"Too lean" means that the engine is not receiving enough fuel for the amount of air coming into the engine. In other words, the engine is receiving more than 14 parts of air for 1 part of fuel. (Internal combustion engines run best at this air/fuel ratio.) While there can be numerous causes for this code to be set, on 2005 - 2008 Toyota Corollas, the repair is usually quite simple. 99% of the time, I have found the cause of a P0171 for 2005-2008 Toyota Corollas with 1ZZ-FE engines to be the intake gaskets. The problem is so common, Toyota even put out a technical service bulletin for it. For a mechanically inclined person, doing this repair can save a couple hundred dollars because the cost of the part to fix this issue is usually not more than $10.
The problem with the Corollas is that the old intake gaskets on the Corollas with the 1ZZ-FE engines is that over time, they harden and lose their elasticity. This allows unmetered air to sneak past the gasket and into the intake manifold, thereby causing a lean condition. Here's how to fix the problem.
First let's take a look at what you seen when you first open the hood.
Remove the plastic engine cover, and remove the intake tube from the air cleaner housing to the throttle body, and you've got this:
Next, remove the two 10mm bolts and two 10mm nuts holding the throttle body onto the intake manifold. Seperate the throttle body from the intake manifold, but you do not need to take the throttle body completely off the car. Just set it to the side with the coolant hoses still attached to it.
Now remove all the bolts holding the manifold to the engine cylinder head. Once you have all the bolts removed, you shouldn't have to fight it out, the manifold should easily pull right out. If it doesn't pull up off the engine easily, you probably haven't removed all the bolts. Here's how the engine bay looks with the manifold removed.
The intake manifold gasket is a one piece rubber gasket. If it is still a factory installed gasket, the gasket will be black. Go ahead and pull the old gasket off.
The new & improved intake manifold replacement gasket is orange. Install it onto the intake.
Once you have installed the new intake manifold gasket, you are ready to install the intake back onto the car. Once you have installed the intake back onto the car, start the engine and spray some carbueretor cleaner around where the intake manifold is. If there is still a vacuum leak present, the engine will idle down. If there is no vacuum leak, the engine rpm will not change as you spray carb cleaner around the intake. Of all the Corollas I have put intake gaskets on, only one required further attention after replacing the gasket. Oh, and don't forget to clear the DTC with your scan tool so the check engine light will turn off.
Most customers at our shop pay $170 - $200 (primarily for diagnosis and labor) for this easy repair on Corollas. If you're mechanically inclined, try to replace the intake gasket yourself, and you just may save a couple hundred Dollars.
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