2006 Aveo engine heating problems...

[HughCage]

Usually the temperature gauge stays at just under halfway. Lately however, the engine gets hotter than that, rising above halfway and continuously upwards. I have not let it get to the red yet, as I turn on the heat when it gets about 3/4 of the way and it cools back down to 1/2.

• The radiator fan is working properly; it turns on and spins.
• There is enough coolant, it is clean, and it does not leak (Reservoir hisses when opened indicating to me it still holds pressure).
• I have changed the OEM plastic thermostat housing for the two-piece metal one about a year ago.
• Mileage is 171k kilometers.
• There is duct tape helping to seal the air intake hose; I am waiting for a new one to arrive.
• Car also did a cross-country trip recently (6000 km), and the problems started happening near the end of the trip. I know this car isn't meant for such journeys, but it won't be taking another trip like that anytime soon. I only need it now for quick trips in the city to and from work.

Turning on the heat does cool the engine down, but as summer is fast approaching I would like to find the problem and solve it ASAP.

Any ideas, or parts to check?

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