Why is the fuel trim correction value too high for the Mercedes GL350?
3 Answers
The excessively high fuel trim correction value in the Mercedes GL350 is a normal phenomenon. The fuel trim value is an important indicator for ensuring power and fuel consumption. The long-term fuel trim value rarely changes in vehicles: The ECU receives signals from the intake air flow sensor, air-fuel ratio, oxygen sensor, and combines them with the current load to adjust the fuel supply. The short-term trim value changes constantly due to load variations, while the long-term trim generally does not intervene except during high power, throttle opening exceeding limits, gear shifts, or sudden load changes. If the long-term trim value keeps changing and the variation becomes increasingly larger, it indicates a vehicle malfunction. It is recommended to first check the catalytic converter. Long-term fuel trim values: If the long-term fuel trim value exceeds 5%, it indicates a fault in the engine system.
My GL350 reported the same fault last year, when I suddenly noticed a sharp increase in fuel consumption and a significant lack of power during acceleration. The mechanic spent two full days diagnosing it and finally found that the mass air flow sensor was clogged with sludge, causing inaccurate air intake measurements. This made the ECU compensate by injecting excessive fuel. Another possibility is that the oxygen sensor on the rear exhaust pipe has aged, sending incorrect data to the ECU and causing it to misjudge the air-fuel mixture as too lean. The most troublesome issue was a vacuum leak—in my case, the intake manifold gasket had aged and was leaking. My suggestion is to first read the trouble codes, then try cleaning or replacing the mass air flow sensor, while also checking all rubber hoses for cracks. If you notice fuel consumption exceeding 15 L/100km on the highway, pull over and inspect immediately.
Running an auto repair shop for twenty years, I've handled numerous cases of high correction values in GL350 models. In 80% of cases, it's either due to oxygen sensor poisoning/failure or a stuck carbon canister purge valve. Last time, a customer used substandard gasoline which caused half of the injector nozzles to clog with gum deposits, prompting the ECU to automatically increase fuel injection as compensation. Another case revealed an aging fuel tank cap seal causing pressure loss and abnormal fuel pressure. I recommend focusing on four key areas: check if the oxygen sensor voltage fluctuates normally, verify the carbon canister purge valve's smooth operation, inspect injector nozzle atomization performance, and finally test whether fuel pressure meets specifications. By the way, an overdue air filter can indirectly cause this issue too - replace it when due.
