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My truck just went in for repairs so I might as well get my honda fixed too..

2001 Honda Civic EX 144000

It overheats and spews coolant out the reservoir ONLY during a nice day (70+ Degrees) I've been driving it like this for a year with no other problems except my starter went out the other day.. Call me crazy but I drove over 350 miles with the temp gauge on the H in 100 degree weather last summer..

It did not overheat once in the winter which makes me think if it was a head gasket it wouldn't have mattered and would have still overheated?

My catalytic converter has also been rattling like nuts for over a year and I had a mechanic "clean" it out...... I read something about catalytic converters can make your car overheat..?

 
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Most definitely. If coolant is being forced out of the overflow, you have a head gasket leak. No question. I had this exact problem and no other symptoms. The exhaust is under far more pressure than the coolant system so when there is a SMALL leak in the head gasket, those gases force there way into the cooling system and displace the antifreeze in any way that it can. The simplest place for the coolant to go is into the overflow tank and then onto the ground once it fills up.

Unfortunately a compression test may not show this problem. My compression was fine according to Honda, but due to these symptoms, I had the head gasket replaced and the head milled to spec and have had zero problems since.

 
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Also, your car DOES NOT HAVE TO OVERHEAT for the head gasket to be bad. If you've gotten to the overheating stage, your leak would be considered moderate to severe.

 
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Here we go again . I'm also most certain its your head gasket, but you can troubleshoot other options (rad cap. etc). You can have a very small head gasket leak and be overheating jponto07, it all depends on how long you let it go on for. A small leak left for 6 months will displace enough coolant to cause the car to overheat, whereas a moderate or severe leak may do it in a week or less. It's not the actual headgasket that causes your car to overheat, its the displaced coolant.

 
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I'm having this same exact issue with my '01 EX.

It shows no signs of a head gasket failure other then the overflow will fill after a few weeks of driving.

I've pressurized the cooling system, and it'll hold all day. I let the car sit all night, and watched the tail pipe in the morning, nothing out of the ordinary as far as I can tell.

It never runs hot, and the heat always works fine.

I've replaced the rad cap, but that didn't stop it.

I'm going to get a chemical leak test to see if combustion gasses are indeed getting into the cooling system.

With 225k miles on it, I'm having a hard time convincing myself to tear into it when there are zero drive-ability issues.

If the only thing negative that is happening is some exhaust gasses getting into the cooling system and then being released into the overflow, is this really big enough cause to replace the head gasket?

 
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I went through the same thing. Its your gasket or your head is warped. Mine was a warped head..