QuestionHi Tino. The check engine light in my 540 is on, the codes are 26 and 27 something about the fuel trim correction went over the limit 24%. Did you have that problem before, what was the fault?
AnswerHello Alain,
You are talking of fault codes given by an OBD scanner. It is further qualified by short term fuel trim or long term fuel trim which translates to the BMW terms of Additive adaptation
and Multiplicative adaptation.
If the number is positive, it means the engine computer is increasing fuel injection levels due to a perceived high level of air in the system (a vacuum leak causing the engine to get more air).
If it is negative, the computer is reducing the fuel intake.
The air/fuel ratio is continously monitored by the engine computer with inputs from the 02 sensors and the air flow meter. Supposing that the 02 sensor is good, the first thing to do is to look for vacuum leaks from bad vacuum hoses. Next is to check the throttle area for bad seals. Next is the end cover valve(crankcase vent valve at the rear of the engine) which could be defective (remove the no.8 spark plug and if it is messed up with oil, chances are that you have this problem). Others just spray brake cleaner at the end cover valve to verify if the engine rpm would change (but this is a potentially dangerous diagnostic method which only
experienced mechanics should do, along with a handy fire extinguisher).
If no obvious faults are found, it could be the air flow meter itself.
Tell me of your progress or lack of if you can.
Tino.
