Radial engines are quite a bit tougher in this game than in-lines. On multiple occasions I've had a damaged and whining Pratt and Whitney turning for at least the 30 minutes it took to get back to base. A Merlin typically seizes up instantly or quits within five minutes.
The black smoke effect you are seeing is just oil burning. Coolant would appear more like a fuel leak if it was modeled.
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