An engine valve works in an intense environment, travelling up and down dozens of times per second in a gaseous atmosphere hot enough to auto-ignite pine wood. The valve slides up and down inside a cylinder head's valve guides with no more than a human hair's worth of clearance. Over time, carbon deposits and other impurities can cause valves to stick and make noise.
Valve sticking will typically manifest as a clicking noise that regularly occurs every two to three seconds at idle. This noise may be especially pronounced at start-up when the engine is cold. The clicking will most likely increase in frequency with as the engine rpm increases, but may or may not get louder. In severe cases, the valve may hang open and cause a cylinder misfire or even hit the piston and damage the engine.
Sticking valves are almost always caused by resin deposits left by oil caught in between the valve guide and valve. These deposits are the result of impurities in the oil overheating in the space between the valves and guides. In addition to resin, cooked carbon deposits can also work their way into valve guides, resulting in a similar phenomena. Extremely worn valve guides can also result in sticking, but are more likely to manifest as engine misfire or spark plug fouling before sticking.
Treating the Intake
Extra valve mass can combine with resin deposits and worn valve springs to cause valve sticking, a condition indicated by gradual loss of power followed by engine misfire and valve clacking. Extra valve mass comes from carbon deposits that build up on the back of the valve, which you can largely eliminate by cleaning your engine with a solvent such as Seafoam or GM Top End Cleaner. The process involves introducing the solvent to the intake tract while the vehicle idles. You may need to repeat the procedure a number of times before the carbon breaks loose.
Treating the Oil
Oil deposits are the root cause of valve sticking, so the simplest solution is to eliminate the deposits by flushing the oil and running a solvent through the engine. Begin by changing the oil and replacing it with a cheap, very thin, single-viscosity 5W or 10W oil. Add a quart of Dextron III transmission fluid or oil-flushing solvent, then run the car at highway speed for about 25 miles. Drain the oil and repeat two to three more times until the oil comes out very thin and clear. Re-fill the engine with a synthetic high-mileage oil that contains seal conditioners to reduce the amount of oil entering the valve guides.