I was bleeding air from my Honda Civic yesterday. Everything was going well until the coolant was still bubbling after 40 minutes of bleeding. I did everything correctly. Turned the heater to max, I checked the lower hose temperature by touching it to see if thermostat opened. It was hot.

After that I drove the car around and with a little bit of hard driving and VTEC. When the car was idle in the garage, I noticed the temperature gauge needle was fluctuating and it was between the hot and middle part of the gauge. Could this be the head gasket? Also I don't have any other choice but to drive it in that condition to the repair shop.

  • What year is the car? Also, I would bet it isn't fully bled yet. The fluctuations like you've said is a symptom of that. I don't know if it's the head gasket, but there are ways to check it. Sep 24, 2016 at 1:26
  • Car is a 96 Honda Civic EK4 with a B16A2 engine. I've changed the radiator to an aluminium radiator. I also bled it for almost an hour, is that sufficient or is there really something wrong with the head gasket?
    – 110100100
    Sep 24, 2016 at 3:10
  • Was there any issue prior to the radiator change? What precipitated the radiator change? Sep 24, 2016 at 10:08
  • The aluminium radiator was given to me for free. The OEM one was dirty inside plus the plastic covering cracked. The slightly higher than normal operating temperature happened since I had the OEM radiator. I thought it would fix the problem by installing the new one. When I bled the cooling system, almost an hour there are still bubbles. I am almost convinced that it's the head gasket.
    – 110100100
    Sep 24, 2016 at 10:43
  • 2
    The problem I have with the head gasket theory is, if it wasn't an issue before it shouldn't be an issue now. The "slightly higher than normal operating temperature" would not be enough to cause the head gasket to blow. You can use a sniffer as shown in this answer to diagnose if it is the head gasket. They aren't expensive and will tell you positively whether it's an issue with the head gasket or not. Sep 24, 2016 at 11:19

2 Answers 2


Bubbles mean that gas is escaping from the coolant. There are four obvious sources that I can think of:

  1. Air introduced to the cooling system as it was worked on.

  2. Air being drawn into the system on the suction side of the water pump.

  3. Water vapor released as coolant boils due to low pressure in the system.

  4. Exhaust gasses getting into the coolant, possibly due to a leaking head gasket.

Since you've been working on the system the first three seem like the most likely. Check your connections and hose clamps, and confirm that if the coolant is hot that you've got a good pressure cap holding pressure on the coolant, otherwise the bubbles may well be due to boiling. This could also be happening if you've got the cap off to monitor progress or to allow venting of the air introduced when you work on it.

  • Thanks for your input. I have some questions. On sources 1, 2 and 3, should I bleed it further? Normally, from what I know, is that cooling system bleeding should only be about 40 minutes maximum but mine was almost an hour (50+ minutes). A block tester kit/combustion leak testing kit is too expensive here in Australia since I am saving up for a few more parts because the car is due for 200k km / 125k miles maintenance. Should I bleed it more you think?
    – 110100100
    Sep 24, 2016 at 23:35
  • 1
    I think they very first thing I would do is to double check the suction side of the water pump. Make sure clamps are tight as well as any other place where air might be drawn in.
    – dlu
    Sep 25, 2016 at 2:23

It's probably a head gasket. And you can drive the car like that as long as there is coolant in the radiator. Keep an eye on the temp gauge though.

  • You're seriously suggesting that OP keep driving a car that you believe has a bad head gasket? Planning on starting a gofundme for a new motor?
    – 3Dave
    Sep 4, 2018 at 20:39
  • Yes. The damage only happens when the car overheats. How do you think people get their cars with blown head gaskets to the shop to get them repaired? Sep 5, 2018 at 6:54
  • Tow trucks. If the gasket is bad enough, overheating can happen very quickly. Shrug.
    – 3Dave
    Sep 5, 2018 at 14:30

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