Abstract: The camshaft tends to change in reaction to pulses which it experiences during its normal operation, and it is permitted to change only in a given direction, either to advance or retard, by selectively blocking or permitting the flow of hydraulic fluid, preferably engine oil, through the return lines (194,196) from the recesses (132a, 132b) by controlling the position of a spool (800) within a valve body (798) of a control valve (792) in response to a signal indicative of an engine operating condition from an engine control unit (808). The spool is centered during normal operation when the hydraulic loads acting on its opposed ends by the action of springs (802, 804) that also act on the opposed ends of the spool are in balance. During periods of operation when system pressure is low, a biasing mechanism forces the spool to its full advance position.