The OM603 was running great with no problems in 3 or 4 long drives I've taken recently. Needle always below 100c no matter what.

I did notice the MB blue coolant in the reservoir darken a bit, and some brown at the bottom of the reservoir tank.

That was bugging me, so I decided to change coolant.

Drained and filled with water a few times to remove all the old coolant, pulled the reservoir and cleaned it too.

Filled with tap and ran it a bit, got some rust again. Squeezing the rad hoses, I could feel the crunch of rust buildup inside.

Since the cooling system was performing very well despite all this, I decided to just try a flush and rinse, fill with coolant, and call it a day.

I couldnt find my leftover citric acid powder so after much research I wanted to try Fleetguard Restore Plus. It has citric acid, and described as chelating also, which interested me greatly since this is how Evaporust works and I've tried that and found it to work very well.

So I got a gallon (smallest size) at the local Cummins place, and in the parking lot drained some water from the rad, added a liter (mixing ratio is 1 liter/10 liters water), and drove home.

You need to keep it at operating temp for 2 hours and I got home before that so I left it running over dinner then drained it all after.