Does not have to be impeller fragments that are causing the blockage.

.....but it´s till i had no water flow out of the exhaust. I changed the impeller set and the pressure was as high, that my hose was cut imediately. Perhaps the hose was old and couldnt handle high pressure, but the water press gauge showed curious data too.
I wanted to test my water press gauge, now its broken

IMO you need a new hose and possible a new water circ pump and double check for restrictions like I asked in the t stat / housing and necks and the exhaust / manifolds and risers - water jackets. just my 2 C s

i have a new hose, but i wont destroy it. I think the circ pump cannot be the problem. When the t stat is closed the engine doesn´t pick up new water. The pump will only pump around and when i´m not in drive (0mls/h) and 5000 rpm there is no problem.
All goes through a small bypass direct to the exhaust when it´s cold.
I ordered a new raw water pump and will make a test drive at the weekend. I´ll reflush it once more and see what will happen.
I´´ll keep you informed.
Thanks so long for helping

I cannot see the pieces getting thru the coolers as the holes are not big...back flush each, but you may have another issue

Since psi guage is in block, only thing after it (block) is heads, intake, t-stat housing, exhaust.

That's where the problem is going to lye.

Are you sure head and intake is possible? When the t stat is closed the engine doesnt pick up new water. It´s only going round (water pump - block). IMO it could only be blocked in the t stat housing or exhaust. I cleaned the t stat housing and sand blasted it as good as i came in. I´ll reflush the exhaust and see what happens. Unfortunately i´ve broken the water press gauge. So i will have to improve.

When your motor is working correctly:

When the tstat is closed some water goes back into engine and much goes into exhaust.

It's a beautiful design, that Merc t-stat housing is.

It recirculates water back to the motor to warm it up quicker and to keep engine water temps more consistent, yet flows water to the exhaust full time.

What he said. The raw water pump is always pumping, otherwise the exhaust would melt down immediately.

Had a couple stuck thermostats cause enough pressure it burst the hose from the circulator pump to the tstat housing