Why is my car overheating?

Hello all,

Here’s what’s happening: I go on a long trip (more than one hour) in my 2005 Honda Civic. I pull off the interstate, and the temp gauge quickly goes up. Within three minutes, the gauge is pegged in the red, especially if it’s a hot day and I’m stuck in slow moving traffic. If I can keep the car moving, the temp goes down. I run the heater to try and dissipate the heat, but the blown air is not hot – sometimes, it’s even cold. The last few times this has happened, I know I’ve run my AC before getting off the interstate, but I don’t remember if that is always the case. This may be happening without AC running at all, and I always turn the AC off immediately after slowing down.

I’ve always been able to park and let my car cool down before anything bad happens, or I get back on the interstate and the air blowing through the radiator cools the car back down. The fluids and fuses both seem to be fine.

Is the thermostat toast? Is there a clog in the hoses? Is the fan on the fritz?

Help! I bought this care for it’s longevity – I don’t want to kill it before its time.

Thanks in advance

I was going to guess the fan, thats what makes the most sence and is easy to test (pop the hood is it on ?? No than thats your problem)… However having no heat has me wondering if you have another issue, not sure what to make of that.

The first thing to check for is if there’s air trapped in the cooling system. This can cause the no heat problem and if there’s air in the location of the coolant temp sensor for the radiator fan the fan won’t turn on. Coolant temp sensors don’t read air temperature.

With the engine cold, look on the engine for an air bleed port for the cooling system. Open the port and add coolant until all the air is purged from the cooling system and where nothing but coolant comes out of the port. Then close the port.


One really simple thing–bugs and other debris may be blocking the fins of the radiator. Take a garden hose or an air hose, and from the engine side direct the air or water stream throgh the honeycomb of the radiator.

If the coolant level in the radiator is right up to the top of the filler neck (when everything is cold), then the next things to look into are…
a cooling fan that is not turning on when it is supposed to
air trapped in the cooling system
the use of water instead of a 50-50 coolant/water mixture in the cooling system.

@vdcdriver I thought water by it self was a bad heat xfer media. That anti-freeze or water with a water weter product made water more efficant… Infact plain water can cause over heating in some cases. Is this wrong?


I think that you are misinterpreting what I wrote.
Re-read it, and you should see that I am listing the possible problems–as I see them–and in addition to an inoperative cooling fan and trapped air, the use of “straight” water would indeed be a problem.

Just as I am not suggesting that the OP disconnect the cooling fan or try to trap air in the cooling system, I am also not suggesting that the OP use “straight” water. Re-read it, please.

The heater blowing cold is because of low coolant level in your radiator, either because of overheating and pushing the water out. or because of a leak and low coolant can cause overheating,

Plugger radiator fins would cause overheating on the highway, not when you slow down.

The first thing to do is to fill your radiator with the engine cold. You can use water if you are in no danger of freezing (just until you have this problem solved. Do this several times with the engine cold each time).

If you car stops overheating, then cet the cooling system pressure tested to find the leak.

If it still overheats, see if your radiator fan is running with the engine hot and the A/C and defroster off.

If the fan isn’t running with the engine overheating- thats is your problem and you have a bad fan relay, fan motor, or fan controller.

If you fan is running, your radiator is full , and it still overheats only at slow speeds I am stumped.

I didn’t mention a head gasket because I have never seen one overheat at low speed and not on the highway.

The only overheating problem I have seen that was really hard to diagnose was on a car that someone had repaired the fan wires and hooked them up backwards- it would only overheat at 45-50 mph.i

I would recommend starting with a pressure test, coolant loss is often times indicative of a pressure problem, but not always. Low coolant pressure can exhibit your problems and it may be as simple as a radiator cap.

One thing that is easy to check: Is the radiator span spinning and whirring away when the temp guage says the engine is hot? If you don’t know, the next time it gets hot, pull off the road, pop the hood, and look at the fan. It should be spinning. If not, there’s a problem with either the fan or the control mechanism which turns it on. If the fan isn’t spinning, you’ll have to fix that first; then if the problem remains, you can continue the debugging process as stated by the good posts above.

Be careful, the fan can turn on unexpectedly, so keep your hands clear and don’t be wearing stuff like can be caught in the fan, like neck-ties, etc.


You are correct I did mis-read… My apologies good sir :wink:

The OP really needs to get this sorted out quickly. Pegging the temp needle to “H” could mean a damaging overheating event. These motors do not handle overheating well, and a blown head gasket or warped head would become a very expensive repair.

The fact that cooling occurs when the AC is on and when you are on the highway (air moving past radiator) indicates your electric cooling fan isn’t working properly. There is a separate cooling fan connected to the AC which always goes on whenever the AC is “on”. Could be the regular electric fan is bad, a sensor is bad, or a relay switching the fan on and off is bad. The lack of heat is likely due to air that got in the system as a result of overheating pushing hot coolant out the overflow vent.

Without further delay get the car checked out. A new radiator cap could solve the problem, or a new fan, or a new relay. All of which is relatively cheap compared to a blown head gasket. Once the problem is repaired monitor the coolant overflow container every morning when the car is cold. If the tank is empty fill it 1/2 way with 50/50 mix of coolant. Once the level stays the same for several days you will have purged all the air from the system and all should be well again.

First thing I would do on this model is turn on the ac and see if both fans come on. If only one fan comes on, then either the fan is defective or something in its electrical circuit is defective. Could be a relay, fuse, thermostatic switch, fan control module or wire.

Then I would check for trapped air per testers comment. I suspect that it is trapped air, but the fan test is easier and quicker to do so I would do it first. It could be both.