I just purchased my 5th Mini Cooper project car, a 2009 MCS. The person that I purchased it from said the car overheated, so he changed the thermostat out. When he put the car back together, it wouldn’t start, so he sold it. When I pulled out the plugs to check compression, I saw coolant in the cylinder 3 and 4. I hand cranked the motor over and got a face full of coolant with what looked like sand in it. I pulled the head off and there was a sand-like substance packed around the cylinder cooling towers and a black plastic like stuff blocking the water port from the water pump to the cylinder head, it looked like something had melted inside of the engine. Oh, and when I took off the valve cover the inside of it was completely melted, like it was on fire a some point.
After a bit of research, I believe that someone tried to fix a leak in the coolant system with powdered coolant system leak fixer and it went way wrong. Has anyone ever heard of this happening? Do you think that there is permanent engine damage?
I was able to clean the lower block up pretty well. I will replace the water pump, timing chain, valve cover, head gasket, and make sure the head is within tolerance (and replace that if needed). Would love to hear thoughts on whether I should replace the engine or not.

 

Unless you see obvious signs the the block is toast I'm thinking the only way to know if you need to replace the engine is to button the engine up and being sure it is ready to run start it and let it run and see what happens.

But before you do this you will have to remove all coolant leak fixer stuff from the cooling system. A backwards flowing flush of the block, head, radiator and heater core is called for.

The engine and head cooling passages and the radiator tubes and heater core tubes want to be free of that leak fixer stuff as they can be.

Be sure you drain the oil and replace it with fresh oil and replace the filter too.

If the engine sounds healthy after running it awhile drain the oil and refill with fresh oil and replace the filter. I'd be tempted -- more than tempted -- to when the engine has cooled down to drain the coolant from the cooling system and replace it with fresh coolant. To this end you could at the start of this just fill the cooling system with water since you know you are going to drain this out after some engine run time.

If the engine doesn't sound healthy well, you can decide it you want to fix the problem or replace the engine.

If you fix or replace the engine you want to be sure the cooling system is as free of that stop leak stuff as much as it can be.

 

The block looks like its in pretty good shape. I will do a warp test of the cylinder head tomorrow to see if I am going to replace that or not. I think I agree with everything you provided. Thanks for the input. I will pressure wash the cooling system to make sure there is good coolant flow through all of the ports. The head scares me a bit because the ports there are so small. Ill put it back together and see if it all tests out OK with no overheating noted. Worse case, I pull the whole engine and put another one in it. I have only lost 2 weekends of time and the cost of the head gasket.

Thanks for the input.