What causes low fuel trim?

During idle when the throttle plate is closed, the vacuum in the intake manifold is high and very little air flow is entering the engine, so even a small amount of un-metered air will have an effect on fuel trim. This causes the fuel trim numbers to become negative during a boost situation.

What causes positive fuel trims?

Here are some common causes of positive long-term fuel trim: Air or vacuum leaks in the intake manifold, throttle body or in a vacuum hose. Defective fuel pump not generating enough pressure or volume. Restricted fuel flow due to pinched hose or plugged filter.

What causes negative long term fuel trim?

Possible Causes Of Rich Fuel Trim Mixture

• High fuel pressure.
• Leaking petrol injectors.
• Malfunctioning MAF sensors.
• Leaks in the exhaust system.
• Loss of compression in at least one cylinder.
• Occasional misfires.
• Defective O2 sensors.

What should my fuel trims be at idle?

Fuel trim should be checked at idle, at 1,500 rpm and at 2,500 rpm. 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 do you reset fuel trims?

To perform a PCM reset

1. Bring car up to normal operating temp.
2. Turn ignition off.
3. Remove ECM main fuse.
4. Turn ignition to “on”, do not start.
5. Wait for system check to complete, numerous errors will be detected.
6. Turn ignition off.
7. Replace fuse.
8. Turn ignition to “on” and wait for system check.

What is normal short-term fuel trim?

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.

How do I check my fuel trims?

To test a rich response, you can feed some propane vapor from a small propane tank into the throttle body or a vacuum hose connection on the intake manifold. This time, you should see a drop in fuel trim readings, with STFT going NEGATIVE, and LTFT creeping downward in response to the rich fuel mixture.

What is the difference between long term and short term fuel trim?

Long-term fuel trim relies on sensors further downstream in the exhaust system, past the catalytic converter. While the sensors’ signal will fluctuate frequently upstream, where short-term fuel trim is measured, the signal at the sensors further down should be fairly constant.

What affects fuel trim?

Fuel trim can be affected by just about anything between the air filter and the muffler, including sensors, injectors, ignition, EGR, the engine’s mechanical condition and even the crankcase ventilation system.

Can a bad catalytic converter cause negative fuel trim?

The downstream sensors only monitor the health of the catalytics. If the readings between the upstream and downstreams are too close or too far apart a code is thrown for catalyst inefficiency. The only way a catalytic will affect fuel trims is if it’s clogged, but under normal operation it has zero effect.

How do I fix my fuel trim Bank 2?

What repairs can fix the P0173 code?

1. Replacing the mass air flow sensor.
2. Replacing the fuel pressure regulator.
3. Replacing the intake manifold gaskets.
4. Repairing or replacing the engine air filter intake system.
5. Replacing a vacuum line connected to the throttle body.
6. Repairing a leaking brake booster.

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.

What should an O2 sensor read at idle?

A good O2 sensor should produce an oscillating waveform at idle that makes voltage transitions from near minimum (0.1 v) to near maximum (0.9v). Creating a lean mixture by opening a vacuum line should cause the sensor’s output to drop to its minimum (0.1v) value.

What causes lean condition at idle?

Your engine runs lean if your air-to-fuel mixture is too light – this means that the fuel in your ignition chamber is being igniting with too much air or too little fuel. Your engine running lean is always a symptom of another problem – it could be a dirty MAF sensor, a damaged oxygen sensor, or damaged fuel injectors.

Does clearing codes reset fuel trim?

During closed-loop operation, the PCM reports changes in fuel trim calculations via the OBD II generic data parameters short-term and long-term fuel trim. STFT for most vehicles will normally sweep rapidly in response to the oxygen sensor. Both STFT and LTFT will normally reset when all trouble codes are cleared.