What is a normal short term fuel trim reading?
So what should fuel trim values be? Assuming that the engine is in excellent mechanical condition, and that all sensors implicated in metering both intake air and fuel, short-term fuel trim values should generally be between positive 10%, and negative 10% when the engine is running at a steady speed.
What does negative short term fuel trim mean?
The short-term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. A negative fuel trim percentage indicates the PCM is removing fuel while a positive percentage indicates the PCM is adding fuel.
When to use fuel trim to identify a problem?
Fuel trim can also be used to identify which bank of cylinders is causing a problem. If the LTFT for bank 1 is 20% and the LTFT for bank 2 is 3%, the source of the problem is associated with bank 1 cylinders only.
What should bank 1 fuel trim be at idle?
If long-term fuel trim (LTFT) for bank 1 is 25% at idle, but corrects to 4% at 1,500 and 2,500 rpm, focus on factors that cause a lean condition at idle, such as a vacuum leak.
How does short term fuel trim ( STFT ) work?
Short Term Fuel Trim (STFT) Short-term fuel trims occur as a direct result of changes in the oxygen content of the exhaust stream. The exhaust stream is monitored by an oxygen sensor upstream of the catalytic converter, and the signal voltage it generates is directly proportional to the oxygen content of the exhaust stream.
When to turn off long term fuel trims?
However, when long-term fuel trims exceed about 10% or so to either side of 0%, there is a problem that needs to be investigated, but note that codes indicating rich or lean running conditions will generally only be set when the deviation reaches about 25% or so.
What causes negative fuel trim?
A negative fuel trim is considered anything at or below the minus 10 percent threshold. Ron addresses the common issues that can cause fuel trims to go negative. Issues like leaking fuel injectors, a defective fuel pressure regulator and defective MAF sensors will be addressed.
What is long term fuel trim?
Long-term fuel trim refers to the measurements these sensors make , which are effectively an average of the modifications the ECU made as a result of short-term fuel trim. That Sounds Like A Ton Of Data.
What is a positive fuel trim?
POSITIVE fuel trim values mean the engine computer is adding fuel (increasing the pulse width or on-time of the fuel injectors) to add more fuel to the engine. In other words, it is attempting to RICHEN the fuel mixture because it thinks the engine s air/fuel mixture is running too lean.
What are fuel trims all about?
In short, fuel trim describes the adjustments that a vehicle’s engine control unit (ECU) makes to the fuel delivery system to keep the air-fuel mixture as close to its ideal ratios as possible. Short-term fuel trim happens as a result of changes in the oxygen levels of exhaust gases.