I have an automatic 1995 Honda Civic with 275,000 miles. As of recently, it has an overheating issue. Overheating and coolant boiling out of the overflow tank happened initially after a hot day (90F) of driving up and down passes for ~5 hours. I know some about cars, but could use advice from someone with more expertise!
TL;DR My Observations

• Radiator cap fell apart and a small spring dropped into the radiator and it hasn’t been removed, though it has been attempted. Radiator cap was replaced after this one broke.
  
• Car only overheats after being driven for a while (1-2 hrs)
  
• Overheats more quickly when driving slowly, idling a lot, i.e. stop and go traffic or city driving
  
• Coolant bubbles out of reservoir tank when overheating, and appears to be boiling. It starts to bubble/boil from reservoir tank before the engine overheats.
  
• Heater in cabin stops blowing hot air as engine temp gauge goes up, and starts blowing hot again as temp gauge goes down. Temp gauge goes down rapidly and seemingly at random when driving on highway
  
• Since the initial issue began, it has passed several extensive shop leak and pressure tests at a reputable Honda Dealership
  
• Radiator bubbles coolant at random intervals when car is running, appears to be air bubbles and not boiling
  

My leading hypotheses:

• There is a small crack in the engine block or the head gasket is beginning to go, but the cracks only open up after the engine heats up significantly. This is where the air bubbling out of the radiator comes from, and this also allows the coolant to boil because it drops the pressure.The air being sucked in from the crack in the engine or head gasket causes back pressure on the coolant and pushes it back towards the radiator and overflow tank, causing the bubbling and eventually boiling as this malfunction heats up the engine.
  
• Exceptionally high pressures reached when driving on hot day, radiator cap fell apart and got stuck and restricted coolant flow which built up pressure and caused a leak or bounced around in the system and caused a leak and potentially damage further through the system. Overheating was caused when small leak caused pressure to decrease and boiling of coolant to occur. Now, when driving after this event, the coolant must reach a certain temperature before it boils and air bubbles get bigger and become obstructive (which is why I can drive short distances without overheating). When bigger air bubbles move through the system, coolant flow is interrupted and the cabin heat stops working, and the engine begins to overheat. When the air bubble passes, or the spring moves from its obstructive position, the coolant begins to circulate again and cabin heat begins to work, and the engine cools.
  
• There is a small spring from the radiator cap in the radiator, but this isn’t an issue. Driving in the hot day caused a small leak to form in a hose or other piece of the coolant flow system, which allows the coolant to boil once it reaches a certain temperature.
  

Long story: After the initial overheating issue, I took it to a Honda Dealership where it was worked on by a technician with 20 years experience on Hondas and other cars. They diagnosed it initially to be the old radiator cap, and it was replaced. The old radiator cap partially fell apart when before or while it was removed, and a spring was missing that fell into the radiator, but I don’t know when it fell in. Could be when it was replaced, could be years ago; I haven’t needed to remove or replace the radiator cap in a long while. After driving for 2 hours after the cap was replaced, again in hot weather (~80) I was in a small amount of stop and go traffic, and it overheated again and when I opened the hood, I could see the coolant boiling out of the reserve tank. I let it cool down, turned up my cabin heat, then turned to drive back to the shop. It began to overheat again and at the same time as the engine temp gauge started to go up, the cabin heat stopped working. As I was looking for a spot to pull over, the gauge suddenly dropped back to normal temps and the cabin heat began to work again. I drove it 1.5 hrs back to the shop with no further overheating issues, until I got off the highway. When driving was slower and I idled at a few lights, it began to overheat again. Thankfully it was close enough that I could just roll into the shop. They tried to remove the spring from the radiator but weren’t able to. They performed a leak test and a pressure test, holding pressure overnight, and both passed perfectly. The shop insists that even though it passed these tests, there could still be a leak in my engine block or head gasket that only opens up at pressures higher than that used to test the engine in the pressure test. Because they also saw air bubbling out of the radiator (not boiling, but bubbling) when they ran the car with the radiator cap off, they insist that the only place the air could be coming from is a crack in the engine or head gasket. I’m not so certain; does anyone have other ideas?

 

Classic signs of a blown head gasket.

 

Initial overhead may have been the radiator cap. However, the repeated overheats likely warped the head to cause the head gasket to fail causing permanent overheating issues until fixed.

 

this would also be my guess ...

 

sounds like a stuck/flakey thermostat is what caused the initial overheat and the spring from the cap is just an aw crap. Unfortunately you MAY have wraped the head or POSSIBLY blew the head gasket driving it with the stuck t-stat. You MIGHT get lucky though, these engines are fairly tough and can take a beating within reason. Replace the thermostat/ bleed the cooling system before you do anything else. Then follow these directions
1 Do a compression test, anything over 135 is fine.
2 If it passes the compression test, with the engine off and cool, take the radiator cap off and start the car. CAREFULLY watch the radiator fill hole. If coolant shoots out the fill hole then its a blown headgasket or a cracked head, if it does not shoot coolant, go buy a lottery ticket cuz luck is on your side
If it is the head gasket, its not the end of the world, just a greasy mess to replace, there are 2 bolts that are a pain to get to, the rest is easy. The intake manifold bolt at the lower back center is a pain, and the lower power steering mount bolt is a pain, after that its just an afternoon to fix.
Price wise
Raidiator (at that milage it needs it) $60
headgasket 30
Head 300
valve cover gasket 15
oil/filter 20
timing belt kit 45.
This is presuming you have the d15b7 engine

Been there done that, for about $500 you will have a running car
Good luck