What can cause fuel trim to be high?

High fuel trim, such as +10 \% and higher is a lean INDICATION.

What does a high fuel trim mean?

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 controls the fuel trim?

The Electronic Control Module (ECM) or Powertrain Control Module (PCM) uses a few sensors to determine how much air is flowing into the engine. The ECM or PCM then uses oxygen sensor readings to find out what the actual air-fuel ratio was, and ‘trims’ the fuel quantities based on this ‘real’ air-fuel ratio.

Do downstream O2 sensors affect fuel trim?

Input from the downstream O2 sensor(s) is used to detect any problems with the converter(s), and to fine-tune fuel trim. If a fault occurs in an O2 sensor or its internal heater or wiring circuit, the OBD II system should detect the problem, set one or more fault codes and turn on the Check Engine light.

What causes negative fuel trim at idle?

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 lean fuel trim?

The lean condition can be caused by a vacuum leak, which introduces more air into the air-fuel mixture, or by a weak fuel system, which does not input enough fuel into the air-fuel mixture.

Is negative fuel trim bad?

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 can cause negative fuel trims?

Negative fuel trim means the engine is fuel rich and the PCM is subtracting fuel for correction. It could be many things, bad O2 sensors, fuel injectors, fuel regulator, EVAP system, MAF, MAP/Baro sensor, etc.

Can fuel trim cause a misfire?

A rich biased O2 sensor may cause inappropriately negative fuel trim on one bank, resulting in a lean misfire. There could actually be too much fuel going to Bank 1, but -35 percent fuel trim is still considered “in fuel control,” so the engine should be running well. However, uneven fueling on Bank 1 could explain it.

What causes low fuel trim in a car?

If the condition exists in all rpm ranges, the cause is more likely a fuel supply issue caused by low fuel pressure, a restricted injector or a defective sensor input from a MAF or engine temperature sensor. Fuel trim can also be used to identify which bank of cylinders is causing a problem.

What does it mean when the fuel trim value is negative?

In other words, it is attempting to RICHEN the fuel mixture because it thinks the engine�s air/fuel mixture is running too lean. NEGATIVE (-)fuel trim values mean the engine computer is subtracting fuel (decreasing the pulse width or on-time of the fuel injectors) to reduce the amount of fuel injected into the engine.

What happens if fuel trim is too high at idle?

Depending on the condition of the low-pressure fuel pump, the fuel trim values may be only slightly elevated at idle, but get worse with greater load. If the fuel pump is in really bad shape, the fuel trim values will be high enough to set a fault code and would not perform very well.

How do you diagnose a fuel trim problem?

Some general guidelines for diagnosing fuel trim and the potential causes for failure can be found below. A useful diagnostic tool is to determine if the fuel trim problem exists in more than one engine operating range. Fuel trim should be checked at idle, at 1,500 rpm and at 2,500 rpm.