My 98 4Runner is throwing a lean code. I brought it to the shop and they replaced the air filter, fuel filter, and clean the mass airflow sensor. A few days later the code came back. They told me that I probably needed a new MAF sensor. However I pulled it out and re-cleaned it, and put it in my 97 4Runner and drove it for a few weeks including over mountain passes and never got a code. I put it back in the 98 runner and after a few days the code came back. It is not running sluggish at all and is idling normally so I don't suspect the fuel pump. What else could cause this? I appreciate your feedback . I also need valve cover gaskets replaced. Could this cause that code?