Help! High temperature and black gunk in coolant?!

[summerhenderson]

Hi there!
Hi all!
I have a t25 Caravelle 1988 water cooled 1.9 diesel.
My temperature gauge sits quite high when the van is running, I’d say about 80/90% of the way up. The van seems to run fine and it never gets so hot that the warning light comes on.
Is this normal/ any ideas or tips which may help me get the gauge back to the centre or what may be causing it?! The light on the dashboard works by the way.

Alongside this I have found black stringy gunge like stuff in the bottom of my coolant tank, it was disturbed when I added more fluid. What is this, it doesn’t look like oil thank goodness?!
I also want to add new coolant but not sure what had been used previously as the current fluid looks like water only, can I just add any new coolant in?
Thanks so much.
Thanks

[Stesaw]

If it was oil it would be floating on top of the water in your tank. If it's not overheating then maybe there isn't an issue? I don't have a Diesel so I don't know how hot they run. 
Maybe the system could do with purging out, the radiator might have gunk in it. Or the system coulda had a leak in the past and someone put some sealant stuff in with hopes to plug it without replacing bits.

 These are just guesses.

[Mocki]

The thing is , has it always run at that temp , or is it a new thing ? 
the gauges on these vans are quite wide ranging , normal is anywhere from just over a 1/4 to just under 3/4 and As you have a non standard engine, it may just be a gauge / sender mismatch . Prob best to get a thermometer to find out what the actual real temp is ?

as for the black stringy stuff , it quite possibly sealant from a previous muppet mechanics ? 

[davidoft1]

The black stuff is probably just settler goo, etc , clean it out , see if it returns , you need to
See if it’s as hot a gauge is suggesting

Does it use much oil or Water ?