Hi Scotty,
I have a 2005 Honda Civic DX with a 1.7l SOHC engine, an automatic transmission, and 183,000 miles.
A few months ago the engine over heated on a long highway trip. The coolant was lower than normal.
I replaced the radiator 5 years ago at 136,000 miles.
I replaced the timing belt, water pump, thermostat (I tested the thermostat before installing), radiator cap, coolant, and chemically flushed the radiator last month at 182,000 miles. I tested for a head gasket leak with a combustion leak detector. The chemical never turned green after 10 minutes of testing. The engine was plenty hot and thermostat was open. There are no visible external coolant leaks. I checked the spark plugs and they all look the same. No white smoke at the tail pipe. There is no milky oil. I've cleaned the overflow reservoir hosing. I've replaced the transmission fluid. I've cleaned the grill of the radiator and condenser. The fans work as expected and cycle on and off normally.
However, the car is still over heats when continuous driving on the highway for more than 80 miles. The car doesn't over heat when driving in town (20-30 minute drives less than 50 miles). I've noticed the overflow reservoir will get very full on drives. The shorter the drive the less full it gets. The longer the drive the reservoir can get 100% full and fill all the way up to the cap. After cooling down the overflow flow reservoir goes back down to a normal level.
I think on long drives the coolant leaks out of the top of the overflow reservoir until the coolant gets low enough to cause the engine to over heat. When I put more coolant in the radiator everything runs great.
What is causing the over filling of the overflow reservoir? Is it a cracked head/head gasket even though all my testing says it's not? Could anything else cause the over filling of the overflow reservoir? Should I try a head gasket sealer (if so which one do you recommend)? Is there a way to check if the radiator is clogged internally or if there is a clog somewhere else in the system? I will be pressure testing the radiator and radiator cap next as well.
Any advice would be greatly appreciated. Thank you for the help and the video.
Do the combustion leak detector again. (assume you did it the right way at first).
(your blue fluid is not 'new old stock' -and not working any more?).
To me, the rising fluid level in the overflow reservoir (more than due to an expanding liquid) points to exhaust gases entering the cooling system, causing ''air' locks' that block further cooling.
Do the cooland hoses swell on/get very firm when the engine overheats?
I've ran the combustion leak detector twice. Both times show negative results. I run the car for about 10 minutes and test for about 5 minutes. The liquid is still blue. If I breathe into the liquid it turns green making me think the liquid is working.
I don't see any swelling in the lines. They do get firmer after running but I would imagine it's due to the system getting pressure. I did a pressure test at 15psi and the lines get firm and feel about the same as when it's running.
Would putting some head gasket sealer into the system as a precaution help seal up minor leaks if there is one? If it would is there a brand that is recommended? Do those sealers hurt anything else in the system?
Also does anyone know what would cause a head gasket to go bad in the first place?
As far as I'm aware this is the first time the engine has every gotten this warm.
Do the combustion gas test again. This time do it with the engine at operating temperature (thermostat open) and holding rpms high.
You'll have to be quick because the coolant will soon want to overflow out of the coolant reservoir. But you want to get the pistons really pumping any combustion gasses through any possible tiny head gasket leak(s) and sometimes idle rpm pressure isn't enough to do that
