A rough idle and even an emissions inspection failure for high HC and CO can be caused by improperly adjusted valves. Also, exhaust valves may become to tight which can lead to valve failures. Our technicians tell us that regular valve adjustment inspections are a must.
Improperly Adjusted Valves May Cause Emissions Failure/Rough Idle on Toyota 4Runner
Average mileage: 221,617 (180,000–289,000)
9 people reported this problem
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rattle on acceleration maybe valves
Idles poorly but runs well at higher RPM's. Won't pass smog due to high hydrocarbon reading.
Rough idle when I'm at a stop... no CEL, though...