Maybe, I'm just not sure what the rated pressure of a heater core might be. I'm sure its designed to take the rated pressure of the radiator cap installed about 20psi max. The garden hose has the capacity to creat about 70psi, I think, could be wrong. Done cautiously with good flow control one might flush the core using that technique. I have seen pictures from GM cars with Dexcool mutations that looked like heavy mud. A good flushing might be the ticket, if none of the other issues are the answer.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician