I thought, as you, that the block or head was cracked (may still be) or that the gasket had failed - but they all look OK.
However, my car was overheating really badly. It boiled after only 5 or 6 miles.
I tried to post earlier but something went amis.
I've got a problem in that my coolant seems to take on a black tint. I've flushed out with 60 odd litres of water to clear everything out
and lots of brown then black silt. The water seems as if the black substance is in suspension, which I tought could be exhaust soot.
The engine is one I bought from a guy on locost builders, which had new crank shells and gaskets, so I assume its in reasonable condition.
I've tried reving it a bit and on a few times foam appeared. When reving it bubbles appeared in the coolant, but as I've flushed and
refilled the coolant several times I don't know if thats just air pockets shifting.
Now my thougths are warped head/defective gasket/crack.
I've checked the head bolts and all are fine. The exhaust is clear, no smoke what so ever (not white, blue or black).
It doesn't get exceptionally hot (I've had it up to 80deg C).
There doesn't seem to be oil in the coolant (i.e. no film on the surface once drained or in the header). The oil has no guiness look either.
Any suggestions?
I take it that there is antifreeze in the water, anifreeze inhibits corrosion, the only time I have seen black coolant it was caused by iron corrosion caused by lack of said antifreeze. There may be other reasons, but can't think of any. Anyone?
I've currently got the head off my MK Pinto in an effort to fix overheating. I used a suction oil extractor to get the last bit of water out of
the water jacket and got some black gloop. I tipped the water out to leave the residue in a container - now it's dried out and looks a bit like
coffee grounds, but actually it must be ferrous oxide. Some is very fine, some is little flakes.
I thought, as you, that the block or head was cracked (may still be) or that the gasket had failed - but they all look OK.
However, my car was overheating really badly. It boiled after only 5 or 6 miles.
Yep lots of black coffee grounds.
But after about 2 hrs the water still has some black dust like suspension.
I expect its been using proper antifreeze, but given I'm currently flushing out I've been using water (once the antifreeze was cleared out)
as its not really practical to flush out with copious antifreeze at home. Once I've got the water behaving like water I'll then switch to
proper coolant.
Quite possibly it's so fine it'll stay in suspension forever - let the water evaporate naturally or by warming it in your household's
least favourite saucepan 
I tipped most of the water out of the container and a couple of hours later there was the mix of fine and coarser bits.
Pretty sure it won't be soot, though.
I left it over night.
There is now a layer of black with what seems like a black/brown tinge to the water.
Sounds like just rust and silt now.
I've ordered a universal sniffer kit from ebay just to be sure.
What's that...? Sounds useful.
quote:
Originally posted by bigpig
... universal sniffer kit ...
be rust dude! have seen it loads of times when people don t run engine on the correct anti freeze. happened in a zetec of my mates hes flush it through must be ten times then put fresh anti freez in it and its still like it!
quote:
Originally posted by MautoK
What's that...? Sounds useful.
quote:
Originally posted by bigpig
... universal sniffer kit ...
Looked on eBay - nothing found.
Fogot to add.
I tried Mr Sniffy and he said there was not exhaust gubbins in the coolant.
Now just waiting on the *$"£$ ebay supplier to post the alloy joiners that I was told went in the post 2 weeks ago.