Short Term and Long Term Fuel Trims Explained

Summary

In this video, the host demonstrates how to diagnose a performance issue in a 2003 Ford F-150 with a 4.2L V6 engine, using fuel trims to identify potential problems. The vehicle exhibits hesitation from a dead stop, but no check engine codes are present. By explaining short-term and long-term fuel trims, the video teaches viewers how to interpret these numbers to pinpoint lean or rich conditions. The diagnosis points to a possible vacuum leak, but further confirmation with a smoke machine is needed. The video offers valuable insights into diagnosing fuel trim-related issues for various vehicle models.

Takeaways

• 😀 Short-term and long-term fuel trims are essential for diagnosing engine performance issues, especially when there are no codes or check engine lights.
• 😀 The air-fuel mixture in an engine should maintain a ratio of 14.7 parts air to 1 part fuel for proper emissions and engine performance.
• 😀 Fuel trims are adjusted by the engine's computer, which controls the fuel injectors' pulse width and uses oxygen sensor feedback to fine-tune the mixture.
• 😀 Short-term fuel trim responds quickly to changes in the air-fuel mixture, with a positive trim indicating a lean condition (too much air).
• 😀 Long-term fuel trim compensates for persistent issues over time and slowly adjusts to account for a consistent lean or rich condition.
• 😀 A typical acceptable range for short-term fuel trim is between -3% and +3%, while long-term fuel trim should ideally stay close to 0%.
• 😀 Vacuum leaks are a common cause of lean conditions, and they can be harder to detect, especially when the leak is after the throttle plate.
• 😀 To diagnose lean conditions, it's important to check both short-term and long-term fuel trims to get an accurate picture of the engine's fuel mixture.
• 😀 Increased long-term fuel trim percentages (e.g., 15-17%) indicate the engine is compensating for a lean condition by adding more fuel.
• 😀 Raising the RPMs can reduce the impact of certain issues, like a vacuum leak, as seen in the improvement of long-term fuel trim when the throttle plate is open.
• 😀 When diagnosing vacuum leaks, traditional methods like spraying starting fluid or using propane might not always work, especially if the leak is inside the intake manifold.

Q & A

What are short-term and long-term fuel trims used for in diagnosing car issues?

Short-term and long-term fuel trims help to diagnose engine performance issues, particularly when there are no error codes or check engine lights. They measure adjustments made by the car's computer to maintain the correct air-fuel mixture, allowing you to detect lean or rich conditions in the engine.

What is the ideal air-fuel ratio for a car engine to run efficiently?

The ideal air-fuel ratio for a car engine is 14.7 parts of air to one part of fuel. This ratio ensures the engine operates efficiently and meets emissions standards.

How does the engine's computer (ECU) control the air-fuel mixture?

The engine's computer controls the air-fuel mixture by adjusting the fuel injectors' pulse width, or how long they stay open. It uses inputs from sensors like the MAF sensor to determine the correct amount of fuel needed for the air entering the engine.

What role do oxygen sensors play in maintaining the correct air-fuel mixture?

Oxygen sensors monitor the exhaust gases and provide feedback to the ECU about the air-fuel mixture. If the oxygen sensor detects a lean or rich condition, the ECU adjusts the fuel injectors' timing to correct the mixture.

What happens when the oxygen sensor detects a lean condition?

When the oxygen sensor detects a lean condition, it sends a signal to the ECU, which then increases the fuel injector pulse width to add more fuel to the mixture, making it richer.

What is the difference between short-term and long-term fuel trims?

Short-term fuel trims adjust the fuel mixture in real-time to respond to changes in engine conditions, while long-term fuel trims gradually adjust to persistent issues over time, such as a vacuum leak or malfunctioning sensor.

What does it mean when long-term fuel trims are consistently high, like 15% or 17%?

Consistently high long-term fuel trims indicate that the engine is running lean, meaning the ECU has had to continuously increase the fuel injectors' pulse width to compensate for an ongoing issue, like a vacuum leak or insufficient fuel delivery.

How can you detect a vacuum leak in an engine?

A vacuum leak can be detected by observing the changes in fuel trims when the engine's RPM is adjusted. If the fuel trims improve when the RPM increases, it could indicate a vacuum leak. You can also use diagnostic tools like a smoke machine or diagnostic sprays (starting fluid, brake cleaner) to find the leak.

Why might the fuel trims improve when the throttle is opened, even though there's a suspected vacuum leak?

When the throttle is opened, the engine has easier access to air, which reduces the relative effect of a vacuum leak that is only present at idle when the throttle plate is closed. This explains why fuel trims might improve when the RPM increases.

What is the significance of using a smoke machine in diagnosing vacuum leaks?

A smoke machine is a more effective tool for diagnosing vacuum leaks because it allows you to visually detect smoke escaping from the intake system, pinpointing the location of the leak. This is particularly helpful for internal leaks, like those in the intake manifold gasket, which are hard to detect with traditional spray methods.