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I've read through the Overheating Common Causes thread stickied at the top of the forum, and a couple others. I think my situation is different from what I've read, although I am not very knowledgeable on cars...

Here is what I know:

1) I lose all my coolant when it is 100 degF outside, and I've been driving on the highway for a bit

2) Pressure test for a few hours, doesn't lose any pressure (also put on a new radiator cap and it didn't fix the problem)

3) After losing the coolant, it looked like it had sprayed all over the engine, not just a particular spot. I think it may have been from the overfill cap. I recall seeing the overflow tank filled with coolant after I had pulled over due to a slightly increasing temperature gauge.

4) Driving thousands of miles since the summer, I haven't had a single problem. The problem ONLY happened when it was ~100 degrees outside, multiple times.

Because the problem doesn't happen without the temperature conditions, I can't troubleshoot it but I'm looking for ideas.

Thanks!

 
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Have you checked to make sure your radiator fan is coming on?

 
Rep Power: 518

Year?
Model?
Miles?
A ton of miles equals a ton of bugs and dirt.

Clogged radiator internals from lack of coolant maintenance.

Or clogged fins on the radiator and condensor. I'd do a bunch of cleaning first, and it could mean pulling the radiator fans to reach in with a garden hose sprayer from the engine compartment side. Washing from the front only pushes everything in tighter. Wash from behind, toward the front.

 
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Sorry, forgot to include the information.

2005 Honda Civic EX. 85000 miles

Radiator fan does come on.

What type of coolant maintenance is usually recommended? Flushing every ___ miles?

 
Rep Power: 518

Read your owners manual. Look in the section called "maintenance".

Wild guess: Probably 5 years on the factory fill, and every 3 years thereafter.

Oh. It's one of THOSE cars.
Head gasket job. Pretty dang common.


I'd also look to cleaning the radiator and condensor (free), and a factory thermostat (cheap).

 
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From what I've read, having a blown head gasket will result in some symptoms, regardless of temperature. I haven't had any symptoms for 5 months of anything wrong...just when summer comes back in full swing.

Can a blown head gasket be the problem but not be causing any other noticeable issues without the temperature?

 
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Did you ever check your radiator like Ezone said to do? Have not thought of that as an issue here in Ohio. States like Texas amongst others' have tons of rocks and definitely could cause that problem.. You are right with the HG, the car would be getting hot regardless of temperature once at normal running temp. Do a pressure test if really worried but you should notice loss of coolant over time if that was the problem.

 
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Maybe, maybe not. I guess the symptom you see would depend on the heat dissipation capacity of however much coolant is remaining in the cooling system, and the amount of added heat load there is at higher ambient temps. A head gasket on these typically causes only a certain amount of coolant loss, then it stabilizes at a level that is a bit under full, and that's enough to have reduced cooling capacity. The cars have very little reserve cooling system capacity, so any little loss of coolant becomes a major problem pretty fast when it comes to trying to shed BTUs in high ambient heat.

If you rarely use your heater, you may never notice it blows only lukewarm at idle.


Go out and check your coolant level after 3 months of normal driving without any overheating. Pull the radiator cap off and look in the radiator. Is it completely filled to the top?


Does the radiator cap hold the proper pressure?
If the engine has overheated, there could be steam erosion damage in the neck of the radiator that keeps the cap from making a proper seal.

How about that thermostat? And cleaning the radiator? Any fin damage?

 
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A cooling system pressure test doesn't normally show the problem (quickly) that these engines have.

I like to apply shop air pressure to the cylinders and watch for coolant level to change. Thats the definitive test for me in this situation, but there are many more ways a head gasket can fail and all require different checks.


There are other relatively simple things that need to be checked on the OP's car first.