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.Google+
Car Problem Reports
Toyota 4Runner Improperly Adjusted Valves May Cause Emissions Failure/Rough Idle
Toyota 4Runner Problem
Average Mileage: 187,926 mi (180,000 mi - 195,853 mi)
Visitor, 1992 Toyota 4Runner, 195,853 mi
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...