Check under the supercharger.


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Not an expert on the 4.2 but I've heard of the hoses under the supercharger being a common issue. I'd check them....not sure if the 4.2 has the same waterpump connector issue that the 5.0l has? Maybe check?

 

take off the SC then order the correct parts.
if you have a valley pan also replace with a new part,
and if you have rear cylinder head water bridge replace it ,
as well as any O rings.
use DC111 on all O ring connections,
this will keep the rubber soft so it wont leak

 

So, my 2007 4.2 has started losing coolant somewhere.

...everything points to a leak in the cooling system. But I can't see any sign of one....

Has the cooling system been repaired/replaced since new?
With 17 years, it would be expected for leaks to be developing somewhere, not a catastrophic problem, but normal wear.
I've replaced all the cooling system parts in my 2003 years ago (like... 10 years ago maybe), come to think of it it's probably time to start thinking of replacing parts again (before anything fails).
AND I've replaced the cooling system parts on my 2012 RR and my wife's 2012 Jaguar XJ with the same engine (both naturally aspirated though) earlier this year, to avoid failure.
When I did remove the old parts (from the 2012s), I did find a couple of the plastic pipes & fittings getting weak & brittle, and a few trace spots of corrosion, almost forming into potential leaks.

Most common spot for small leaks is the coolant pump and it's connections or weep hole where it will leak by-design when the bearing wears.

 

Thanks everyone. I'll try and look more closely around the water pump. Sounds like I'll be taking the supercharger off though!

Has the cooling system been repaired/replaced since new?

I don't believe so. The car is approaching 180k and it's probably time to proactively replace a few components (water pump, belts/tensioners, alternator). This is actually the first engine problem I've had in 5 years of ownership - it's been super reliable & I've never even removed the engine cover before.