I expect that a number of you have a car that has an engine which has a nasty habit of making a noise, as if a whole bunch of road workers with pneumatic drills are in the top of the engine, on cold startup. This noise is caused by one or more of the hydraulic tappets not working as they ought to. The hydraulic tappets were fitted to the majority of VW engines from around 1986 onwards (model dependant). There are a number of things that can contribute to the downfall of the tappets. These are infrequent oil changes, use of a non V.A.G./M.A.N./Knect oil filter without a non return valve, and a blocked oil pick up pipe in the sump.
The solution therefore is to care for the engine a little more. The small oil ways in the tappets are prone to clogging up with oil debris, which can come about by using poor oil or skimping on the oil changes. Remember that most V.A.G. engines need an oil change AT THE LATEST every 10,000 to 12,000 miles. If you do a lot of small journeys, where the oil does not reach operating temperature, about 80 degrees or higher, then the oil must be changed more frequently.
When you change the oil it is also advisable to use an Engine Flush, such as the one produced by Wynns (trade only but can buy it across the counter in some garages), as this can also get a lot of oil residue and crap out of the engine. This is placed into the engine before draining out the old oil. Do read the labels properly, as once this has been put into the engine, the engine must only be used at idling speed for about 10 to 15 minutes before draining and the filter MUST be changed. This alone can help a lot of the tappet noise problems. Please note that this is a flushing fluid and NOT flushing oil.
Contrary to some belief, this type of product does no harm the engine. In fact if you have an engine with hydraulic tappets you should be using this at every oil change. You will find that most of the VW garages now use this type of product which an oil change.
Only use a good quality semi or fully synthetic oil, from the major manufacturers such as Duckhams, Castrol, Valvoline, Millers, Mobil.
Even if you have always used a top quality oil, you may still have some residue in the oil system and sump.
As I mentioned previously, only use a genuine V.A.G. oil filter, or one from M.A.N. or Knecht (0C47 for general fit VW/Audi filter). These two last manufacturers make the filters for V.A.G., and you should find that they are cheaper than the ones from the dealers. These filters also contain a non-return valve which some of the cheaper filters do not have. This is to prevent the oil that is up on the top of the cylinder head from falling back down to the sump when the engine is switch off.
It might also be a good idea, if you have a bit of time and a few tools, to drop the sump from the engine (after you have drained to oil out!), and release the oil pump pick up pipe. This should be thoroughly cleaned out, blown through if possible with an air line, and replaced. You will be surprised just how much sludge can be held in this item, despite the use of top quality oil and flushing fluid.
If you have been able to follow all of this drivel, then you might have an engine that will have quieter hydraulic tappets.
If your tappets only tap occasionally on startup then you are probably quite lucky. If your tappets knock for a number of seconds on startup then you should think of taking some of the above action. If you have noisy tappets when the engine is hot, then you have one or more fully shagged (technical term) hydraulic tappets which you will have to replace.
There are also some oil system aditives which can help to quieten down the tappets without changing the oil immeditately (Hydraulic Lifter Concentrate). You simply add this to the oil, run the car for about 500 miles and then change the oil. This is only to be used in extreme cases. If this does not work then tappet replacement is the only answer. Don't try to work out which ones have gone either as this will take you forever. It's much simpler in the long run to replace all of them (Sorry 16v/20v owners).