What are hydraulic lifters used for?
Due to the heating of the engine, individual parts of the valve mechanisms expand and lengthen. Therefore, in order for the valves to maintain their tightness, despite their slightly elongated rods, there must be a small play or gap between the camshaft and poppets. This backlash - the so-called valve clearance - in older car models had to be adjusted at regular intervals. In the Audi A4, this is done automatically every time the valve is actuated by means of so-called hydraulic valve clearance equalization, which is carried out by poppet tappets. Hence the name «hydraulic pushers».
Actuation of valves by means of hydraulic tappets: oil under high pressure is shown in red. The pink color indicates the amount of oil not used to actuate the valves. The numbers indicate:
- 1 - camshaft cam;
- 2 - space of the oil collector;
- 3 - return valve;
- 4 - oil supply;
- 5 - high pressure chamber;
- 6 - valve stem;
- 7 - pressure spring;
- 8 - cylinder;
- 9 - piston;
- 10 - hydraulic pusher.
Noises in the valves
Knocking in the valves in the area of the cylinder head may indicate one or more defective hydraulic tappets.
But noise in the valve can also occur with serviceable hydraulic pushers if:
- shortly before starting the engine, the oil supply to the hydraulic pushers was not yet provided;
- the vehicle has previously been driven with full load at a high outside temperature;
- the car was not driven for a long time. The noise should disappear when the engine reaches operating temperature;
- When the oil level is extremely low, the oil pump sometimes sucks in air.
Determination of a defective hydraulic pusher
1. Start the engine and let it run until it reaches operating temperature.
2. Raise the speed for 2 minutes to approximately 2500 rpm.
3. If noise in valves does not disappear, then switch off the engine and dismantle a head of the block of cylinders.
4. Turn cylinder No. 1 to TDC of the ignition moment.
5. Press all poppet pushers of cylinder No. 1 down with a wooden wedge. If, with the hydraulic pusher pressed, it is possible to insert a template 0.20 mm thick between it and the camshaft, replace the pusher.
6. Repeat the same process on the other cylinders. To do this, set each cylinder to TDC. This is the position where all the camshaft lobes of the cylinder under test are equally facing upwards.
Tip: To replace a defective hydraulic poppet, the camshaft must be removed. After installing a new poppet pusher, do not start the engine for 30 minutes! The poppet must «sit down». Otherwise, the valves will hit the piston.
To check the hydraulic tappets, press with a plastic or wooden wedge (1) on an unloaded hydraulic pusher (2). Metal tools that can easily scratch the surface of the pusher should not be used.
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