If the coolant flow is restricted or non exsistant it will cause hot spots in the bores which will also ignite any fuel/air mix in the bores. I have seen hot spots in bores chewed out by the rings requiring a new block they were that bad. (Mick may remember 1 im talking about.)

Just draining the coolant and flushing with water wont be sufficient, even trying to do a reverse flush wont be enough. I recommend going to your rad shop and asking what chemical they use to do a cooling system clean, the stuff i use is very good, so far its turned 2 apparently good rads into not so good rads but has cleaned the cooling systems very well (moved alot of the crap in the block, the cooling system flushes were asked for by the customers) ........

Do the comp test etc first before anything else to be sure and i'll pm you the cooling system cleaner i use.

cheers Scott

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http://www.youtube.com/watch?v=nDMkVq1jRGU

Can also be a carbon buildup in the combustion chambers that stays red hot for a few seconds after switching off causing the fuel to ignite.

Pete