An overnight investigation by Mercedes has determined that Nico Rosberg’s issue in FP3 at Monza was caused by a coolant leak that contaminated the engine.
The problem forced Rosberg to make a hurried switch back to the older spec engine for aualifying, a move that contributed to him only making fourth spot.
A team spokesman told this writer that it was too early to determine whether Rosberg’s orginal upgraded engine will still be useable at a future event.
The team had been concerned that it would find something that might compromise Lewis Hamilton’s Sunday. However it is confident that there will not be a repeat, and nothing is being changed on the World Champion’s car as a result of the investigation.
The part design that failed has well proven and had been in use for some time, and was thus not associated with the Monza upgrade package.
Adam Cooper's F1 Blog 
Why did Rosberg get an old engine? He might as well have had a new engine of the old version. They should be able to last the rest of the season. Strange like they wanted to make it difficult for him. I know it is just me not knowing all the facts but can anybody give the reason?
Think about it. If they gave him a new old one he would be committed to that for four or five races or get a penalty by changing to a new new one. And all that time HAM would have a new new one and be more competitive. So they took a punt on the old old one lasting…
Hmmf so they can not put all of the engines in a pool and use the Monaco engine on Monza etc. If engine 1(old) and 2(old) had lasted for 11 races 3(new) and 4(new) could surely last for the 8. And if engine 3 new is damaged the new 4 will have to last anyway. Or was engine 3 new not one of the 4 yet? This is just as I see it but I am not an A student in mat