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Old 05-10-2020, 09:54 PM
Joe's Garage Joe's Garage is offline
Chief Ponti-yacker
 
Join Date: Oct 2016
Location: Las Vega$, NV
Posts: 640
Default 99% of the time, your situation is caused by combustion gases entering the coolant.

Actually, it's probably really 100%.

Leaking gasket (doesn't need to be blown). Did you re-torque the heads?
Cracked head or block or ??

You can verify with one of those test kits where you drain all the coolant, put in straight water with their chemical additive and then let the engine idle.

If combustion gas is present in the water, the chemical changes color.

Good luck.