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Help me diagnose this engine problem-please

Hi all, I'll cut streight to the chase.
My 2001 156 selespeed lost all its coolant whilst doing 70ish and I didn't notice for another 7 miles (about 5 minutes) until I saw the temperature and she stated making pinging/tapping noises. I've now fixed the cooling system and it stays at the correct temperature and the oil level was at the max before and somewhat about the max after so no oil starvation has occurred.
The problem is I've got a definate tapping/knocking from the top end, it's not so noticeable at tick over but at all other revs you can hear it. It's there under load and coasting/engine breaking and when hot and cold. I've changed the oil and filter and all spark plugs because the plugs of no. 1 cylinder were black/sooty/wet but the others were fine. The new plugs have been in for les than 10 miles but the plugs of no. 1 cylinder are already darker than the rest, I can also see that the crown of no. 1 piston is wet through the plug holes. I've also got a vibration at tick over that you can feel in the cabin but I know that all cylinders are firing because I've disconnected each coil pack one at a time and you can tell the difference.
Can anyone please offer any advise what could be the problem.
Oh, the engine only has 58000 genuie miles on it cause I've had it from new.
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Whats the compression on nr1 cyl ?
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If the car got very very hot the small end could now be suspect or some scoring has been applied to a piston which would show by smoking at highish revs and a fouled plug. Trouble is its only easy to determine the problem by stripping it down.A compression test may help but old worn engines can have different compressions on each cylinder even if they are not failing.Incidentally try swapping coil packs and if number one still has a dodgey plug odds are its number one cylinder with the issue.
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I suspect that in reality you are looking at getting another engine as they really don't like getting so hot that they start making funny noises. Head gaskets fail, heads warp and blocks distort plus there will probably be damaged caused by the oil being cooked. By the time you have stripped it down, done all the investigation work, done any repairs needed (new cam belts kit as well) it'd be easier, quicker and cheaper to just chuck another engine in.
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