFT (Long-Term Fuel Trim) stays at +18% but STFT (Short-Term Fuel Trim) hovers near zero, it usually means that the ECU has already adapted to a consistent lean condition. The short-term is constantly making small adjustments around the learned value.
Common causes include:
-
Small vacuum leaks (especially intake manifold gaskets, PCV hoses, brake booster line)
-
Dirty or failing MAF sensor
-
Weak fuel pump (low pressure but not fully failed)
-
Exhaust leaks ahead of upstream O2 sensor
You may want to start with a smoke test to check for vacuum leaks. Cleaning the MAF sensor is often a simple first step too.
Quote from AutoFixerMike on June 16, 2025, 11:11 am???
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Quote from Deleted user on June 16, 2025, 11:14 amShort-term fuel trim (STFT) reacts instantly to sensor changes, adjusting fuel delivery in real-time. Long-term fuel trim (LTFT) tracks ongoing trends and stores corrections over time. Together, they help identify if a problem is temporary or systemic.
Short-term fuel trim (STFT) reacts instantly to sensor changes, adjusting fuel delivery in real-time. Long-term fuel trim (LTFT) tracks ongoing trends and stores corrections over time. Together, they help identify if a problem is temporary or systemic.
Quote from LadyGreaseHands on June 16, 2025, 4:27 pmShort-Term Fuel Trim (STFT) makes quick, real-time adjustments to the air-fuel mixture based on current sensor data. Long-Term Fuel Trim (LTFT) is a slower adjustment based on trends over time. Together, they help the engine maintain the ideal fuel balance for performance and emissions.
Short-Term Fuel Trim (STFT) makes quick, real-time adjustments to the air-fuel mixture based on current sensor data. Long-Term Fuel Trim (LTFT) is a slower adjustment based on trends over time. Together, they help the engine maintain the ideal fuel balance for performance and emissions.
Quote from CarGuyBrad on June 17, 2025, 2:58 pmShort‑Term Fuel Trim (STFT) is the engine computer’s real‑time correction to the air‑fuel ratio—adjusting instantly (many times per second) based on the oxygen sensor to compensate for lean (+) or rich (–) conditions.
Long‑Term Fuel Trim (LTFT) is the adaptive, averaged adjustment built over time—like a memory offset—used to reduce the magnitude of STFT, keeping STFT centered around zero. Together, STFT + LTFT yield the total correction applied .
Short‑Term Fuel Trim (STFT) is the engine computer’s real‑time correction to the air‑fuel ratio—adjusting instantly (many times per second) based on the oxygen sensor to compensate for lean (+) or rich (–) conditions.
Long‑Term Fuel Trim (LTFT) is the adaptive, averaged adjustment built over time—like a memory offset—used to reduce the magnitude of STFT, keeping STFT centered around zero. Together, STFT + LTFT yield the total correction applied .
Quote from WrenchWizard on June 18, 2025, 11:07 amShort-term fuel trim adjusts the air-fuel mixture in real-time based on sensor feedback, while long-term fuel trim makes slower adjustments to compensate for trends over time. Both help keep the engine running efficiently.
Short-term fuel trim adjusts the air-fuel mixture in real-time based on sensor feedback, while long-term fuel trim makes slower adjustments to compensate for trends over time. Both help keep the engine running efficiently.
Quote from ScanToolGuru on June 19, 2025, 11:04 amShort-term fuel trim (STFT) adjusts fuel mixture instantly to match changing engine conditions — like when you press the gas pedal.
Long-term fuel trim (LTFT) shows how the engine computer has been adjusting fuel over time to keep the engine running smoothly.In simple terms: STFT = quick adjustments | LTFT = learned long-term adjustments.
Short-term fuel trim (STFT) adjusts fuel mixture instantly to match changing engine conditions — like when you press the gas pedal.
Long-term fuel trim (LTFT) shows how the engine computer has been adjusting fuel over time to keep the engine running smoothly.
In simple terms: STFT = quick adjustments | LTFT = learned long-term adjustments.