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I have a Nissan March (Micra) 2003 and I had a thermostat problem (was jammed) and my friend changed it for me almost 9 months ago. Ever since then, I need to top up the water every few days. If I'm going long distances, this is more frequent. There is no apparent leak and no sign of water going into the engine through the head gasket (I have frequently checked the engine oil gauge but no sign of water). However, it seems like the system is overpressurised and the water comes out from the reserve tank as there is stain around it. Do you guys have any idea what might be wrong?

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Some engines (not sure if this is one) will need to be burped or have a air bubble removed for proper cooling. There are several ways of doing this.

If you do not have good flow of the coolant, because of an obstruction (air bubble) in some engines they will overheat. You are having overheating, or at least localized boiling of coolant, which is causing the bumping of fluid out of the overflow/reservoir bottle.

This localized overheating may not show as a warmer engine temperature on any gauge, as it is localized.

Some Chrysler minivans were notorious for this, and a fill procedure was published which required the vehicle to be parked on a certain slope, with incremental filling of coolant. An alternative method involved drawing a vacuum on the cooling system, which caused the air bubble to expand, and much of it's bulk would leave the engine, resulting in increased coolant flow.

I am just providing the most likely approach I might take, given your description. Since there are many factors, some which are not mentioned in your question, I cannot provide certainty that this is the cause. But it is a likely cause based upon my experience, and can cause the symptoms you are describing.

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Do you guys have any idea what might be wrong?

There is probably air in the system. Nissans used to be tricky to bleed the air after changing a thermostat, waterpump, timing belt or timing chain on certain engines. Older Maximas and Sentras used to be known for it.

You can bleed and fill the system as follows. The idea is to get the radiator higher than the heater and let gravity do the work of flushing the system and displacing the air in components like the heater and hoses.

  1. jack-up the front end of the car as high as possible
  2. set the heater on high heat
  3. run the car without a radiator cap
  4. insert a large-reservoir funnel
  5. top-off the funnel with fluid
  6. run car for 15 to 30 minutes
  7. refill the funnel as required

It will take about 10 minutes for the thermostat to open. You will know when the thermostat opens because the radiator cooling fan will start cycling on and off. After about 20 or 30 minutes the the cooling system will mostly be air/bubble free.

Waiting for the "thermostat to open" is important. The thermostat is just a cutt-off valve to the radiator. When the thermostat is closed coolant does not flow to the radiator. When the thermostat is open coolant is fully circulating through the radiator.

You are finished bleeding the system when the system does not draw fluid from the funnel for 5 or 10 minutes after reaching temperature. In this case "reaching temperature" is about 185° F, and you know the system is there when the cooling fans start cycling.

I used to know guys who would wedge-open the thermostat with aspirins to save the 10 minutes waiting for the thermostat to open. The fluid would eventually dissolve the aspirins. I never did it though. If you want to do it, drop your thermostat in boiling water so it opens.

You should probably replace the radiator cap when you replace the thermostat or waterpump. The cap's gasket gets old and brittle, and can cause a leak. The leak is usually closer to venting steam rather than a traditional coolant leak that puddles.

Some motors, like the old VG30's, had a 10mm bleeder screw on the manifold near the firewall. They rarely worked as expected and we usually did not bother with them. Plus, they could leak after closing them, so we did not want the additional problems.

The funnel to use is similar to Radiator Coolant Filling Funnel Kit. The important part is the size of the funnel's reservoir. You want a large reservoir so the system can draw fluid as needed. You can probably get away with a regular 99¢ funnel, but you will have to watch it and ensure fluid is available to draw from.

If you are unlucky then you could have a coolant leak. External leaks are pretty easy to spot because of the fluid on your garage floor. Internal leaks are trickier because the fluid gets sucked into oil or gets burned. For an internal leak check the oil or spark plugs. Spark plugs will be kind of clean as the coolant cleans the plug.

I can only say "Nissans used to be tricky..." because I worked for a Nissan dealership in the service department in the 1990s. I don't know how they are nowadays.


... it seems like the system is over-pressurized and the water comes out from the reserve tank as there is stain around it

Replace the radiator cap. It is the relief valve that allows fluid to flow from the radiator to the reservoir. (It also allows the system to draw from the reservoir).


There is no apparent leak and no sign of water going into the engine through the head gasket (I have frequently checked the engine oil gauge but no sign of water).

Also check the exhaust and plugs. A non-trivial leak will cause white smoke in at the exhaust. You can also check the plugs. If there is a small leak at the head for a particular cylinder, then the plug for that cylinder will be cleaner than the others.

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First, how are you determining when to add more coolant? Do you get an indicator lamp, or is there a fill line in the coolant reservoir that you are looking at? I know, it sounds silly but it could be that you're simply overfilling it and the excess is being forced out.

If that's not the case, it could be that your coolant cap isn't holding pressure properly. The cap's pressure rating is usually marked somewhere on the cap and can be tested in a few seconds if you can find someone with the proper tool.

If your coolant cap is holding pressure and you're sure that you're not overfilling it AND the reserve tank is still overflowing, you almost certainly have a combustion leak into your cooling system. The absence of water in your oil doesn't mean that you don't have a leak in your cylinder head or gasket.

Common ways to detect whether you have combustion gas leaking into your cooling system are a Cylinder Leakdown Test (with air pressure), or a fluid test (usually called Block Tester or Combustion Leak Tester) basically a kit that has a blue-colored fluid that will turn yellow in the presence of combustion gases.

Hope that helps.

  • I hope I'm not doing it wrong but when I'm going short distances the fluid in the reserve tank almost stays at the same level but when I open the coolant cap, the water has gone down and I need to fill it up with for say 150 ml every 2-3 days. But when I go long distances both the water levels go down. – akam553 Aug 28 at 23:30
  • How high are you filling it? If you don't leave some air space in the pressurized area, the liquid will expand as it warms up and force its way around the cap's pressure seal. 150ml is a negligible amount of coolant. You need let it go a bit longer and see if it will stop losing coolant at some point yet still remain mostly full. – Jim Perris Aug 28 at 23:43
  • Just to clarify, you should be able to look into the top of the radiator and see the top of the aluminum core. The coolant level should be just above that level. If it falls below that, you have another problem beyond just overfilling. – Jim Perris Aug 28 at 23:50
  • I fill up so it spills, afraid of messing up with the air tightness. – akam553 Aug 28 at 23:57
  • Yeah from time to time it falls below the core! – akam553 Aug 28 at 23:58

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