What is the brown sludge in my water coolant and why has my engine lost power on my 05 Renault megane coupe cabriolet

For manual try Renault owners forum's

inside or outside? passenger foot wells.
inside id heater coil
out side is easy, that which is (name it) leaking is bad or needs service.

get the OBD checked by a suitable mechanic save time and hassle

Is it over heating? If there is oil in with the coolent, It could be a cracked head gasket on the engine, Or evan a crack internally in the engine block,

This is the result of the newer type of long life coolant breaking down chemically, it forms a brown sludge, flush the system and you will be OK

electric aerial megane cabriolet 2002 - Renault Forums ...

www.renaultforums.co.uk > ... > Ask the Experts > Electronics
Dec 8, 2013 - 3 posts - ‎2 authors electric aerial megane cabriolet 2002 Electronics. ... 2002 Megane 1.6-16v cabriolet, well on drive as no garage lol. tynesidelad is ... Take it out and check it ,hopefully enough room to remove it from below,(is it motorised?). Megane mk1 cabriolet
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replace thermostat it may help tell me if it does

There is no real solution to this problem apart from actually taking it to the renault garage and explaining to them what the problem is, from there onwards they will actually do a water test on your car it can actually be anything in between small adjustments on the windows of your car or resealing the roof glass.

check spark plugs and coils

Your clutch is either wet with oil , or you are riding it oo much and making it hot. Push it all the way in when you are stopped. Don't hold it part way, or you will burn it out.