Rust in Fresh Water Cooling System

[toddtat]

Hi All -
Have an issue with rust colored residue in cooling system. No particles when strained through a paper towel, just rust colored. Settles to the bottom of tank when cold, and mixes into a dirty colored liquid when running and system in use. Recently drained system, pulled heat exchanger for cleaning and added new coolant, and the dirty material is back. 7.4 Gi engine. Any tips on how rust or residue could get into FW cooling system would be greatly appreciated.
Thx!

 

[Thalasso]

Are you using a coolant. There shouldn't be any as the coolant has rust inhibitors

 

[toddtat]

Yes, using a 50/50 coolant. That is why I'm scratching my head. Thx for any advice!

 

[muc]

If you didn't flush the system, it's possible the coolant that you used isn't compatible with the anti-freeze that was in it.

 

[Scott Danforth]

If you mix green and orange coolant, you get a brown sludge

 

[QBhoy]

Hi.
A sensible go at it would suggest or suspect you have a bit of cross contamination with the 2 cooling systems. Usual suspects as follows.

Good start would be at all interfaces between the two.
That would be any coolers which may have seals gone or have corroded through.
If you have a hot water system ran off the engine but thinking your domestic water wouldn’t be that bad.
Any cooled manifolds perhaps.