If the radiator cap is in good condition and the coolant is nonetheless decreasing with no visible leaks from the outside, the problem is indeed serious. This may be caused by a defective cylinder head gasket. At this stage, you should consult a specialist.
If the radiator is leaking, replace it with a new or remanufactured one. If the radiator has been repaired by the owner or a workshop, it must be pressure tested to at least 1.4 kg/cm2. To do this, close the outlet port with a suitable plug and use another port to supply compressed air to the radiator at a pressure of 1.4 kg/cm2. Immerse the radiator in a bath of 90°C water. Leave it for 15 minutes and pressurize again to 1.4 kg/cm2. Watch for bubbles indicating leaks. If the radiator has not been tested after a pressure repair, it should be monitored very closely for several weeks after installation in order to detect possible leaks in time.
Radiator mounting elements (1998-2002 models)
1 - radiator;
2 - upper radiator mount;
3 - radiator cap;
4 - upper radiator hose;
3 - expansion tank hose;
6 - expansion tank;
7 - elements of the condenser fan of the air conditioning system;
8 - casing;
9 - fan motor;
10 - radiator tube (only models with automatic transmission);
11 - fan;
12 - bottom mount.
Radiator mounting elements (2002-2004 models)
1 - radiator;
2 - upper radiator mount;
3 - radiator cap;
4 - upper radiator hose;
5 - expansion tank hose;
6 - expansion tank;
7 - fan of the condenser of the air conditioning system;
8 - fan motor;
9 - casing;
10 - fan;
11 - lower radiator mount;
12 - radiator tube (only models with automatic transmission);
13 - upper radiator mount (only models with automatic transmission).
Withdrawal
Drain the coolant as described above.
Loosen the upper radiator hose and lower radiator hose clamps and remove them.
Disconnect the hose from the expansion tank.
Disconnect all electrical wires from the radiator fan and fan on sensor
Disconnect the cooling system pipes preventing the radiator from being removed from the vehicle.
After unscrewing the top mounting bolts, remove the radiator together with the radiator fan (not all models).
Remove the radiator from the engine compartment by first removing it from the lower radiator mounts.
Flushing
Flush the radiator. If the radiator is removed from the vehicle, flush it in the opposite direction as well. Rinse the outside of the radiator with a hose with a strong jet of water. If you suspect that the radiator is partially clogged, flush it with a chemical solution before removing it from the vehicle. If nothing has changed, check if the water passes through the radiator normally. In half a minute, 20-25 liters should pass.
Leaks can be temporarily eliminated by using special additives to the coolant or by covering part of the radiator with special mastic. However, it is better to replace the radiator.
Installation
Radiator installation is made in an order, the return to removal.
Check for cracked hoses, replace if necessary.