Join Date: Mar 2010
Location: Stoke On Trent
Car: Rover 45 1.8 iXS Auto - Now Dead.
A tappet is a cam follower - it is what connects the valves to the camshaft cam lobes. Its job is to take up the slack between the 2 parts. The slack varies when the engine is hot as parts expand with heat. It fills with oil under pressure from the oil pump and its little piston is pushed outwards against the valve stem and takes up the slack. If the pistons in the tappets stick or get seized, they wont work properly and will not take up the slack, or not allow enough slack, causing compression loss around the valve seats due to the valves not quite being able to close properly.
The best bet is for you to change the oil and then add some Wynns hydraulic valve lifter treatment from the motor factors. Leave this several weeks to allow it to try to clean the tappets and valve gear.
If this doesnt work, then you will have to strip the head and either overhaul the tappets, or replace them - not forgetting there are 16 of them, and they are not cheap!