What does long-term fuel trim malfunction mean for Volvo S40?
2 Answers
Long-term fuel trim is a numerical parameter with a value range of -23% to 16%. Long-term fuel trim is derived from short-term fuel trim values and represents long-term fuel supply correction. A value of 0% indicates that no fuel supply compensation is needed to maintain the PCM-commanded air-fuel ratio. If significantly below 0% (a negative value), it indicates the system is running too rich and fuel supply should be reduced. If significantly above 0%, it indicates a lean condition, and the PCM will increase fuel supply for compensation. Depending on different engine management systems, adaptive correction values may be stored in either the PCM's volatile memory or non-volatile memory. If stored in volatile memory, the memory is erased when the ignition is turned off, and the correction value returns to 0 upon restart. If stored in non-volatile memory, the memory remains even when the ignition is turned off, and upon restart, it returns to the previously stored correction value. The memory is only erased when the battery is disconnected or the PCM fuse is removed, returning the value to 0. Note: Both long-term and short-term correction values can be compared to the injector opening time. Values greater than 0 indicate increased opening time, while values less than 0 indicate decreased opening time. Fuel correction only occurs in closed-loop operation; during open-loop operation, the parameter value remains fixed.
I learned some knowledge from car forums. The long-term fuel trim malfunction of the Volvo S40 sounds quite complicated, but it actually means the engine computer system is monitoring the data fed back by the oxygen sensor and then finds a persistent excessive deviation when adjusting the fuel injection volume over the long term. Simply put, the car keeps trying to correct minor issues with the fuel injection system but can't fix them anymore. This malfunction is commonly caused by a faulty mass air flow sensor, clogged fuel injectors due to carbon buildup, or vacuum leaks in the intake system. When driving, you'll notice a significant increase in fuel consumption, weaker power, and possibly strange exhaust odors. It might also fail emission tests. I recommend using a diagnostic scanner to read the specific trouble codes to pinpoint the issue. If you're doing it yourself, you could try cleaning the fuel injectors or checking pipe seals. However, Volvo models are more electronically sophisticated, so it's best to have a professional technician inspect and repair them to avoid worsening the problem and causing engine damage. For daily prevention, regularly changing the air filter and using some fuel additives can reduce the likelihood of this issue occurring.
