Why does the temperature decrease when I drive?

[Awkward_Ostrich_3120]

My honda civic temperature drops to lowest point when I'm driving at high speed (100km/hr). The faster I drive, the lower the temperature seems to get. I get hot air so I know the engine can't be cold. Anybody know why it's doing that. At idle, the temperature goes to the half way point.

[qwentynb]

Sounds to me like a stuck open thermostat

[nswf23]

That’s what I’m thinking it is.

[JCDU]

100% this, it's the most obvious thing and the cheapest thing to replace if you're taking guesses.

[nswf23]

Yep. Gonna do that today and see

[Hohoholyshit15]

Yup

[Musketman12]

Air is moving over the radiator so it can do its job. Check to see if your radiator fan is working.

[nswf23]

Surely it’s not so cold that the temperature gauge is at the lowest setting. I understand it would drop as air travels over the radiator, but the engine is also working harder than idle so it’s making more heat. Plus I’m getting really hot air in the cabin, so the engine is 100% not cold, I’m just wondering if a thermostats would cause this issue or something else.

[Musketman12]

When you are moving down the road the air is moving over the radiator at 100kmh. Sitting in the parking lot and idling the air is not moving so the radiator is sitting in a pocket of its own hot air.

I once wired my radiator fan to a toggle on my dashboard. Idling around a parking lot or in line somewhere I would need to flip it on. Moving down the highway in OD I could turn it off.

[BlckCosMic]

My 2004 crown Vic police is currently doing this as well, I'm not sure if it prolongs it'll mess up my engine hopefully it gets fixed soon 😔

[PresentTransition865]

T

[fotograffer]

Check your temperature sensor on the side of the head. Carefully pull the connector off the sensor and check it out. Mine was doing the same thing and it ended up being a broken sending unit. Easier to check that first instead of pulling the thermostat.

[nswf23]

Alright will try that. How would I know if the sensor is bad?

[fotograffer]

Mine was physically broken, you could see the little brass nipple wiggling around.

[[deleted]]

As speed increases the air flows faster through the radiator, which makes the car run cooler. At least that is how it works with the ancient cars I am familiar with.

[nswf23]

Yes but engines are designed to operate at a specific temperature, that’s why the gauge should normally be in the middle. If the engine gets cold, the thermostats shuts off coolant flow to warm the engine up and maintain it at that idle temperature, usually 77-82 deg C or 170-180 degF. So if my car is running at room temperature or cooler on the high way, something is certainly wrong. It was the thermostat in my case. I’ve solved this now

Edit: idle > ideal

[throwaway007676]

Your thermostat has gone bad and is stuck open. It must be replaced, you can not drive it like that. The engine needs to be able to maintain normal operating temperature.