93 Civic LX 1.5 liter overheating during slow or stop, midrange of gauge on highway. Radiator and thermostat new one year ago.

Last night the temp gauge was very near the top of the scale before I heard the fan turn on, and it seemed to be running fine and moving a good amount of air. I would have thought that the fan would have actuated much sooner. This leads me to question the accuracy of the gauge.

Strangely, the temp gauge rises rather rapidly from normal operating temperature, and then once underway it drops just as rapidly. It is not swinging wildly mind you, but certainly, especially on the drop, moving more quickly than I would think that the water could cool. This also leads me to question the accuracy of the gauge.

I have searched around the forum and could not really find a definitive answer on a problem such as this. I have read head gasket (I have no white smoke), bleed your system (I will, but after radiator and thermostat were replaced a year ago the temperature held steady in all sorts of weather, and the system was not opened since then), and thermo switch (of which I am unsure of the location).

Has anyone had this scenario and nailed it definitively? Any other help that can be offered before I am able to delve into it in earnest this weekend? For instance, is there a sensor that feeds the signal to the gauge that has been known to go faulty? Can a headgasket be so marginal that it won’t dump plumes of white smoke through the exhaust but will cause a mild overheating at slow speeds? Etc. Any help is appreciated.

Ed Fisher
Dallas, Tx

 

Sounds more like the fan switch is balky. Once the fan came on, the gauge dropped back to normal, right? That suggests the engine was actually hot.

The fan switch is on the thermostat housing. You can unplug the switch and jump the wires in the plug-- then the fan should run all the time the key is on regardless of coolant temperature. Test drive like that and see if there is any tendency to overheat. Don't leave the fan running all the time permanently because it will eventually wear out the motor.

 

i believe you need to check your grounds. - on battery going to trans and going to body. ground from body to front of motor, and the ground on the thermo housing.
when there isnt a good ground connection the temp gauge will act funny.
the temp sending unit is located just underneath the distributor it has 1 wire leading to it. when the temp is showing overheating does it seem to you like it is overheating?
when a car overheats you can usually smell it.

 

Thanks. I will definitely try the jumper to fan and check the grounds. It has just now gotten to be somewhat repeatable to where I can replicate and test. Will check back in and give the fix when I find it, or, ask for more help if I don't.

All the best,

Ed Fisher
Dallas, Tx

 

Update: I jumpered the fan switch to allow full time operation and the temp gauge still rose. I checked the coolant level, and lo and behold, it was low. I filled it and did what I hope was an adequate job of bleeding the system. Now the question begs to be asked: Where did the coolant go and why? I have seen no puddles or indications that the system is leaking. Is there a way to check the headgasket without a gas analysis? I believe that if the headgasket is to blame it is a VERY slow leakage.

Thanks, especially to mk378 for his always calm and logical approaches, and his valuable time.

Ed Fisher
Dallas, Tx

 

I had a headgasket problem and it was slight enough that I just fill the coolant every few days until it got a lot worse. I could see very small bubbles in the reservoir when I first had the problem. I also had some positive air when I open the rad cap in the morning.

 

Thanks for the above reply GWL. I did finally find a small amount of coolant had escaped after several days of checking, and one morning did have a positive pressure in the radiator. New headgasket is on order.

Ed Fisher
Dallas, Tx

 

Confirmed head gasket leak causing small amount of coolant to escape over time. Leaking water jacket on #3 cylinder. Thank you all for replying.