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How to clean blockage between radiator and reservoir

I have 2003 Civic, 125K. I am having an issue that the hose that connects radiator and coolant reservoir keeps disconnected at the top of radiator. I am sure this is due to pressure build up in radiator, and radiator cap functions. I also have seen lower level of coolant in my radiator but reservoir is full (of course the hose I mentioned above is connected!). I took my Civic to a shop and they bled radiator but didn’t fix the problem. I am certain there is a blockage somewhere in between reservoir and radiator. Would flushing the cooling system fix the problem? I just want to give bit more specific information to my shop. Thank you for your advice!!!

I would replace the radiator cap and install a small hose clamp or a zip tie on the reservoir hose. The hose can be blown through when it’s disconnected to make sure it’s clear.

Of course if you still have trouble you’ll have to have the radiator and thermostat checked, but experience tells me you have engine head gasket replacement in your future.


That was the first thing that came to my mind when the OP mentioned “pressure build-up” in the radiator.

Start by replacing the hose. It’s cheap and new flexible rubber will grip the barb better.

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Thank you for all the inputs.
How would engine head gasket be potential cause of the issue? I just thought the “pressure build up” is due to coolant water temperature increase in the radiator, and then I also thought the radiator cap functions well, as it would have open the valve at the cap to release the pressure or coolant water to the reservoir. As the hose or somewhere in between the radiator and reservoir has been blocked, the pressure has nowhere to go so that the the hose at the radiator to the reservoir disconnected. If I install a hose clamp as suggested, and if there is still a blockage, would the build up pressure rupture some other parts of cooling system? Thank you for advice!

It’s easy - try and blow though that hose with the reservoir cap removed. I bet there’s no blockage. This hose is not supposed to see any significant pressure, so if it’s popping off there is likely a lot of flow, typically from a blown head gasket. Any good shop should be able to diagnose it.


Try replacing the radiator cap. It should relieve any high pressure buildup regardless of a failing head gasket. They usually fail low but stranger things have happened. Its cheap to try.

I’d listen to the folks here. Sometimes things don’t work the way you think they do. They should check for combustion gases in the coolant, look for air bubbles, etc. to diagnose a head gasket.

Many people here jump very quickly to the head gasket, and it could be the issue, but it could also be that the rubber hose has gotten stiff and lost its elasticity. A buck for a new hose or hundreds for a new headgasket? Which makes sense to do first?

One other thing, If the hose goes to the side of the reservoir instead of the cap, then the reservoir has a molded tube from the hose barb to the bottom of the reservoir. It can get plugged up at the bottom. Blow some air into the hose barb to clear the tube, the remove the reservoir and wash it out.

Do the simple stuff first. BTW, it’s not the radiator cap. The radiator cap holds pressure inside the radiator, the hose is on the outside.

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A headgasket breech could be allowing combustion gasses to get blown into the water jacket blowing excess heat into it AND pressurizing it. I’m not saying you have a bad headgasket, just answering the question. If with the radiator fill cap removed and the engine running you see bubbles coming out the fill hole, suspect a bad headgasket. There are simple tests to find out, including a “lab test” (my words) that checks the coolant for the presence of hydrocarbons and a “cylinder leakdown test” that checks the ability of the cylinders to hold pressure.

I suspect (hope) you’ll actually find a tired rubber hose that can no longer hold the barbs well, as Keith suggested. Rubber dries out and it also “cold flows”, losing its ability to compress the barbs.

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It could well be just what the OP speculates, the hose is clogged or breaking down and acting as a one-way valve. That happens to flexible brake hoses too, with the same effect.

If I had that problem I’d remove the hose and use compressed air to blow through the hose from end to end. Or I’d hook up my hand held vacuum pump and pump air through the hose that way. If it seemed clogged at all I’d just replace it with a new one.

Another idea, do a little experiment. After installing a new radiator cap, use a different hose with clear plastic so you can see through it, as a test-hose, that you know is totally open and place the non-radiator end in a separate plastic bottle you’d install by hook or crook. Start the engine and see if fluid is coming out of the radiator and going into that bottle through the hose as the engine heats up. And does the fluid go back into the radiator as the engine cools down after the engine is shut off?

Ok, here is update.
I had gone back to the guy who initially bleed the system, told him the hose had came off. He called me saying he did pressure check and found no leak and then he put a hose clamp at the hose connection.
After driving abut a week, one day when I came back to home, I saw vapor coming out of under the hood. I open the hood and found the coolant water was pushed out of the radiator cap. It was so obvious for me to know the hose is clogged. I went back to the guy and ask him just to replace the reservoir and the hose, he said he would, but he called me back and said he’s gotta do the head gasket and the quote was $1,700. How controversial to his previous pressure check test he did a week ago and I would rather buy a similar Civic for $1,700 and sell this one for parts. I declined the head gasket work and searched youtube on how to replace the hose and reservoir tank. Thank Youtube I found a clip that shows it would only take 5 minute of work to get it out.
I took the hose and the reservoir out, as expected, found really bad clogging at the bottom of the tank and hose inside. I didn’t even need a new hose, neither the tank, I just clean the gunks out and flushed them with clean water. It’s been for another week now. Problem fixed. Thank you all. I am done with the guy…

Congratulations on your success and sincere thanks for the follow up post.
As regards the guy who said he’d have to change your headgasket for $1,700, be sure you tell all your friends, coworkers, family, relatives, and everyone you meet your story. It deserves to be told. The world needs to know.

There are inexpensive tests to determine definitively if a headgasket is breeched. Anyone who tells you this over the phone without doing the diagnosis deserves IMHO to be exposed. Sorry if some feel this is “hard nosed”, but I find his response appalling.