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Old 1 Week Ago   #1  
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Cooling system problem

Hi guys, my Honda civic 1999 ex has cooling system problem. When the temperature gauge reach at operated temperature; around 85C, the radiator fan did not turn on. If I drive the car for too long, I afraid it will get overheat. There are some troubleshoots that I went through.

1) Test the radiator fan itself; connect directly to the 12v battery (passed)
2) Check the 10A fuse and relay of radiator fan (passed)
3) Unplug connector from coolant temperature sensor. Use jump wire to jump the connector and see if the fan turn (passed)

The troubleshoot# 3 indicated coolant temperature sensor is faulty and i will get it replace. I am wondering if there is anything else i left out that prevent radiator fan from running.

My other question is cooling system itself. How does coolant run? Does it go from the engine to upper hose, through the radiator, and then down lower radiator hose? When I run the engine, the upper radiator hose was really warm and lower radiator hose was cool. As the temperature reach 85C, the upper and lower hoses got really hot plus the radiator fan is not running. Does this indicate the thermo stat is still completely closed/faulty? I am thinking of replacing it if it is the case.
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Old 1 Week Ago   #2  
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Re: Cooling system problem

There may be an air pocket in the coolant within the cylinder head that's possibly preventing temp sensor from reading correctly. Bleed the coolant of air per the service manual.

You can remove the thermostat and test it..put the thermostat and a high-temp thermometer in a pot of room temp water then boil the water. The thermostat should open up at the temperature it's designed to. If you end up buying a new one purchase a Honda Genuine (OEM) only.

Here's a good read about coolant parts: https://www.carparts.com/classroom/coolingsystem.htm


Coolant flow video:

Last edited by Megalodong; 1 Week Ago at 05:28 AM.
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Re: Cooling system problem

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Originally Posted by Megalodong View Post
There may be an air pocket in the coolant within the cylinder head that's possibly preventing temp sensor from reading correctly. Bleed the coolant of air per the service manual.

You can remove the thermostat and test it..put the thermostat and a high-temp thermometer in a pot of room temp water then boil the water. The thermostat should open up at the temperature it's designed to. If you end up buying a new one purchase a Honda Genuine (OEM) only.

Here's a good read about coolant parts: https://www.carparts.com/classroom/coolingsystem.htm


Coolant flow video:
https://www.youtube.com/watch?v=V7inC4lOpGs
I forgot to mention that I did bleed the coolant air to check system circulation; got air out, but the fan didn't turn on.Thank you for reply
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Re: Cooling system problem

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Originally Posted by thaipham161 View Post
I forgot to mention that I did bleed the coolant air to check system circulation; got air out, but the fan didn't turn on.Thank you for reply
Service manual bleed method is:
* Raise front of car on jackstands as high as possible (top of radiator higher than engine)
* Set climate control to max heat with fan off
* Idle engine with radiator and reservoir cap off until cooling fan cycles twice. Fill radiator or reservoir as needed. Bubbles should progressively dissipate from larger to smaller bubbles and be almost non existent (at idle) by the end of bleed procedure.
*Allow the engine to cool down fully, inspect radiator and reservoir levels and fill as needed to proper levels.

In addition to the service manual during the bleed procedure I like to rev the engine to 2500 rpm occasionally to help force bubbles/air out of the system and to visually inspect the coolant flow inside the radiator to access if the water pump is functioning properly.


Did you bleed it per the service manual?
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Re: Cooling system problem

Fan at only 85c? Incorrect.

Fan should turn on when the fan sensor hits 199F. 199F = 93C
The gauge and ECT can show quite a bit higher than that before the radiator fan turns on, because

Fan temp sensor does not have to mirror the gauge nor the ECM sensors due to its location.

The fan will not turn on until the temp of the liquid coming out of the cold end (bottom) of the radiator is 199F.
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Re: Cooling system problem

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Originally Posted by Megalodong View Post
Service manual bleed method is:
* Raise front of car on jackstands as high as possible (top of radiator higher than engine)
* Set climate control to max heat with fan off
* Idle engine with radiator and reservoir cap off until cooling fan cycles twice. Fill radiator or reservoir as needed. Bubbles should progressively dissipate from larger to smaller bubbles and be almost non existent (at idle) by the end of bleed procedure.
*Allow the engine to cool down fully, inspect radiator and reservoir levels and fill as needed to proper levels.

In addition to the service manual during the bleed procedure I like to rev the engine to 2500 rpm occasionally to help force bubbles/air out of the system and to visually inspect the coolant flow inside the radiator to access if the water pump is functioning properly.


Did you bleed it per the service manual?
yeah, I did the same procedure as you mention, got few bubble out from reving the engine. From my observation, the coolant inside the radiator didn't go down a lot, probably 2'' off from the top of radiator cap. Does this mean the coolant is circulate? I noticed both upper and lower hose were really hot, but radiator fan didn't turn on at all.
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Re: Cooling system problem

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Originally Posted by ezone View Post
Fan at only 85c? Incorrect.

Fan should turn on when the fan sensor hits 199F. 199F = 93C
The gauge and ECT can show quite a bit higher than that before the radiator fan turns on, because

Fan temp sensor does not have to mirror the gauge nor the ECM sensors due to its location.

The fan will not turn on until the temp of the liquid coming out of the cold end (bottom) of the radiator is 199F.
Oh, I was estimated the temperature because the gauge temperature was around half way when i ran the engine. So, what exactly operation temperature if looking at the gauge meter? Like pass half way since you mention that it show higher? I got it to the point where upper and lower hose got really hot. Does this determine if thermostat is closed/open?
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Re: Cooling system problem

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I got it to the point where upper and lower hose got really hot. Does this determine if thermostat is closed/open?
That would indicate the stat opened and hot liquid is flowing through the radiator. Now you'd need to be able to accurately measure the temp of the liquid in the bottom hose.




I'd plug in a good scanner and monitor the ECT reading in a data list. If fan turns on at 200-205F, I'd say it seems reasonably close to 199F (remember they don't have to mirror the other sensors).
If fan doesn't run by then, I'd probably let it run up to 212F or so before wondering if the fan temp sensor is a problem.
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Re: Cooling system problem

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Originally Posted by ezone View Post
That would indicate the stat opened and hot liquid is flowing through the radiator. Now you'd need to be able to accurately measure the temp of the liquid in the bottom hose.




I'd plug in a good scanner and monitor the ECT reading in a data list. If fan turns on at 200-205F, I'd say it seems reasonably close to 199F (remember they don't have to mirror the other sensors).
If fan doesn't run by then, I'd probably let it run up to 212F or so before wondering if the fan temp sensor is a problem.

What kind of scanner did you use? And thank you for the tip
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Re: Cooling system problem

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What kind of scanner did you use? And thank you for the tip
Any scanner that can accurately display live operating data.








Maybe an accurate thermometer too.
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