Why Car Running Rich, Negative Long Term Fuel Trim

If the engine is running properly, fuel trims should be close to zero. However, some run high, like five percent. A scan tool can read the fuel trim. A negative long-term fuel trim means that the ECU has to decrease the injector pulse because the car is running rich. The vehicle needs less petrol in the blend. 

How To Diagnose Negative Long Term Fuel Trim

A scanning tool can diagnose negative long-term fuel trim. The scanner will give a number. If the number is negative, then the vehicle is running rich, with too much fuel in the mixture. 

How To Diagnose High Negative Long Term Fuel Trim

The sensor will not diagnose if the negative long-term fuel trim is high. But, you can watch the sensor share the negative trim numbers. If the negative trim goes past -10, then it is excessively high. The negative number shows you how much fuel the injectors are not sending into the mix. So, there is too much fuel in the blend. 

Possible Causes Of Rich Fuel Trim Mixture

If the vehicle is running rich, there are several underlying reasons, including:

• 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  

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Thanks a lot 🙂