This is a 2003 Honda Civic with about 140k on it. I've had this car checked out by a mechianic and the thermostat was replaced had a pressure test to check for air pockets, nothing. Had a test for a bad head gasket with some sort of chemical test which came back negative. But the car still overheats. I was told by this mechanic that there is a pinhole-sized leak in the head gasket somewhere that leaks into the engine very slowly. I have to drive about 60mi every day and as I drive at high speeds the car is fine it doesn't overheat. It's only when I am at idle or driving slowly that the gauge begins the rise. On these daily trips I have the heater blowing the entire time because I know it acts like another fan to take heat off the engine. But when I am at a red light all the sudden the air which is on the hot position will turn cold and then the gauge will start to climb. Is this normal for a blown head gasket? or when the coolant gets low. By the time I get to my destination is the coolant is basically gone and it's about the same time I am close to my destination that the air turns from hot to cold and overheats. Please any ideas would be fantastic!

Thank you

  • You have a leak somewhere, otherwise you would not be losing coolant. If you were loosing that much coolant, I'd expect to see the tell-tale white plume coming out of the tail pipe, though. 60 miles is a pretty good distance, but it would take a substantial leak to drain that much coolant in that amount of time. As a stop-gap measure, you may try a bottle of Bar's Stop Leak (or some other stop leak). This would slow/stop the internal leak for the mean time so you could plan to do some major work on the head gasket. You'd need to do a complete coolant flush afterwards, though. Commented Jan 28, 2015 at 12:14
  • Do you think a leak in the head gasket could cause this much coolant to be lost in 60mi w/i an hr? Without pummels of white smoke and a milky dipstick? I was told it was the head gasket but something isn't adding up. Thank you
    – AppleFix88
    Commented Jan 28, 2015 at 14:48
  • 1
    I think it is a distinct possibility. It's going somewhere. If you aren't seeing it on the ground, it has to be going out the tail pipe. Also, if it's leaking into the cylinder does not mean it is leaking into the crankcase (in ref to the milky dipstick). Commented Jan 28, 2015 at 17:16
  • I wonder where this leak is though, this is the oddest thing. No pummels of smoke and no milky dipstick, wouldn't that mean it must not be going into the engine? also had a chemical test done through the rad cap and came back negative. I looked under the hood today after it began overheating and noticed that around the overflow cap there was antifreeze sprayed all over the place particularly downwards from the overflow and on the air intake.
    – AppleFix88
    Commented Jan 28, 2015 at 21:13
  • Replace your radiator cap. Commented Jan 28, 2015 at 21:16

2 Answers 2


Paulster is right that coolant could be leaking into the cylinder rather than into the crankcase. This would explain no milky dipstick. But your mechanic said you only had a pin hole leak in the head gasket so I'm wondering how much coolant could leak out of such a small hole and cause so much coolant loss.

One idea I have is that perhaps your water pump is defective but it's not completely shot. I'm not sure if there's a possibility of partial loss of effectiveness for a water pump, but my idea is that when you drive fast the pump is able to move enough coolant through the cooling system AND higher speed air through the radiator cools that coolant flow more efficiently. When you slow down, the pump slows and pumps less volume and the radiator doesn't cool at slow speeds as it does at high speeds. Higher speeds may enable the pump to also pump water through your heater which has an additional cooling effect as you know. Not so at slow speeds, therefore no heat from the heater and insufficient coolant flow through the system leading to overheating.

An overheating engine would cause water to be pushed past even a functioning radiator cap. Seeing the coolant around the air intake as you did may be what happens when you've had coolant loss and this is what was blown out after you slowed down at the end of your trip.

In my experience it doesn't take but a couple of miles driving when the engine is warm to get very hot coolant. I think you're losing a lot of coolant through poor circulation. A pin hole leak in the head gasket seems just too small to vent that much gas into the cooling system but wiser heads may know differently. Pressure in the cylinders is very high, of course.

The presence of coolant inside the engine compartment is an important clue. I bet steam pushes that quantity out not venting exhaust from the supposed pin hole leak.

One thing you can try when the engine is cold and its got all its coolant, is take the radiator cap off and start the engine. After the thermostat opens, you should be able to see some current flowing at the top of the radiator. If it starts to boil over and you don't see that flow, that might point the finger at the water pump. If it were a head gasket leak, I'm thinking you'd see that in the form of bubbles after the thermostat opens. You'd see bubbles, current and then boiling (because of unpressurized system).

  • 1
    Had a master ASE tech examine the car it was a bad head gasket after all.
    – AppleFix88
    Commented Feb 3, 2015 at 23:29

I had all this going on with my car 96 accord I replaced cooling sensors, radiator cap, fans, thermostat, and water pump, I thought about changing out the radiator but just went for the head gasket and solved the problem

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