After the cooler spent night in citric acid it got few rounds with MrMuscle. While waiting MrMuscle to do its job I changed the spark plugs.
This was the worst one, it was quite loose so that's why it was so dark. All of the plugs were worn and on their last leg. You could measure their gap with calender. Still had some time to kill, so headlights got some attention. First with 2000 grit sanding sponge, then 4000 and last with cutting wax.
Much better... Next time have to buy a polishing set and clear coat.
So after much waiting and several flushing, cooler slowly started to flow some water. If you zoom in, this is where we started.
And finally it was time for pressure washer. It work'd well, just adjusted the water spray to pointy and shot dark shit on the wall. After 15 minutes washing cooler was brand spanking new inside.
It runs really smooth, it has never ran so well in our ownership. In slow speeds it used to have slight judder, did read that clogged EGR does that, but can't really say is it thanks to new spark plugs or clean EGR. Waiting to see if it makes difference to MPG.