Parts for car recovery and other

More BMW issues. Sometimes I wish I had my dad’s gift of not hearing (or is it not acknowledging) car problems. 

Michele’s car has a tick that’s at a frequency of  ‘RPMs / 2′ so at idle it’s ‘(720 RPMs / 2 ) / 60 secs’ or 6 ticks per second. There’s only a couple possibilities for ticking at that frequency: a valve lifter or a valve. Piston pins or rod bearings would tick once per crankshaft revolution, or 12 times per second. Valve lifters are activated once every two crankshaft revolutions as follows:

piston goes down with intake valve open and exhaust valve closed for the intake cycle intake valve closes and piston goes up for compression (potential click of intake valve or lifter) spark plug ignites and piston goes down for combustion with all valves closed piston goes up as exhaust valve opens for exhaust cycle, exhaust valve closes when piston reaches the top (potential exhaust valve lifter tick)

Hence the term “4 cycle engine”. So why do the valve lifters tick? The lifters are hydraulic, a steel cylinder with a central plunger filled with oil that acts as a cushion between the valve stem and the camshaft lobe (lifters are also called “cam followers”). The valves are spring loaded, so when the concentric cam lobe pushes down the lifter, the lifter in turn pushes down the valve stem to open the valve. If the lifter is working correctly, the hydraulic pressure keeps the clearance between the camshaft lobe, the lifter surfaces, and the valve stem at around zero throughout the complete range of the valve opening and closing. But, if the lifter is worn or has small dirt particles caught in it, the hydraulic oil pressure in it will leak out too fast, and when the valve spring drives the valve up and closed, a clearance will open between the lifter top and the cam lobe. When that clearance catches up and closes, there is a tick. Simple eh?

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