Originally Posted by
kpatz
In many cars this is normal, if the car sits in traffic for a while especially on a hot day, or if you park for a short time on a hot day then restart, the temp will be higher than normal.
Here's the scoop: When the engine is cold, the thermostat closes, so coolant only circulates through the engine, not through the radiator. When the engine warms up, the thermostat opens, and coolant now starts hitting the radiator, which cools the coolant. The thermostat is designed to maintain a set temperature by opening or closing slightly so that the temp. gauge stays more-or-less in the same spot, under normal circumstances.
Under certain circumstances, the temperature can rise above normal. For example, when idling for an extended period and not moving (parked with engine running, or stuck in traffic for 10-15 minutes or more), the air around the radiator will get hot. When this happens, the gauge will creep upward until a preset threshold is reached, then the radiator fan will kick on, pulling cooler air over the radiator and causing the gauge to drop down a few clicks, then the fan will shut off. You'll see this cycle repeat over and over on a hot day when idling. This doesn't happen when driving because the air naturally flows when the car is moving, and the fan doesn't have to kick on at all. In many cars the radiator fan is also the A/C fan, so you can test the fan's operation by turning the AC on and see if the fan runs then.
Also, if the engine is warm and you park the car on a hot day, the heat from the engine will transfer into the coolant which is no longer being circulated. For a period of time, until the engine cools, this coolant will be hotter than the normal temperature the thermostat maintains when the car is running. This will cause a hotter-than-normal reading when you start the car up after a short period of time, but it will drop back down as the coolant circulates and (if needed) the fan runs, or airflow from driving will bring the temperature back to normal.
In cars with mechanical instead of electric fans, the gauge tends to remain more stable since there's always airflow over the radiator. Also, some cars with electric fans run them continuously, varying the speed as needed instead of cycling them on and off, with the same effect as a mechanical fan, a more stable temperature gauge.
When you have a problem, is when the gauge reads higher than normal under normal driving conditions. If you're cruising down the highway, or even in town if you're not stopped a lot, or the weather isn't hot and the gauge still reads higher than normal, then you have some problem with the cooling system, such as low coolant, a faulty thermostat. Also, if the gauge ever goes into the red zone, turn the car off! If it goes into the red when idling but not when driving, it's probably the radiator fan isn't working.
I used to have a Saturn SL2 (similar size/build quality to your Civic) and it did the same thing. When idling for more than 10-15 minutes or so, especially on hotter days, the gauge would creep up toward the 3/4 mark, and just before hitting 3/4, the fan would go on (computer controlled) and run until it was a bit higher than 1/2 way (it dropped fairly quickly with fan running), then it would stop, and repeat as needed. When driving, the temperature would drop back to the level the thermostat would maintain, which was at or slightly below the 1/2 mark. On my Audi A4 I drive now, the gauge stays in the middle no matter what, because this car varies the fan speed instead of just switching it on and off at certain thresholds.
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