For both the 3.4L V6 and 2.7L 4-cylinder:
* Valve Cover Gaskets: Leaks are a frequent problem, leading to oil leaks and potential engine damage if left unattended. This is often noticeable as oil stains on the engine.
* Oil Leaks: Beyond valve cover gaskets, other sources like crankshaft seals and rear main seals can leak, again necessitating repair.
* Timing Belt/Chain (depending on engine): The 3.4L V6 has a timing belt, requiring replacement at recommended intervals (usually around 90,000-105,000 miles). Failure can lead to catastrophic engine damage. The 2.7L 4-cylinder has a timing chain, which is generally more durable but still can stretch or fail over time.
* Water Pump: A failing water pump can cause overheating, leading to serious engine problems. Often tied to timing belt replacement on the V6.
More specific to the 3.4L V6:
* Intake Manifold Gaskets: These can leak, causing rough running, vacuum leaks, and potential misfires.
* Exhaust Manifold Leaks: Cracks or leaks in the exhaust manifold can lead to reduced performance and exhaust fumes in the engine bay.
* Throttle Position Sensor (TPS): A malfunctioning TPS can affect engine performance, causing poor idle, hesitation, or stalling.
More specific to the 2.7L 4-cylinder:
* Head Gasket Issues: While less common than on other engines, head gasket failures can still occur, especially with overheating.
Important Note: The age of the vehicle is a significant factor. Many of these problems are more likely to occur as the 4Runner gets older and experiences higher mileage. Regular maintenance, including oil changes, inspections, and following Toyota's recommended service schedule, is crucial to preventing many of these engine problems. Ignoring minor leaks or unusual noises can lead to much more expensive repairs later on.
