2008 Toyota Sienna with a 3.5L V6 engine was towed to our shop due to running poorly and sputtering. The owner asked us to diagnose the problem and advise what repairs were needed.
Why is my Toyota Sienna running rough?
The most common cause for a rough-running Toyota Sienna is a vacuum leak. The next most common issue would be worn spark plugs. A defective airflow sensor will fail to correctly regulate the air-to-fuel mixture and cause rough running. Another cause of the Toyota Sienna running rough is carbon buildup in the intake ports which restricts airflow to the engine.
Problem
This 2008 Toyota Sienna 3.5L V6 had a rough running engine that sputtered when accelerated, had a lack of power, and the check engine light was on. The vehicle had less than 100k miles on the odometer but had received little routine maintenance. There were no details on when or how the problem started.
Diagnosis
Our Toyota service technician connected the vehicle to our engine management system scan tool and found error codes stored in the computer. The check engine codes indicated a random engine misfire on cylinders one and two with a lean fuel mixture. A test drive with the scan tool connected showed no misfire but a lean fuel condition causing rough engine running and poor performance.
A visual inspection of the engine found a broken vacuum hose fitting on the engine air cleaner housing. This vacuum leak caused a lean fuel mixture making the engine run very poorly.
Often vehicle problems are complex and have more than one cause. Diagnosis is a step-by-step process where each problem is corrected or repaired before the next part of the problem can be discovered. With this vehicle, our technician recommended replacing the broken air cleaner top piece first and then continuing testing to find if there are other contributing factors to the poor engine performance.
Next, the engine was checked for other vacuum leaks and none were found. Then the fuel pump pressure was checked and found to be good. Another test drive with the scan tool connected showed that the problem had been solved. In this case, repairing the vacuum hose leak solved the rough-running problem.
A visual inspection of a spark plug showed signs of excessive wear and age so the technician recommended spark plug replacement and dirty air filter replacement.
Problem Cause
Vacuum leaks become common as vehicles age. Vacuum hoses become hard, brittle, and break or crack from years of high under-hood temperatures and being exposed to oil and fuel. The small plastic hose fittings are fragile and the plastic becomes brittle as the vehicle ages. Unfortunately, the parts can be extremely expensive and due to the vibration and high heat in the engine compartment usually prevent successful, lasting repair of small plastic parts.
Repair
Air cleaner housing top replacement. Connected all vacuum lines going to and from the air box. Air filter element replacement. Cleared check engine light error codes and test-drove the vehicle to verify successful repairs. The engine ran well and no further error codes were found.
2008 Toyota Sienna 3.5L V6 Spark plug replacement. Our technician removed surrounding parts to gain access to the spark plugs, removed old spark plugs, and replaced them with new spark plugs. The vehicle was test-driven for quality control to make sure it was ready to be returned to the owner.
Repair Cost
Repair prices vary greatly depending on the vehicle model and options, the condition of the vehicle, and the diagnostic time required. It is best to contact us for a detailed repair cost estimate.
This repair cost, including diagnostic work, air cleaner housing replacement, spark plug replacement, and parts, was approximately $2,000, with 62% parts and 38% labor. The new air cleaner housing was almost 50% of the cost of this repair.
Toyota Sienna Repair Service
Hoover Auto Repair in Ann Arbor, MI is a Japanese auto repair specialist and provides complete Toyota repair service. Please contact us if you have any questions.



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