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Hey guys, new to the civic world. Recently bought a 2004 Honda Civic with 200k miles. It’s my car I drive back and forth to work. For the first 8 or so months everything was perfect. Recently the car has been shifting hard into third gear, almost feels like it’s down shifting. Once it shifts hard my car instantly starts to overheat. If it shifts normal my car never overheats. It seems to happen more in the morning than in the afternoons. I’ve had the thermostat replaced, new radiator, new hoses, new cap. After all this replaced it still happens. I’m lost at this point. Please help if you have any input. Thanks!

 
Rep Power: 225

Head gasket failure is extremely common in these Civics, read up in the thread below. Is the check engine light on? What codes?

https://www.civicforums.com/forums/3...reference.html

 
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No CEL no codes. My mechanic left it under pressure for almost 48 hrs and it never dropped. He thinks the head gasket is fine.

 
Rep Power: 225

They can pressure test the coolant system all they want, doesn't prove the head gasket isn't bad. Did you actually read the thread I linked? These cars don't fail normal head gasket tests. The first post in that thread has two different tests you can run to prove if yours is bad.