I have an LS in my Civic and I just flushed the coolant and changed the radiator hoses since it hasn't been done in a while. I did the flush the proper way (draining from radiator and from engine drain bolt) and flushed a gallon of distilled water through it before filling it with Honda Type 2 coolant.

Everything seemed ok at first. No leaks or anything...but then I noticed that as the motor ran up to operating temperature (upper hose hot and valve cover hot to the touch), the temperature gauge in the car still read cold and the radiator fan never came on, even after letting it idle for 10 minutes, and then taking it for a 10 minute drive. I noticed that the lower radiator hose was much cooler than the upper radiator hose (not sure if this is abnormal or not but just thought I'd mention as many details as I can)

What could be causing this?

*See my latest post for solution

 

Did you bleed the system of air pockets?

 

I let it run for a while without the radiator cap on. I think it was a sufficient amount of time for the air to have all bled out. I can try letting it run and open the bleeder valve if you think that might help.

 

http://www.summitracing.com/parts/LIL-24610

 

did you turn on the heater when filling with coolant? have you check to see if the sensor on the head came unpluged? maybe try another sensor.

 

Yea had heater on the whole time so it would circulate...the motor was running for 20 minutes, that should have been plenty of time to get things heated up. I guess I'll just replace the thermostat and sensor since doesn't seem to be any definitive answer.

 

if the temp sensor is bad the gauge would have behaved that way before the flush. it's a 13$ part from honda, so you can't lose very badly replacing it.

 

you can test the sending unit out of the vehicle by measuring it's resistance cold, then using a hair dryer and measuring it hot. see if it falls within spec.

 

With the car off and cold crack open the bleeder valve and put fluid in the radiator until only water comes out of the bleeder valve, then close the valve. Then start the car up and turn the heater control to hot (the heat doesn't have to be blowing). Do this with the radiator cap off. Allow the radiator fan to cut on twice, you can rev the car to 2k RPM's while doing this to speed up the process. Fill as necessary.

 

i don't understand why people are pointing towards bleeding the system. air in the system will not cause his temp gauge to stay at the bottom

 

After replacing the thermostat and coolant temperature sensor and rebleeding the system, the problem still persisted. I then proceeded to inspect the temperature sender switch (on the block, to the right of the coolant temperature sensor) and noticed the plug was all gunked with oil. I went ahead and bought a new sender switch (this goes to the gauge, this is not the switch that sends the signal to the fans) since it was only $6 and also cleaned out the harness plug. This fixed the problem.

Posting this in case anyone in the future searches and has the same problem.

FYI on the 3 different coolant sensors, again for the sake of those that may search in the future.

Coolant temperature switch - this is the large sensor that is located in the thermostat housing. When the thermostat is open, coolant flows past this sensor. If it is warm enough, the sensor will send a signal to your radiator fans to kick on. If your motor is running hot and the fans are not coming on, this might be the sensor to replace. The next logical choice would be to also inspect the fans themselves for failure.

Coolant temperature sensor - this is the larger of the two sensors located under the distributor (closest to the firewall and has 2 wires). This is the sensor that sends your actual temperature readings to the ECU. It is the temperature from this sensor that controls your idle speed. Cold = high idle and more fuel. If you are getting bad gas mileage and your gauge reads normal temps, then this is the sensor to replace.

Temperature sender switch - this is the smaller of the two sensors located under the distributor (closest to the radiator and has 1 wire). This sensor sends the temperature reading to your gauge cluster. If everything in your coolant system is operating normally but your gauge is either always reading hot or always reading cold or has other erratic behavior, this is the sensor to replace.

 

After you add coolant make sure you "massage" your upper and lower radiator hoses until there are no air bubbles popping up from your radiator fill hole.