paulh2602601 Posted July 10, 2024 Share Posted July 10, 2024 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. Quote Link to comment Share on other sites More sharing options...
BENZE220AMG Posted July 10, 2024 Share Posted July 10, 2024 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. Quote Link to comment Share on other sites More sharing options...
paulh2602601 Posted July 10, 2024 Author Share Posted July 10, 2024 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? Quote Link to comment Share on other sites More sharing options...
BENZE220AMG Posted July 10, 2024 Share Posted July 10, 2024 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? Quote Link to comment Share on other sites More sharing options...
Starcar2022 Posted July 10, 2024 Share Posted July 10, 2024 Are there any modifications or adjustments made to enhance the performance of the car? Are you using conventional catalytic converters? Quote Link to comment Share on other sites More sharing options...
paulh2602601 Posted July 10, 2024 Author Share Posted July 10, 2024 The automobile is completely unmodified. Quote Link to comment Share on other sites More sharing options...
paulh2602601 Posted July 10, 2024 Author Share Posted July 10, 2024 After disconnecting the purge valve switch, the trim levels decreased. How can I verify whether the valve is open while it is not receiving power? Quote Link to comment Share on other sites More sharing options...
Starcar2022 Posted July 10, 2024 Share Posted July 10, 2024 Remove the pipes and attempt to exhale air through it to see whether it is clear. Quote Link to comment Share on other sites More sharing options...
TechnoTrailblazer Posted July 14, 2024 Share Posted July 14, 2024 Yes, that is correct, remove the pipes Quote Link to comment Share on other sites More sharing options...
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