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The vehicle is experiencing a fuel trim issue with the C63 model.


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Greetings, I am the proud owner of a 2012 C63 AMG.

I am seeing negative LTFT values ranging from -16 to -18 at idle. Despite replacing both MAF sensors and installing new OEM air filters, the negative LTFT values persist.

Do you have any recommendations for identifying the underlying cause of this issue, even though I have already replaced the MAFs and filters?
Thank you.

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A significant negative trim value indicates that the O2 sensor is detecting an insufficient amount of oxygen in the exhaust. This is the result of an excessively concentrated combination.

The most probable reason is a fuel injector that is leaking, however there might be other factors contributing to the issue.

Given the prevalence of leaky injectors or instances of them being stuck open, it would be advisable to prioritise investigating the M156s as a potential source of the issue.

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The injector was replaced with a different set, but it had no impact. The long-term fuel trim (LTFT) value remained in the negative range. Additionally, both mass airflow sensors (MAFs) have been replaced.

Yesterday, I unplugged the evaporative emission (EVAP) switch while the vehicle was running, causing the fuel trims to decrease to around -7. I am unsure whether this is linked to my current issue. However, after reconnecting the EVAP switch, the fuel trims returned to -16.

Is it feasible that an intake manifold leak is responsible for this?

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Leakages in the intake manifold result in positive fuel trims.

If the purge valve remains open, it will introduce more fuel into the mixture, resulting in negative trims as shown. Verify whether it is accessible even in the absence of electricity. When not powered, it should not allow any airflow.

By increasing the engine speed to 1500rpm while the vehicle is in neutral and maintaining this level, do the trim levels gradually return to a normal state?

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