Kohler KDI 2504TM Workshop Manual page 44

2
The operating principle of the hydraulic tappet is based on
the incompressibility of the liquids and on controlled leakage.
The oil under pressure enters the tappet chamber A,
providing a constant supply of oil in the low-pressure
chamber.
Through the non-return valve, 4 the oil can only access the
high-pressure chamber B and exit via the clearance between
the piston 3 and the tappet body 5 (controlled leakage).
The chamber B is filled when the rocker arm is on the base
radius of the cam and the spring 6 keeps the piston 3 against
the valve stem, thus eliminating any system play. Thanks to
the spring extension, the tappet "extends", creating a small
depression in the chamber B, making the non-return
valve 4 open, and allowing the oil in the chamber A to pass
to chamber B, restoring the proper amount of oil required to
eliminate any play in the valves.
2.16.4.2 Difficult operating conditions
For proper operation on the hydraulic tappets it is essentia that the low pressure chamber of the piston 3 is
always full of oil.
In some conditions this may not occur (due to the fact that the oil leaks away when the engine is switched
off, which can also partially drain the tappets). This situation will be the cause of clearances that will result
in a characteristic noise similar toa ticking sound.
1. When the engine is cold, the tappet filling time could be very long if the oil used is not suitable for
the specific environmental conditions
2. If the engine is very hot: at idle speed, oil pressure may be low, and small air bubbles could form in
the circuit. Because of this, this compressing the tappet slightly and producing valve play which is
responsible for the ticking sound. On account of this, the tappet compresses slightly giving rise to a
valve clearance, thus generating a slight ticking sound, which however disappears rapidly (MAX 10
seconds) once normal operating conditions have been restored.
Anyway the duration of ticking Anyway the duration of ticking sound must be MAX 30 seconds. If not, the
problem is surely due to the poor quality of the oil, wear or impurities that, transported by the oil, can
infiltrate between the ball valve and its seat inside the piston, compromising the operation of the tappet
itself; In these cases, the only solution is to replace the oil or hydraulic tappets.
The prolonged persistence of the ticking sound or abnormal noise must be investigating in order to prevent
any malfunctions; if necessary, replace the hydraulic tappets and engine oil.
EN 44
TECHNICAL INFORMATION
(Tab. 2.2)