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Old 11-24-2008, 09:55 PM   #1
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Civic VX not circulating coolant

So Ive been troubleshooting a high and bouncing idle and have read about air in your radiator/hoses being one of many possible culprits. Before bleeding the radiator of air bubbles I wanted to make sure it was actually circulating coolant. I took it for my morning commute, which is about 30min 20miles mostly freeway and at my destination, popped the hood and felt the radiator hoses. The top hose was warm but the bottom hose was still cold. The car is not overheating, just having idle problems and is getting less than stellar gas mileage. Now Im looking to troubleshoot this problem, other than buying a new thermostat, what else would cause something like this. Any input is greatly appreciated.

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Old 11-25-2008, 03:07 AM   #2
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I had similar symptoms with engine racing at idle reported here:'ve
and here:

Ultimately it was a bad MAP sensor in my case.

It could also be dirty or malfunctioning IACV, or vacuum leak, etc, but not air bubbles in coolant.

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Old 11-25-2008, 04:53 AM   #3
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Your top hose should be hot and the bottom hose warm. You might start it cold with the radiator cap off. If the coolant is circulating before the engine warms up your thermostat is either missing or not working properly.

My hands are pretty tough, with my engine running I can hold my hand on the top hose for about 5 seconds before it begins to hurt. Thats 180 degrees. The bottom hose will be much cooler depending on how well your radiator is working, usually about 120 degrees.

Is the temp guage reading normal? If it is not getting up to its normal operating range, it,s a good indication of a bad thermostat.

Nissan thermostats fail in the open position, so the engine runs too cold and mileage will suffer greatly.

If your coolant was not circulating, your engine would overheat in a short distance, certainly before 30 minutes.

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Old 11-25-2008, 06:18 AM   #4
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The top hose is only warm, I can hold onto it with no problem. The bottom hose is cold. The temp gauge is reading about 1/3 the way up, def not overheating. Now that I think of it, I dont think Ive ever heard the fan turn on, even while idling in my driveway, working on the car.

Edit: I lied, that ish was hot, but the bottom hose again is still cold.
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Old 11-25-2008, 08:25 AM   #5
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If you're not overheating, I don't think you have a problem. You said it was mostly highway driving, which is pretty effective at keeping a radiator cool, especially this time of year. If the top hose is hot, theres nothing wrong with your thermostat or water pump. I agree with djenyc, its not a coolant issue.
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Old 11-25-2008, 03:58 PM   #6
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I agree that it sounds like your coolant system is working fine.

Check to make sure your throttle body cable is properly adjusted, that your IACV is clean, your throttle body is clean, check for vacuum leaks (sometime they form at the PCV elbow), and that your ignition timing is properly adjusted.

Those are all possible reason why you could have the issue with your idle. Good luck.

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Old 11-25-2008, 10:08 PM   #7
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While cleaning out the IACV I lost a bit of coolant. Any suggestions on how to remove the air in the lines if the coolant doesn't even circulate.
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Old 11-26-2008, 02:45 PM   #8
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the IAC hose should be small enough to pass the bubble without any work on your part.

again, coolant IS circulating or you would have overheated long ago. a closed thermostat won't let the upper hose get warm AT ALL. coolant not circulating will blow the engine in just a few minutes. if the weather near you is like it is here (about 20f this morning) the upper hose won't get that hot and the lower hose certainly won't be warm. as already stated, either your thermostat is stuck open, missing, or your engine just isn't putting out that much heat.
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Old 11-29-2008, 02:32 PM   #9
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I recently had my t'stat replaced with one from the Honda dealer because the old one didn't keep the system warm enough. It rose much more slowly than I expected from driving other cars and even after warmed up as much as possible (say, a half hour on the highway) the upper hose was only warm to the touch and not hot.

The big tipoff in my case was black soot on the bumper near the exhaust. I figured the computer was forcing it to run rich due to being cold. I wiped it off and it didn't come back after getting the t'stat replaced.

With new t'stat in, the upper hose is hot to the touch. I can keep my hand there but it's pretty darn hot. Temp gauge (mine is a '97 HX) goes to dead horizontal on the gauge every time.
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Old 11-29-2008, 04:27 PM   #10
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Another thing that could be causing poor circulation is a bad water pump. If the coolant hasn't been changed in a long time it can become acidic. I've seen the steel impellers corroded completely off of water pumps before.


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