Car has the heater core bypassed and it overheated?

Hi,

The first thing is to make sure the engine coolant is full. Next, if the coolant is full, I need to know if this happens only in stop and go traffic or does it also happen when driving at highway speeds?

Next, pay attention to see if the cooling fan is turning on when it starts getting warmer. If it does and the coolant level is full, I would suspect the thermostat is bad and not allowing coolant to flow.

Here is a link that explains how in general to replace a thermostat:

https://www.2carpros.com/articles/replace-thermostat

Here are the directions specific to your vehicle (2.2L 4 cylinder engine) for thermostat replacement. The attached pictures correlate with the directions.

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2004 Chevrolet Cavalier L4-2.2L VIN F
Procedures
Vehicle Engine, Cooling and Exhaust Cooling System Thermostat Service and Repair Procedures
PROCEDURES
Thermostat Replacement

Removal Procedure

pic 1

1. If equipped with an automatic transmission, remove the exhaust manifold.
2. Drain the cooling system.
3. Remove the thermostat housing to water pump feed pipe bolts.

pic 2

4. Remove the thermostat housing to water pump feed pipe.

pic 3

5. Remove the thermostat.

Installation Procedure

pic 4

1. Install the thermostat.

pic 5

2. Install the thermostat housing to water pump feed pipe.

pic 6

Notice: Refer to Fastener Notice in Service Precautions.

3. Install the thermostat housing to water pump feed pipe bolt.
Tighten the thermostat housing to water pump feed pipe bolt to 10 Nm (18 inch lbs.).
4. If equipped with an automatic transmission, install the exhaust manifold.
5. Fill the cooling system.

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If you do replace the thermostat, that is the time I would recommend flushing the cooling system. Here is a link that explains how that is done:

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

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Let me know if this helps. If you can think of anything other that may help me narrow this down, let me know that as well.

Take care,
Joe