i've had some recent issues with my mk2 gti 8v. apart from ongoing idle speed problems, i recently changed the distributor cap and rotor arm, only to find the previous cap faulty and therefore running the engine on 3 cylinders. since replacing these parts i've been getting a constant ticking (mechanical), which increases speed in relation to rpm, from what seems to be the head area. what is valve lash? could this be my problem or what about fuel pressure or mixture? any help would be appreciated.