Hi,
l have a peugeot 306 1.9 diesel 2001 when l went to check the water for the rad l noticed that it was full of black oily sludge.
What is the best way to flush this out and how did it get there?

sounds like the head gasket has gone.has the car over heated at all

Head gasket has probably gone. Even if you do clean it out you will just keep getting the same problem, head gasket needs to be replaced. Happened to my girlfriends 106, so l assume it is the same with yours.

Suspect your head gasket has gone is the first place to start - oil does not pollute the water under normal circumstances - remove the oil filler cap and see if there are any signs of a yellowish white sludge in the cap - if there is then you can almost certainly suspect the head gasket.

Sorry mate, the head gasket has gone.
Try not to drive it until it is fixed as you could do more damage.

Sell It NOW.

Oh dear,
If there is oily sludge in the radiator, then the head gasket has popped.
If it is a case of sludge with white oily bits, then it has been over-heaten.
The next bit is - has the oil on the dipstick got droplets of water ?

If so then it is need of a total fluid drain, a new head gasket, or possibly a head skim if it is no longer flat.
That should have been bolted down properly at the previous service / inspection.

Water & oil live close to each other in an engine, & should not mix.
There is Peugeot, or Pudjits as they r called in the trade.
l would sell the darn thing in the local rag as quickly as possible.
Press the middle of the bonnet. It is made of bacofoil.

Now is your chance to buy a DECENT automobile.

check your water gauge to see if its running hot, sounds like your head gasket has gone