Two common causes are intake gaskets and cylinder head gaskets.
Most times this can be isolated by doing a compression or cylinder leakdown test. Otherwise, the intake needs to come off before removing heads....Carefully inspect the intake gaskets around the water passages for signs of coolant bypass or deterioration. If you find any, likely that is the problem. If you find any engine oil in the cooling system, that points to a head gasket.
Reason for this common problem is simply that aluminium belongs in race cars not on street cars...It's growth rate is too radical and causes too much lateral movement, as well as swelling and shrinkage for gaskets to remain sealed. Gasket manufacturers have dealt with this pretty well but it has not been solved completely.
Intakes leak at a rate of about 4 to 1 compared with head gaskets from what I've seen, so most likely that's where you will find the problem.
Coolant can cause bearing damage so after completing the repair, make sure you change the oil BEFORE starting the engine. In severe cases, I use cheap oil, change it, run it at idle, then change it again, to purge any that remains inside lifters and galleys. Then use a good quality oil when finished. (don't forget the filter!)
good luck
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