So I've been having this weird issue. I always let my 99 ex warm up before I drive it. I start driving temp gauge is at the normal position. It's cold up here in jersey. I turn the heat on 2 or 3 notches. Temp gauge slowly drops down almost to the "C." I turn the heat off and the temp gauge slowly rises back up to normal position.

Any ideas what could be causing this?

 

Possibly faulty ECT temp sending unit. Try bleeding air out of coolant
Read some of these google hits: https://www.google.com/#q=dash+temp+gauge+drops+when+heater+is+on&*

 

Thermostat stuck open.

 

Keep the heater settings running longer and what happens?

 

The blower fan being turned on with the setting on hot will pull away heat from the coolant as it passes through the heater core. That's a simple explanation of it.

Does the heart work flawlessly for long periods of time, even with the engine temperature down to "C"?

 

Shouldn't bring down to "C". I've never had any car I've owned do that and I've probably owned tweny cars.
It pulls away heat but, heat and pressure is always being generated to replace it as long as the engine is running.

OP's description is lacking though. He doesn't say the length of heater use, length of drive time, and whether or not it ever reached operating temp.

 

Check system to see if its low on coolant.

 

the heat does seem to work fine even when the temp gauge drops. Like I said I'll turn the heat off and it will go back to Norma temp. A few minutes later I'll turn it back on the temperature gage will remain at the normal position. It seems to only happen the first time I turn the heat on.

Coolant is filled and bled properly. No air in the system. ECT sensor was giving out proper readings.

 

when I see the temp drop to the Cold area while I'm driving I usually turn the heat Off, let the gauge come back to temp. Then turn the heat back on and it works fine. Drive to work is about 30 min.

 

I would leave the heater on, why turn it off?

 

because I'm not sure what exactly is happening. I'll leave the heat on next time it drops and see what happens

 

I have noticed something weird with the coolant overflow tank. I flushed and re filled the coolant the other day after working on the car. The overflow tank was slightly under the max line. After a day of driving the coolant in the overflow tank is way low. Only filled up about 1/5. Is this normal? I didn't see any leaks and there was no air in the system.

It has has done this before and has driven fine for weeks with no problems ( the coolant in the overflow tank stays low) I don't know if I should keep filling it.

 

Taking your word on the coolant system being bled properly, I would suggest filling the reservoir to the max line when the engine is completely cold and see what happens after a week of driving.

If it drops, then you may have a coolant leak/consumption issue you're not aware of.

 

I'd check if your carpet is showing any discoloration because of the coolant, either green or blue. It could be a slow leak.

 

When the engine is completely cold, take the radiator cap off and check. The radiator should be full to the top. If there is air space in the radiator you need to refill and bleed the air again.

 

If the coolant temp ever drops below normal when the engine is running, then this is your problem. Replace the thermostat.

BTW: It's normal for the coolant to drop after work has been done on the cooling system. Air gets trapped in the system and it takes quite a few drive cycles to work all of it back out.

 

Pretty normal after doing a drain and fill or other type of coolant system work for reservoir level to go down after a heat and cool cycle because air pockets are still being displaced..especially if the system wasn't bled with the front up high on jackstands and heater temp control turned to max heat + fan off

If you're getting further coolant loss: Rent (free) a coolant system pressure tester at Autoboned, O'Blowme or Advanced A-hole and do a pressure test while the engine is cold and another one at operating temp. Do not pressurize the system above spec (IIRC 13-16 lbs.) as stated in service manual. The pressure rating may be labeled on the radiator cap (if OEM). Once pressurized start looking and feeling for leaks on all coolant and heater hoses, connections,temp sensors, heater core inside cabin. Remove the air intake hose, air box and distributor to get a better view and reach. the heater hose underneath the distributor is especially suspect because oil leaks from the distributor o-ring cause that hose to deteriorate faster than others. If, you suspect a heater core leak a pressure test when engine and heater core is cold is better than checking at operating temp.

Hey.. imagine this ETCG has a video of him doing a pressure test on surprise! (not) a 1999 Honda Civic:

 

Thank you for the advice! I will do the pressure test just to be sure.