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Home Workshop: Aprilia cooling system maintenance (liquid-cooled engines)

On Minarelli and Aprilia engines, the water pump is mounted externally on the alternator cover. On Piaggio engines, the pump is mounted inside the crankcases and is driven by a shaft running off the oil pump.

The heated coolant passes upwards to the thermostat in the cylinder head and through to the radiator. The coolant then flows across the radiator core, where it is cooled by the passing air, then down to the water pump and the cylinder water jacket where the cycle is repeated. The thermostat is fitted in the system to prevent the coolant flowing through the radiator when the engine is cold, therefore accelerating the speed at which the engine reaches normal operating temperature. A coolant temperature sensor, also mounted in the cylinder head, transmits information to the temperature gauge on the instrument panel. On Minarelli engines, a coolant circuit from the cylinder head supplies the heater mounted on the side of the carburettor.

COOLING SYSTEM – draining, flushing and refilling:

DRAINING

Remove the cap from the coolant reservoir slowly (see illustration). If you hear a hissing sound as you unscrew it (indicating there is still pressure in the system), wait until it stops.

Remove any body panels as necessary to access the water pump and hose unions on the right-hand side of the engine. Position a suitable container beneath the engine, then loosen the clip securing the lower coolant hose to the water pump, or the pump union on Piaggio-engined models and detach the hose (see illustrations). Allow the coolant to completely drain from the system.

FLUSHING

Flush the system with clean tap water by inserting a garden hose in the reservoir filler neck. Allow water to run through the system until it is clear and flows cleanly out of the detached hose. If there is a large amount of rust or sediment in the water, remove the radiator and flush it out separately.

Reconnect the coolant hose to the water pump and secure it with its clip.

Fill the cooling system via the reservoir filler with clean water mixed with a flushing compound. Make sure the flushing compound is compatible with aluminium components, and follow the manufacturer’s instructions carefully. Fit the reservoir cap.

Start the engine and allow it to reach normal operating temperature. Let it run for about five minutes.

Stop the engine. Let it cool for a while, then remove the reservoir cap.

Drain the system once again.

Fill the system with clean water and repeat the flushing procedure.

REFILLING

Attach the coolant hose to the water pump and secure it with its clip.

Remove the appropriate bodywork to access the cylinder head.

On Minarelli and Piaggio engines, a bleed valve is located either on the upper edge of the cylinder head or on the top of the coolant hose union (see illustration). Pull off the valve cover, then loosen the valve and attach one end of a suitable length of rubber hose to it. Place the other end into the coolant reservoir.

On Aprilia engines, note the location of the bleed screw on the top of the thermostat housing (see illustration).

Fill the system all the way up to the MAX level line on the reservoir with the proper coolant mixture.

Note: Pour the coolant into the reservoir slowly to minimise the amount of air entering the system.

On Minarelli and Piaggio engines, as you fill the reservoir, air will bubble up from the hose attached to the bleed valve. When the system is full and no more air is coming out of the tube, tighten the bleed valve and remove the hose. Fit the valve cover and install the reservoir cap. Where fitted, loosen the bleed screw on the water pump to release any trapped air, then tighten the screw securely (see illustration).

On Aprilia engines, loosen the bleed screw to release any trapped air, then tighten the screw securely. Install the reservoir cap.

Start the engine and allow it to idle for two to three minutes. Flick the throttle twist grip part open three or four times (so that the engine speed rises), then stop the engine. Loosen the bleed valve or screw to allow any air trapped in the system to come out, then tighten it securely.

Warning: Make sure that the scooter is on its centrestand and that the rear wheel is off the ground before starting the engine. If necessary, place a support under the engine to prevent the rear wheel contacting the ground.

Let the engine cool then remove the reservoir cap. Check that the coolant level is still up to MAX level line. Top-up the reservoir if necessary, then refit the cap.

Check the system for leaks. Do not dispose of the old coolant by pouring it down the drain. Instead pour it into a heavy plastic container, cap it tightly and take it into an authorised disposal site or garage (see Warning section at the beginning of this article).

This workshop routine is taken from the new ’Aprilia – SR50, Rally, Sonic & Habana/Mojito 1993-2009’ manual (Haynes 4755). Visit www.haynes.co.uk for more information.

WARNING

Do not remove the reservoir cap when the engine is hot. Scalding hot coolant and steam may be blown out under pressure, which could cause serious injury.

Do not allow antifreeze to come in contact with your skin or painted or plastic surfaces of the scooter. Rinse off any spills immediately with plenty of water. Antifreeze is highly toxic if ingested. Never leave antifreeze lying around in an open container or in puddles on the floor; children and pets are attracted by its sweet smell and may drink it. Check with the local authorities about disposing of used antifreeze. Many communities will have collection centres which will see that antifreeze is disposed of safely. Antifreeze is also combustible, so don’t store it near open flames.

CAUTION

At all times use the specified type of antifreeze and always mix it with distilled water in the correct proportion. The antifreeze contains corrosion inhibitors which are essential to avoid damage to the cooling system. A lack of these inhibitors could lead to a build-up of corrosion which would block the coolant passages, resulting in overheating and severe engine damage. Distilled water must be used as opposed to tap water to avoid a build-up of scale which would also block the passages.