My Laguna has the same issue (97 1.8 RN) and on lookung into it on thinternet it seem that the most likely reason is that the matrix gets clogged up with collant goo. The solution seems to be to remove the matrix hoses from inside the engine compartment on the bulk head and to flush the matrix for quite a while with a hose (with a bit of water pressure). The outcoming water should contain sludge and some calcium deposit. Word of caution. the hoses are retained with a clip which is like a half an arrow head through a loop. This can be removed by pressing on the arrow head whilst pulling the retainer through the loop but be careful not to break. Supposed to be ok when you know how. Refitting is much easier. On removal make sure you dont lose any sealing rings.
Need to say that I have not doen this myself yet but this is the described solution I found.
Pete O
January 2007
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