Engine overheats on highway?

The radiator can do that; however, the oil wouldn't have gotten dirty as a result. Did they maybe find a creamy substance in the oil that would indicate a bad head gasket?

As far as the heat not working, if there wasn't enough coolant in the engine, it won't produce heat.

What I recommend is if you can, replace the radiator yourself. You should be able to get one for under 200 dollars. I feel 400 in labor is nuts.

You don't need to do anything. As far as the oil, wait until you can check it.

You don't need to do anything. As far as the oil, wait until you can check it. If there is coolant near the water pump, it most likely bad too if it is the original one.

Let me know if you have other questions.

You need to provide some details or observations, otherwise there is no way to diagnose this over a computer. Bypassing the heater core will make any overheating worse. That's a small radiator that you just took out of the system.

What are the indications it's overheating? Do you see bubbling in the coolant reservoir? If so, does that occur within a couple of minutes of starting the cold engine or not until you've been at highway speed for a while? If there's bubbling in the reservoir, is there steam too? Does the upper radiator hose feel really hot? It should be too hot to hold onto for very long. Feel across the radiator for cold spots. Is the shroud around the radiator fan missing?

If the heater core is leaking, it can allow air to get into the system. If that air pools under the thermostat, (same as if a cylinder head gasket is leaking), it will prevent the thermostat from opening. Thermostats must be hit with hot liquid to open.

When the engine is hot, feel across the radiator to see if there are cold areas indicating some of the tubes are plugged. If the entire surface is hot all the way down to the bottom, the radiator isn't giving up its heat to the surrounding air. That is due to corroded and crumbling cooling fins. Also try pinching the heater hoses. The heater core poses some restriction to coolant flow. With it bypassed, coolant may be circulating too easily through those hoses and is not going through the radiator.

Start with a cooling system pressure test for any external leaks, also test pressure cap, replace thermostat and get the radiator core cleaned, not a back flush. Start here and let me know if you have any improvement.

If heater is not producing hot air, you may have a waterpump problem.

Otherwise perform a compression test, then cooling system pressure test.

That sounds a little high, but I don't know where you live. Replacement will be around 100.00 in parts and the book calls for 7.7 hours labor.

Let me know if you have other questions.

That's a pretty good indication of a blown head gasket, probably caused by a previous overheating. I would have it chemical tested to verify that.