I have been chasing a gremlin in a cooling system of another car I have, its an 03 camry on which I recently had to redo the threads in the block for the cylinder head. (they stripped out) - btw I bought the car with the bad condition and was towed back home.

after the rebuild, the car is running perfect except for one issue which is the cooling system.

during the rebuild I replaced the water pump as the old one was squeaking and was crusted up at the weep hole. Filled up zerex pink coolant. No air bubbles etc. Car runs for a month perfectly and I notice the coolant is a bit dark - a couple of weeks later the water pump fails catastrophically its literally shooting a stream of coolant from the weep hole - caught in time about 300 yards from my driveway.

pushed the car home, removed the water pump - got a replacement under warranty - same GMB pump and I removed all coolant - even pushing out the old coolant from the heater core, It was quite dark - there is something floating in it which does not settle immediately, but takes about 2 days to settle out. - Even a coffee filter does not fully clean it out and the "mud" that collects on the filter paper is like clay but black in colour.

a fresh fill of 1.5 gallons of zerex pink again with new water pump and cap - car is good for about 3 months and now today the water pump is weeping again - coolant is also black again with the same mud.

point to add - I drained about a pint of coolant from the radiator drain and it was clean as new - whatever this mud is, it kind of is on the top side of the radiator. - I cannot see any deposited on the metal of the radiator from the cap opening but the overflow tank has the mud, so much of it that it clogs the pipe nipple on it.

engine temperature is normal on the gauge - as they are buffered on toyota for about 15 degrees C, I checked it with a scan tool and see coolant temperature between 86-95C no matter how hot it is outside or if the A/C is off or on.

Anyone experienced this?

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)