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A few years ago, my Z had a similar problem. Even though the radiator cap spring is not seized, I've found the seal can fail and allow hot coolant to overflow. Try replacing the cap and seeing what happens.

 

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My failure happened during warm weather, so the heater was not an issue. it could be the water pump is the problem, but that's just pricey speculation.

 

Same issue here currently. Please update the thread with whatever steps you take and the results so others can benefit. Is your lower rad hose on the driver side getting hot after you reach operating temp?

 

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Check Thermostat. I had almost exact same issue and replaced thermostat and my problem was solved. Thought it was my heads too. It can't hurt to try since the part is real cheap. Also you need to purge the heater hose at the firewall also.


Hope this helps.

**Edit** read you might have white smoke? If so then yeah its the most likely venting into the cylinders. Do a compression test to see which side is the culprit. Can pick one up for cheap on Amazon.

 

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