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What could cause pressure in radiator preventing overflow being pulled back in

Customer Question
My '04 Civic Coupe EX...
My '04 Civic Coupe EX (D17a6) has started filling the radiator overflow to top - not pulling it back into the radiator. When I check the engine cold, it still has a lot of pressure in the system. It starts quickly cold, and idles very smooth. I am unable to detect anything different about the exhaust (no evident white smoke) and the oil is normal (no milky appearance). When I refill the radiator, and then start the engine to top it off, it will slowly blow out a bubble like I'm used to seeing right before the thermostat opens but it is still cold. Doesn't seem too unusual at this point. I bought the car used, 15 months ago, have not overheated it yet that I am aware (according to the factory gauge); it has 87K miles on odo so far. What could be pressurizing the cooling jacket? I'm not seeing the usual signs of head gasket issues.
Submitted: 8 years ago.Category: Honda
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Answered in 7 minutes by:
4/16/2009
Mechanic for Honda: Honda Jim, ASE Certified Technician replied 8 years ago
Honda Jim
Honda Jim, ASE Certified Technician
Category: Honda
Satisfied Customers: 16
Experience: 20 + years as a Honda Tech / Honda Master Technician /ASE Certified Master Technician
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Is the engine stone cold and the radiator hoses are still hard?

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Customer reply replied 8 years ago
yes, last Thursday morning it was 49 degrees out (after sitting all night) when I checked it and was totally surprised of the significant amount of pressure that escaped. way more than just a "ssst", more like a "phsssssssst" - a good 2 seconds or so. It will push all the remaining liquid out of the overflow tube and bubble some in the overflow tank before I get the cap fully removed.
Mechanic for Honda: Honda Jim, ASE Certified Technician replied 8 years ago
It sounds pretty likly to me that you have a bad radiator cap.

You could take the cap in to have it tested or you could just replace it. They are pretty inexpensive.

If you replace the cap and you still have this issue we can look at other things but replacing the radiator cap needs to be the first step.
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Customer reply replied 8 years ago
seems too simple of an answer, there seems to be too much "left over" pressure in the system, could that be from the anti-freeze getting too hot?
Mechanic for Honda: Honda Jim, ASE Certified Technician replied 8 years ago
You have said that the gauge has not indicated that the car is running hot.

The radiator cap is what regulates the pressure in the cooling system.

A radiator cap is actually a pressure release valve, and when the pressure reaches the set value (usually around 15 psi), it lets coolant escape into an overflow tank. Then, when the radiator cools, a vacuum is created and the coolant is pulled back into the radiator.


I am not saying 100% that this is the problem only that it is very likly and that this is the first thing that needs to be checked / replaced to properly diagnoise the problem.


Honda Jim
Honda Jim, ASE Certified Technician
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Experience: 20 + years as a Honda Tech / Honda Master Technician /ASE Certified Master Technician
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Customer reply replied 8 years ago
I forgot to mention that I have had to siphon the coolant out of the overflow tank to refill the radiator, about 3 times over the last 2 or 3 weeks. It seems to be filling the overflow tank faster now, which seemed awfully strange to me. Thus the question to this forum. I'll replace the radiator cap. Not sure if the accept button will send this response.
Mechanic for Honda: Honda Jim, ASE Certified Technician replied 8 years ago
I got your answer.

Let me know what happens after you replace the cap
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Customer reply replied 8 years ago
Relist: I still need help.
::: No Change :::
Still overfilling the overflow tank after replacing the radiator cap. Still large amount of excess pressure after engine is completely cooled down. Oil is still normal, still no noticeable change in the exhaust. I spent a little more time watching the radiator from cold to full operating temp - while cold the coolant will "burp" like several times (level starts just below the neck, then rises to the top when large bubbles will appear and the level goes back down). You can see when the thermostat opens as there becomes a lot of movement and some swirling at the base of the neck. At that point, very small bubbles will appear giving a foam like appearance as the level will again rise in the neck until the cooling fan engages, then the level will go back down. Since I still have a mechanical throttle, I can raise the engine RPMs while the cooling fan is running and the coolant level will stay somewhat stable. When the fan goes off, the level will rise again and will push out a few ounces of fluid spilling out, but then go back down and stay down as the level is now much lower. Side Note: The Radiator and antifreeze were replaced March '08 due to a slight mis-hap; the anti-freeze is still clear and green.

Other Note: I drive 40 mins each way to work every day, 19 miles interstate driving, 7 miles at 45-50 mph on 4 lane roads.
Mechanic for Honda: Honda Jim, ASE Certified Technician replied 8 years ago
Is the return hose still attached to the cap of the overflow tank?

There should be a hose attached to the underside of the cap.
That hose should dip into the coolant in the overflow tank. That hose can sometimes fall of, if hat happens the coolant cannot be drawn back into the radiator.
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Customer reply replied 8 years ago
The hose doesn't attach to the overflow tank cap. The hose starts from the radiator neck and follows around the side of the radiator and down the outside of the overflow tank (there are some fingers on the side of the Tank that hold the hose in place) where there is an inlet tube at the very bottom of the tank that it attaches to. I mentioned last time that when I open the radiator cap, some of the excess gas that escapes can also be heard bubbling through the coolant in the overflow tank. (I have to hold the radiator cap down as I open it to keep the coolant from spraying all over me.) My first thought about this problem a few weeks ago was to check for a hole in the overflow hose, so I did inspect it and found no hole or blemish. The problem seems to be centered around the fact that there's too much pressure in the radiator to pull the liquid back.
Mechanic for Honda: Honda Jim, ASE Certified Technician replied 8 years ago
Well, at this point it must have a bad headgasket.

It is possible to have a small head gasket leak that will pressurize the cooling system but coolant will not move back into the Cylinders as the engine cools, therfore no smoke/rough running on startup.

It is odd that the coolant is not pulled back into the radiator and that the system retains pressure over night, but I have seen that happen before with either a bad radiator cap or more rarely a bad headgasket.

Until it gets worse and starts showing other normal bad headgasket signs about the only way to verify the problem is to pressurize the cylinders with a cylinder leakage tester and shop air pressure and see if you can get bubbles into the cooling system.

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