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2006 Toyota Avalon needs antifreeze 2x a week

When you switch on the AC you get cool air but only last for a brief moment and the engine get overheated. Antifreeze burns off but no leaks detected. .
When you switch on the Heat the same thing happens.
Sounds of liquid moving around in engine when driving. I have to refill antifreeze twice a week and travel with an extra Gallon of Antifreeze .
The Mechanic says it needs a new engine ???.
Without both the Engine runs fine.

If there are no visible leaks, then the most obvious cause of the problem is a breached head gasket.

You might just need to have the head gaskets replaced. However, if you have allowed this situation to persist for an extended period of time, then it is possible that the engine has been killed by the contamination/dilution of its oil with antifreeze.

It may well run “fine” for a while… until it doesn’t.
You need to ask your mechanic for a specific explanation of what he found when he examined your engine.

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Without both what?

I can understand why the engine overheats when you turn on the AC. I would suspect the radiator fans are not coming on. I don’t understand why it overheats when you turn the heat on but not with the heater off. I can see where you might have a leaking heater core and not notice it. That would explain the sloshing water sound. The core is leaking water into the compartment with the heater core and evaporator core and is draining out the evaporator core drain. This drains out under the car at the bottom of the firewall any you could easily miss it.

Are there any signs of coolant puddling under the car in the morning? Any signs of coolant leaking from the water pump or other places in the engine compartment. If not, after the engine is fully warmed up is there any white-ish smoke coming from the tailpipe at idle? What if a helper follows you as you accelerate up a freeway on ramp, any white smoke out the tailpipe then?

What explanation does your shop have to say you need a new engine? Having to add coolant isn’t a sufficient reason. At least not until the cause is diagnosed.