I am working on a 1988 Accord (CA) that has been sitting for some time. There was a problem with the cooling system - air pockets and bad water pump bearing - so I replaced the pump and drained the coolant.
The coolant that came out was a brown, rusty colour. I looked at the radiator hoses (which, incidentally, are in very poor condition) and noticed about a 1mm of brown, rust-coloured sludge on the bottom of the hoses. I did a flush of the system with a cleaning product combined with demineralised water and again drained the system. It came out the same colour. I flushed the system again with just demineralised water about 5 more times (filling the radiator up, bleeding the air, running with heater on until hot, draining), but keep getting the same colour coming out - not appearing to get any cleaner. In fact, after 3 drains it appeared to get a little darker.
At this point I'm worried if it's oil intruding into the cooling system, but it doesn't appear "creamy" or thick like oil in coolant photos I've seen before. The substance left on my catch-tray after draining seems kind of pasty and dries almost powdery. Furthermore, the drained water doesn't seem to separate like oil and water probably should (even after leaving a bottle sitting for a few days). It doesn't feel thick or greasy and it doesn't smell - it feels like normal water, only brown.
I checked the dipstick, too, and the oil there seems normal. I also had the rocker cover off recently and didn't notice anything unusual there, either.
The car appears to run fine in all other respects - no issue starting, no smoke from exhaust. I do strongly suspect though, due to the condition of the hoses and old thermostat, that the last owner ran the system with normal water and let it sit for a while.
Could this be oil in the cooling system? Or is it just really old and rusty?
You can see the image below a series of flushes, the first being on the left.