Hydraulic Tappets

To check the hydraulic tappets, press the unloaded hydraulic tappets (2) with a plastic or wooden wedge (1). Do not use metal tools that could easily scratch the surface of the hydraulic tappet.

Purpose of hydraulic tappets
Due to engine heating, the components of the valve train lengthen. Therefore, a small clearance must exist between the camshaft and the bucket tappets so that the valves close fully when the engine is warm, despite their slightly longer stems. This clearance, the so-called valve clearance, had to be adjusted at set intervals on older vehicles. On the Audi 80 this happens automatically with every valve movement, thanks to the so-called hydraulic valve clearance compensation located in the bucket tappets. Hence the name "hydraulic tappets". For a description of operation, see earlier in this chapter.

Valve noise
Knocking noises in the cylinder head area (see also earlier in this chapter) indicate one or more defective hydraulic tappets.

Valve noise may, however, occur even with fully serviceable hydraulic tappets if:

The tappets are not yet supplied with oil – the engine has not warmed up sufficiently.

The vehicle was recently operated at full load at high outside temperatures.

The vehicle has not been operated for a long time. In this case the valve noise should stop at the latest when the engine reaches operating temperature.

When the oil level is very low, the oil pump sometimes draws in air.

Locating a defective hydraulic tappet
PROCEDURE

Start the engine and let it run until the electric cooling fan switches on for the first time.

Increase engine speed to approximately 2500 rpm for two minutes.

If the valve noise does not stop, switch off the engine and remove the cylinder head cover.

Set the No. 1 cylinder to top dead center (TDC) of the ignition stroke.

Press both bucket tappets next to the No. 1 cylinder down with a wooden wedge. If the tappet travels more than 0.1 mm, it must be replaced (tappets cannot be repaired).

Repeat this procedure for the other cylinders. To do so, set the corresponding cylinder to TDC. TDC is correctly set when the cams of the checked cylinder point upward evenly.

Replacing a hydraulic tappet
PROCEDURE

Remove the camshaft.

Pull out the old bucket tappet.

Lightly lubricate the tappet guide with oil and insert the tappet.

Install the camshaft.

After installing a new bucket tappet, do not start the engine for 30 min! The tappet must first settle. Otherwise the valves may contact the pistons.