I have 150k on my 03 Civic it has overheated several times. I can drive the car around town without it overheating, but when I drive it on the highway for an hour and it will overheat. I thought I had a blown head gasket turns out I do not. I pulled the car apart and pulled off the cylinder head. I took it to a machine shop and they checked it for flatness I got an okay from the machinist. I was told that if it was a cracked head that one of the cylinders would be steam cleaned. Nothing of the sort here. I am now left perplexed, what else could cause an overheating issue. I've had the water pump, stat, rad and fans checked and replaced. Two master techs suspected a small crack in the head gasket. Should i get a second opinion on the cylinder head being cracked? How about a cracked cylinder wall or a cracked block? How might one check for such things, such as tests. I poured blue devil engine sealant in this car before and it was a huge mistake. When I pulled off the head there was what looked to be white rock salt all in the cooling jackets and coolant holes. I got a lot of that out but not sure how to get the rest that must be in the cooling system. I pulled a big chunk of the sealant that was blocking up the thermostat. Before the sealant was put in the car, the car could run for an hour on the highway before overheating after the sealant it would run for a few minutes and overheat. This is probably due to the thermostat being completely blocked off by sealant. Here are pictures of the head gasket, the cylinder head, and the block.


Thank you

  • Which engine? When was the last C.E.L. on? Is it on now? Did you use a factory thermostat? New factory head gasket? Is the highway exhaust blue? Stick or Automatic? And, what was the last thing you did to the car/engine before this problem?
    – Ted Pants
    Aug 4, 2017 at 2:05
  • @TedPants bought the car used and it overheated. So no telling. The check engine light is not on. I have a aftermarket stat, why would that be an issue? the gasket was OEM. Not sure about the exhaust being blue. It's an auto.
    – yre
    Aug 4, 2017 at 4:59
  • Thermostats aren't all created equal, it's POSSIBLE that it remains the cause. If the block looks good and the coolant isn't being consumed or burned, it's probably coolant flow or an internally damaged radiator. The automatic transmission has a pesky torque converter that CAN create excessive heat on the highway before it fails. The converter heat can do more than just overheat an engine, it can break your transmission too, so hopefully its the stat. An engine light, if operating, might tell you that the engine is running too lean on fuel = way too hot. Make sure its working!
    – Ted Pants
    Aug 4, 2017 at 14:56
  • I put gasket sealant in the car, I took off the cylinder head and examined the stat housing and found that the sealant had clogged up the stat port blocking accurate flow. Where else should I check to make sure that the flow is correct for this car? Thank you
    – yre
    Aug 4, 2017 at 15:40
  • Which gasket were you trying to seal with that product? How many leaks and where?
    – Ted Pants
    Aug 4, 2017 at 20:58

1 Answer 1


my 2003 honda civic has 211000 miles on it and has been overheating on fast highway driving, sporadically, for 10 years now. the other day it overheated, just as i arrived somewhere, in winter, so perhaps the problem is getting worse, which is great for EZ diagnosis. i have decided that it might have a hairline crack in the head that sometimes swells with heat (high speed driving) to lose coolant in a slight amount , which burns off and therefore is never seen. rather than spend a lot of time on what might be a fruitless search and replacement of various parts, i am going to be more proactive about checking the coolant levels before i start out , and by carrying enough 50-50 mixture when traveling.

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