Table of contents: What are hydraulic tappets used for? ↓ Valve knocks ↓ Finding a faulty hydraulic tappet ↓ Replacing the hydraulic tappet ↓
What are hydraulic tappets used for?
Due to the engine heating, the components of the valve train are stretched and lengthened. Therefore, there must be a small gap between the camshaft and the plate tappets, so that even when the engine is hot, the valves close tightly, despite their slightly longer stems. This gap, the so-called valve clearance, had to be adjusted at certain intervals in older cars. In the Audi 100, this action occurs automatically with each valve cycle thanks to the so-called hydraulic valve clearance adjustment, located in the plate tappets. Hence the name "hydraulic tappets".
Valve knocks
Knocking noises in the cylinder head area indicate one or more defective hydraulic tappets. However, valve knocking may also occur with fully functional hydraulic tappets if:
- the tappets are not yet supplied with oil - the engine has not warmed up enough.
- the vehicle had recently been operating at full load in high outside temperatures.
- the car has not been used for a long time. In this case, the valve knocking should stop at the latest when the engine reaches operating temperature.
- when the oil level is very low, the oil pump sometimes sucks in air.
Finding a faulty hydraulic tappet
Start the engine and let it run until the electric cooling fan starts for the first time or, on a 5-cylinder engine, the coolant temperature gauge reaches over 80°C.
Increase the speed to approximately 2500 rpm for two minutes.
If the knocking does not stop, stop the engine and remove the cylinder head cover.
Set the first cylinder to TDC of the ignition moment.
Press down both plate tappets near the first cylinder one after the other with a wooden wedge. If the tappet moves more than 0.1 mm, it must be replaced (because the tappets cannot be repaired).
Repeat this procedure with other cylinders. To do this, set the corresponding cylinder to the TDC position. TDC is set if the camshaft cams on the controlled cylinder point upwards uniformly.
Replacing the hydraulic tappet
Remove the camshaft.
Pull out the old plate tappet.
Lightly lubricate the guide of the new pusher with oil and insert the pusher.
Reinstall the camshaft.
After installing a new plate tappet, do not start the engine for 30 minutes! It must first "sit down". Otherwise, the valves may collide with the pistons.
First turn the engine over manually.