The valve gear incorporates hydraulic lifters which keep the valve clearance within a limited working range from the camshaft bearing seat to the end of the valve stem. Each lifter has two storage chambers and one high pressure chamber. A return spring in the high pressure chamber acts on the sliding piston so that any clearance between the lifter and the cam is eliminated. At the same time, the high-pressure chamber expands and makes up the oil volume to compensate for leakage losses through the passage between the piston and the cylinder which occur at actuating pressure. Under some conditions, the hydraulic lifters may have a chattering noise. In most cases, this noise will be short lived. This may occur after the car has been parked for longer than 48 hours. When the car is parked for a long period of time, oil drains out of the high-pressure chambers and allows air to get in. When the car is started, it will take about 15 minutes for the air to get pushed out of the lifters and the valve chatter to go away. Engine speed should be kept under 3,000 RPM until the noise goes away.
NOTE: It is very difficult to determine which hydraulic lifter is causing the clicking noise when only one fails. You can attempt to isolate the noise by using a stethascope on the cylinder just above where the lifter is but even this solution leaves something to be desired. The easiest way to assure yourself of finding the problem is to replace one lifter bank at a time worth of lifters (maybe not the cheapest way though). By doing this you may replace the one bank and if he noise does not disappear you can move them to the other side to cure the problem. Be absolutely sure that the problem is a lifter issue and not a problem with the oil pickup being clogged which would cause and issue with poor oil circulation to the lifters.
Geert wrote: To determine if the clicking noise is due to a lifter problem or the oil pick ups being clogged, you simply drain some engine oil, fill the equivalent with diesel oil or engine oil cleaner, let the engine run at idle for 20 minutes (better let the engine warm up before you start this procedure, this is more effective). Change the oil and filter immediately
afterwards, replace by full synthetic oil and a new quality filter. You can run the engine for 3K to 5K miles and go back to the regular oil. With me this work perfectly, immediately after the cleaning procedure the chattering noise disappeared, and never came back! You can save a lot of time and money with this procedure!
NOTE: We do not recommend driving the car or revving the engine up to high RPM doing this as it could break clogs loose and disperse them throughout the engine. Keep the idle low and let the chemicals do the work. Then drain several times to be sure the broken particles do not remain in the engine.
Pricing for VALVE LIFTERS HYDRAULIC (engine parts internal)