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Rusty water in Radiator


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75 02.  Stock motor.  Ran warm (between middle and red line of water temp gauge) so I replaced thermostat and installed brand new aluminum radiator from Top End Performance, about a year ago.  She "generally" runs cool when full of coolant/water (temp gauge stays halfway between cool and middle of the gauge, even after driving her for over 100-mile trips).  Lately, noticing car temp gauge is right at middle of gauge, and occasionally above.  Every time this happens, which is about once every few weeks, I check radiator level when cold.  A couple of times, water level was not visible (below the level of the radiator core).  I add water and/or coolant, and the temp gauge goes back to where it is typically.  I tried checking the radiator a couple of time when the water temp arose, just after car has cooled down a bit.  The water level was either below the core or just at the same level as the core.  


It looks like I'm going through water, but the tale-tale signs of a leaky water pump or blown head gasket are not there (no little drip marks; no white smoke; no smell of antifreeze; no visible signs of water leak.  I've seen condensation coming out the tail pipe, but I can't imagine I blow out that much water through the exhaust.  Last few times checking water level, the water has been rusty.  No idea why.  Brand new hoses (all of them) and brand new aluminum radiator were installed within the last year.  


Any one come across similar issues?  Any idea why water in brand new radiator is rusty?  

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Mercedes makes a citric acid based coolant flush, I had rusty coolant 2 years ago. Two flushes later it's fine. Temp runs at 4 o'clock, 3 o'clock in traffic.

Andrew Wilson
Vern- 1973 2002tii, https://www.bmw2002faq.com/blogs/blog/304-andrew-wilsons-vern-restoration/ 
Veronika- 1968 1600 Cabriolet, Athena- 1973 3.0 CSi,  Rodney- 1988 M5, The M3- 1997 M3,

The Unicorn- 2007 X3, Julia- 2007 Z4 Coupe, Ophelia- 2014 X3, Herman- 1914 KisselKar 4-40

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If you did not flush the system completely and use some kind of cleaner you will still have some rust coming loose from the engine block after you replaced the radiator.  Try the above MB coolant flush (it really works) be prepared it could cause your water pump to leak or other places that rust build up is acting like a sealant.  Put a catch can on the radiator overflow tube so you will know if it is blowing the water out (usually running hot or a headgasket problem) if the water is not in the catch can and you can't find an external leak you need to check and see if there is water in your oil. 

1970 1602 (purchased 12/1974)

1974 2002 Turbo

1988 M5

1986 Euro 325iC

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