Is your engine feeling shaky? Is the engine light ON? Is your car lacking power during accelaration? Is your fuel consumption higher than normal? Do you experience oil leaks or an excessive oil consumption?
Any of those symptoms could indicate either mechanical or an electrical problem with your engine. Do not let your car run poorly. It is best to have it checked out right away to prevent any possible irreversable damage to your engine.
If your car is over 5 yrs old it may be time for a regular engine tune up such as spark plugs and wires, filters, injector cleaning, engine flash and many other things that will help to keep your engine running like brand new even after 500000km!
On most vehicles with timing belts it is recomended to replace the timing belts, tentioners (and in most cases) water pumps at 160000km to prevent the engine damage.