The “Check Engine Light” is on. When I brought it in for the annual inspection (NY), the diagnosis was “lean fuel mixture”. The fuel filter was replaced (the car has over 110,000 miles on it), and some adjustments were made in the computer, or the sensors (I don’t remember which). It was found that one adjustment was effective at low speeds, and the other one was necessary for 2000 RPM. A valve cover gasket was replaced because of an oil leak, but I don’t see how this could have contributed to the problem. I drove it for about 10 miles and the light went on again. Any ideas as to what the cause might be?
Can you post the OBDII code that caused the Check Engine Light to come on? If you don’t have it on the repair order, places like Autozone will check it for free.
Including the year of your Avalon may help as well.
What year is this car?
First guess is a leaky intake manifold gasket: common Toyota problem.
@Bugsyg an exhaust leak in front of the upstream oxygen sensors will also generate a lean code
Low fuel pressure will generate a lean code
Misfires can generate a lean code, because of the excessive (unburned) oxygen
Restricted fuel injectors could generate a lean code (and misfires)
Can you hear any hissing (that could be a vacuum leak)
If your car uses a mass airflow sensor, it might need to be cleaned
One more question.
Lean on both banks or one bank only?
Lean fuel mixtures are due to lack of enough fuel or too much air. Typically, the solution is to check for dirty injectors that throw an unequal amount of fuel from cylinder to cylinder or check for vacuum leaks that let unmetered air into the intake manifold. Both conditions create a situation where the computer sensors can read a lean condition that cannot be corrected with it’s built-in adjustments.
I believe the mechanics could make further adjustments, but, until the injectors and vacuum leaks have been ruled out, the situation willget worse again. If your car has a plastic upper intake manifold, the intake seals could be bad, and the source of your problem. But that is just a guess.
The ECM codes should provide the clues needed to get started at least. Rather than speculate about the cause at this point, the OP should first take advantage of the self-diagnosis feature Toyota designed into the car.