PRE-RELEASE
Hydraulic tappets
| Hydraulic 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 cam to the valve. Each tappet has two storage chambers and one high-pressure chamber, which is sealed against one of the storage chambers by means of a spring-loaded ball valve. A return spring in the high-pressure chamber acts on the sliding plunger so that any clearance between the tappet and the base circle of the cam is eliminated. At the same time, the high-pressure chamber expands and makes up the oil volume to compensate for leakage losses in the gap between the plunger and 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 most probable cause of the noise and preliminary action which may be taken to eliminate the noise.
Valve noise
| 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 tappets' 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.
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| 2. |
Valve chatter on starting the engine after an oil change. If valve chatter did not occur before the oil change it will disappear of its own accord when the engine's oil pressure reaches its normal operating level.
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| 3. |
Temporary noise on starting a cold engine. This noise is perfectly normal and will disappear of its own accord when the engine's oil pressure reaches its normal operating level.
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| 4. |
Valve chatter when the engine is started after work has been carried out on it involving turning it 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.
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| 5. |
Noise after fitting new valve tappets. Cause and rectification, see point 4.
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| 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 cause of this noise is low oil pressure in the hydraulic valve tappets at idling speed.
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| 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 suction pipe.
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| 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 the tappet. The easiest way to identify a faulty tappet is to switch off the engine, remove the camshaft cover and press on all tappets not under load. The tappet that feels light (springy) or spongy is the faulty one. Replace the faulty tappet or tappets.
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