The real cause of injector rattle?

[powerandtorque]

If you've just replaced the 'stat with a new Peugeot 83 degree one and the gauge is still showing 90 degrees, I'd be double checking that the gauge isn't lying. That, and that you've definitely fully bled it.

PP2000 or any half-decent diagnostic tool should show you the coolant temperature value.

All the HDi's I've owned or driven have always showed 80 degrees or a smidgen over on the gauge, which would tie up with an 83 degree 'stat.