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Hydraulic valve tappets

Hydraulic valve tappets

Description

The valve gear incorporates hydraulic tappets which keep the valve clearance within a limited working range of 2.05 mm (0.081 in), which is between 18.75 and 20.8 mm (0.738-0.819 in) from the camshaft bearing seat to the end of the valve stem.

Each tappet has two storage chambers and one high-pressure chamber. The opening of the port between the high-pressure chamber and the adjacent storage chamber is controlled by a spring-loaded ball valve. A return spring in the high- pressure chamber acts on the sliding piston so that any clearance between tappet and cam is eliminated. At the same time, the high-pressure chamber expands and makes up the oil volume to compensate for leakage losses through the passage between the piston and the cylinder which occur at actuating pressure. Under some conditions, the hydraulic tappets may give rise to a chattering noise of varying intensity. In most cases, this noise will be short-lived and no remedial action will be necessary.

Details are given in the following of the most common types of noise, the conditions under which the noise occurs, the likely cause of the noise and preliminary action which may be taken to eliminate the noise.

Noisy valve operation


1. Valve chatter after the car has been parked for longer than 48 hours.

During a period of prolonged parking, oil drains out of the high-pressure chambers and allows air to get in. For a period of about 15 minutes after the engine has been started, valve chatter will be heard until the air has been purged from the valve tappets.

Engine speed should not be allowed to exceed 3000 rpm until the noise has ceased.

See point 4.

2. Valve chatter on starting the engine after an oil change.

If the noise from the valves did not occur before the oil change, it will disappear of its own accord as soon as the oil reaches normal operating pressure.

3. Temporary noise on starting a cold engine.

This noise is perfectly normal and will disappear of its own accord as soon as the oil reaches normal operating pressure.

4. Valve chatter on starting after work on the car that has involved turning the engine over by hand or on the starter motor.

The noise is due to one or more of the tappets having been drained of oil and will therefore disappear when the engine has been running for a while. At worst, it may take up to 15 minutes at 2000-3000 rpm for it to disappear. At lower engine speeds it will take much longer to expel the air. At higher engine speeds there is a danger of damaging the valve tappets.

5. Noise after fitting new valve tappets.

Cause and rectification, see point 4.

6. Noise occurring after a short period of idling when the engine is hot. If the engine is run at 1 500 rpm or more, the noise will disappear after a while.

The noise is due to low oil pressure in the hydraulic valve tappets when the engine is idling.

7. Noise which occurs at high engine speeds and disappears when the engine has been idling for a shorter or longer period.

This noise is due to an excessive amount of air in the oil at high engine speeds. The ingress of air is caused by leakage on the suction side of the oil pump, such as a leak in the inlet pipe.

8. Noise from an individual tappet, regardless of how the car is driven.

The most likely cause is a particle of dirt having become trapped in the non-return valve of one or more of the tappets. The best way to identify a faulty tappet is to switch off the engine, remove the camshaft cover and press on all tappets not under load. A light or spongy feel indicates a faulty tappet.

Replace the faulty tappet or tappets.