Likely Causes: Engine intake gasket
Manifold Intake Gasket types
If your engine is fitted with the earlier type shim steel tin gasket then leakage to both the inlet manifold and the water jacket is almost certain. It should always be replaced with the composite inlet manifold gasket type because of overheating issues that may arise in long traffic jam situations as well as causing engine running issue due to weak intake mixture.
Symptoms of Leaking Intake Gaskets
As with head gasket leakage normally you will suffer no more than the need to occasionally top up your coolant system. This will become more so if you make longer journeys in very hot weather situations, perhaps in longer traffic-jams, then overheating will be a major issue. You will also notice worsened engine idle ability when hot.
The best Choice is to replace the intake manifold gasket with the composite type. Or replace the one you have already. This will also give you the opportunity to take a good look inside your engine to asses its condition especially with regard to the camshaft condition etc.
Leaking or blown intake manifold gaskets can not only cause persistent engine overheating but also instigate other more serious engine set up and efficiency problems and should be replaced especially if yours is the original fitment from new or if it's of the shim steel type itís a great opportunity also to take a simple but deeper look into your engines actual condition.