I'd had a slow water leak for a while - until the radiator housing finally cracked, then it got big. Took it to the shop where he put in a new radiator, flushed out all the old black crap including the overflow and the coolant looked great. Clear light green.
A few days age the temp rose on me. Between the overflow and the radiator itself I poured at least 1-1/2 gallons of water in there and the worst part is the cap was full of something like rust flakes. What happened? Speculations, please!

 

Before the development of coolants that could be mixed, the orange Dexcool when combined with the classic green stuff would cause a preciptation of gunk. Also I believe the original (20 years ago) Dexcool if not replaced when it got old would also allow junk to precipitate out.

This junk would clog up cooling systems but also settle in the nooks and crannies of the block cooling jacket.

It is hard to remove properly.

When I had to replace my block heater, I found crud there too after I thought I had done a very thorough flushing. Thankfully it was pretty inert but seeing it there still surprised me.

So, it is posdible that you still have more crud in the cooling system.

For instance, it took quite a bit of effort, to get the crud out of my ovetflow tank and this was so after I removed it to wash in a tub with hot water and a long brush.

 

It was such a joy seeing that clear coolant, hare to hear the honeymoon is over.
I don't see water underneath so I guess it's getting burned up. The engine is running so smoothly though, I don't mind, just need to watch the level.

 

Something yo watch for is the potential for a lower intake manifold (LIM) gasket leak that can allow coolant into your lifter valley and then into your lubrication system. It can ruin a running engine in minutes. This was a fault with the late 90s 4.3L motors.

 

I don't think its getting into the oil but I don't know enough about the manifold system to say where it actually is going.

 

I remember a member here who found a leak in a hose by using a coolant system pressurization tool. He found a pinhole that directed water onto an exhaust manifold which evaporated the water leaving no sign of the leak.

 

Years ago they used to sell a water injection system that made the engines run cooler and smoother. Maybe that's what's happening 🤔

 

Personally I hate Dexcool. Whenever I got a newer vehicle I would do a complete flush of that crud and convert back to the green. But in doing that I would have to flush, fill with the green and come back in a month to do it again because the green would break that sludge loose from the inside. After the second flush it would stay the green.

 

I did a test of modern dexcool and green stuff that were advertised as compatible. I mixed them together in a small bowl and even put small bits od aluminum and steel. I found no reaction at all.

 

There is nothing wrong with Dexacool EXCEPT that so many people have no idea about the lack of compatibility with older coolant formulations. My blazer had top offs mixed and got the brown sludge. Our Malibu went 110,000 miles on OEM Dexacool (8 years) and when I did the scheduled flush and refill everything was pristine. The very few top offs that the Malibu needed were with Dexacool. I think where they screwed up was not making a bigger deal about sticking with the OEM coolant with more warnings etc. I am sure that the thought was not to scare people with skull and cross bones logos everywhere and stick figures catching on fire but it may have saved a lot of people a lot of grief. That sludge is a major PITA.

George