Hello there.

My Stilo 1.9 jtd multi wagon takes ages to get warm inside when fans on with full temp setting.

It's my first Fiat so I don't know if they're just slow at heating up or what. I heard that if your thermostat has gone your MPG will also plummet??

A diesel engine takes significantly longer to warm up than a petrol. Lots more metal to heat up first.
Running the fan on a high speed before the coolant is warm will extend the warm-up time.

Locate the thermostat and top hose from engine to radiator.
From cold, start engine and allow to tick over. Keep feeling the top hose to assess warm-up. Ensure hands are kept away from moving parts, and away from the cooling fan, it may cut in.
It should remain cold for some time, with a little heat creep from the engine, then suddenly feel warm as the stat opens.
If it warms up steadily, stat is stuck open.
Also check the heater hoses to see how quickly, if at all, they warm up. Might have an airlock/empty heater hoses. Look for bleed screws.

If stat is stuck open, warm-up will take longer and more fuel will be used during this time, although this is less noticeable on a diesel if it has mechanical injection.

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