Sludge in cooling system

It can be caused either by mixing the wrong kinds of antifreeze together or alternatively its oil. Oil would tend to mean your head gasket is on its way south. Suggest you get a pressure test done on the cooling system, that should tell you.

I.

alo wrote in news:liiff1peukmnile7tl7u36featnkn2oe8f@

Negligence with regard to changing the coolant regularly. A very bad thing with these engines, I'm afraid. Has it had a new head gasket, by any chance?

Drain the coolant. Remove the thermostat. Flush the engine through both ways with a hosepipe. Do it with the heater set at cold first, then again set at hot. No point in flushing excess crap through the heater matrix. Repeat same process for radiator. That should clear most of it. As it's summer, you could add a flushing agent and refill it with just water, then run it for a week and change it again with anti-freeze. Normal anti-freeze needs changing every two years because after this it stops protecting the engine from rust, hence your rusty brown water.

BTW, don't forget to replace the thermostat after you've flushed the engine.

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Most likely oil. That engine is notorious for failing head gaskets. Fix before it gets worse.