I've searched the threads and have tried a good amount of the solutions others have posted and checked for the symptoms mentioned.

2004 Honda Civic 1.7L with 80,000 is overheating and losing about an entire reservoir of coolant between driving anywhere from 30 to 200 miles. The level drops to way below minimum and stays there whther the car is hot or cold. Also the level of coolant in the radiator doesnt always stay at the top. No white smoke from tail pipe or under the hood. No residue around radiator cap. No problems in the oil either.

Replaced thermostat, coolant sensor, and radiator cap. Independent shop claimed my car failed the block test and I may need head gasket. Honda dealership performed pressure test several times, no issues detected. Tested head gasket, no issues detected. They found no leaks and saw nothing that should be replaced other than what had already been done. Bled the system refilled it, gave the car back, said it had air pockets got rid of them and the problem persisted. When the temperature rises it goes toward the red zone but I've always shut off the vehicle before that point. However, the temperature fluctuates pretty quickly so i turned on the heat and one time the heat began blowing cool air. The gauge moves up and down within seconds, possibly a little quicker than the actual temperature can change??

And randomly my power locks on my driver door stopped working altogether via remote and door lock switch. And my headlights seem to dim slightly when I hear my cars fans come on. Its very slight almost unoticable, but being its my car I'm very attentive to the smallest things. Could be unrelated, just figured it's worth mentioning.

However, I just noticed coolant splattered under the bottom of my car on the side where the radiator and coolant reservoir is located. (Not sure why Honda didn't see green residue).

Idk what to make of all this information combined, especially the conflicting results of the tests performed. And both shops saying I didnt have an external leak yet coolant underneath my car.

I still have the warranty with Honda Care so hopefully the problem gets taken care of with little cost to me, the hard part is figuring out whats goin on! I'm getting fed up...I love this car but I'm tempted to trade it in for the '08

 

I would agree with the shop that said there is a leak in the engine such as the head gasket. What you are experiencing is consistent with a relatively minor leak. Sounds like it is small enough to leak only while actually driving. The pressures in the engine then are much higher than anything you can do testing with the car sitting still.