Craig Truglia is an ASE A6 and A8 certified technician who presently works as a service writer for Patterson Autobody, a repair facility in Patterson, N.Y. A former shop owner and editor of several automotive repair magazines, Truglia combines his Columbia University education with the real-world experience he sees daily in the automotive repair field.
Diagnosing lean problems has become increasingly complicated over the years. Back in the day, all one had to do was adjust a carburetor. Now, thanks to the increased computerization and concurrent lack of standardization in the automotive industry, it requires the mastery of several different systems, with many manufacturers having different nuances.
However, one thing has stayed the same over the years: Lambda. Lambda never changes and it always represents fuel system perfection. If we understand Lambda, no matter how much the sensor feedback technologies end up changing, we will be able to adjust and diagnose vehicles.
What is Lambda? Lambda is a perfect air-fuel ratio in which there are practically no unconsumed hydrocarbons (HC) in the fuel. Now, in reality, no engine is going to burn absolutely perfectly, which is a big reason why even in “good” running engines there will be more HC pre-cat than post-cat, but we are talking about a difference of a few parts per million (PPM) of HC.
For all intents and purposes, if you have a Lambda of 1.0 you have a perfect running engine. If you go below 1, you begin running rich. If you go above 1, you run lean. Anything within 0.97 to 1.03 is normal, but if you go above these numbers and the vehicle has a code for fuel trim or a converter issue, it is worth taking a closer look. However, don’t be hyper sensitive. If the vehicle is running fine and has a Lambda 1.08 or 0.95, that could be “good enough.”
Just remember how it works: above 1 is lean and below 1 is rich.