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im working on my daughters 03 civic si 1.7liter canadian model . car started blowing cooler air at idle and hot at speed . i have been reading the head gasket posts but have diff/but some similar symptoms.no filling of overflow bottle no antifreeze smell or signs of leakage but had to add about 4 oz of antifreeze to rad ... then a test drive with a scangauge attached for real time temps.it ran 192-205f at 50mph and the heat on then with it turned off 205-210f i noticed it was only getting bout 20 mpg at this speed not good.then i accelerated up to 70mph and the temp went up to 235f so i pulled over and turned on the ac to get the fans running and the temp came down to 205f so the main fan isnt comming on as needed . my first question is where and which sensor and or relay controls the main fan .i am gonna change the rad cap,thermostat and do all the other checks before the head gasket because it should be done anyways regardless.the upper rad hose is hot the bottom is luke warm after idling for ten min at 205f and no fan but cools quickly by getting them on by the ac mode.car has 370,000 kms and runs great just did the timing belt ,water pump,idler pulleys and tensioners bout 15,000 kms ago.sorry bout the long post and thanks for any help....lee

 
Rep Power: 517

I'm assuming the timing belt wasn't installed a tooth off....incorrect, especially late timing can cause an engine to run hot, and lose gas mileage, while many people wouldn't notice the slight loss of power from the timing error.



You probably don't need any sensors. The fan switch senses coolant temp from the bottom radiator hose, you said that was cool so the fans do NOT need to run.


I'm confuzzled by the mixing of metric and standard here too LOL

Driving down the road above 50+KPH in cool weather there should be plenty of air flow to keep the radiator cooled without needing the fans to run, your ECT measured temp should have been in the 202F range or lower at any fast speed. Never 235F.
If the stat was stuck closed you'd have blazing heat from the heater, not cold air. Both of these descriptions together indicate most likely "low coolant" to me.

Low coolant level leads to localized hot spots or overheating (mostly within the cylinder head areas, the hottest location in an engine) which can be seen on the temp gauge and scangauge (because the ECT is located in the upper rad hose outlet) but by the time that hot liquid passes through the radiator it's become cool enough the fan switch doesn't see it as hot.

The cooling system could still be quite low even though you see the radiator appears full. You need to get trapped air burped out of the cooling system first.....then refill the radiator to the top, and adjust the level in the reservoir to the max line (cold engine) and maybe mark it with a sharpie for future reference so it's easy to see.

I like to use a cooling system funnel and raise the nose of the car as high as I can safely get it, in order to make the radiator the highest point in the cooling system.
Set heater temp to high, but leave heater fan off.
I run the engine at 2-3000 RPM to increase internal water pressure and velocity until I'm sure it's all burped out. (if the bubbles never quit, that indicates a likely head gasket problem)

AFTER the cooling system is ensured to be completely filled and free of trapped air, then you can let it run to see that the fans do work correctly when the temp sensor tells them to run.

Then figure out why it was low in the first place.



You're wanting to change sensors and the thermostat, none of which may be a problem (but cheap thermostats are a problem) and you will then likely refill and burp the cooling system to a level sufficient to mask the problem for what might be several hundred miles. Eventually the problem will return if you don't figure out why the level was low.

 
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thanks for the info and sorry if i mixed up the metric and standard measurements the timing belt was checked twice after several rotations so it think its ok and the mph was suppose to be 50mph not kms and with it getting that hot at 70 mph im assuming that the main fan didnt come on till i used the ac to get it to work. the thermostat is of unknown origin/condition and the cap is honda but could use replacing.

 
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nowthat you have me second guessing myself other than ripping it all apart is there a quick easy way to check to see it its out of time? thanks lee

 
Rep Power: 517

I'd think that if burping trapped air out resolves the 'running hot' situation at least temporarily, that would lead suspicion away from incorrect timing.

Almost nobody has ever mentioned (posted here) anything about timing belt off by a tooth causing their engine to run hot, but you commented about low mileage so i tossed it out as a possibility. (though you really didn't indicate anything about actual documented long term mileage drop)


You said you pulled over and stopped before you turned on the AC. Stopping the car removed the load, that stopped the overheating. (high load burns more fuel)
The fans helped reduce the temperature quicker of course, but stopping the car is why it quit overheating. (or even slowing down reduces the load enough to make the temp go back down, if you had slowed to 50 the temp should have gone back to the previous temp)
Both of these are still indicative of low coolant level.

The cooling system only holds a little over a gallon, any loss or displacement of liquid can quickly compromise the efficiency of the system.

 
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thanks again ezone for the advice im gonna try to check out these things tmw ...lee

 
Rep Power: 178

Did you check your thermostat switch (thermoswitch)? Do the paper clip trick and see if the fan turns on then your switch is bad. I had to change that switch twice one was the oem and the other was aftermarket. Now I'm using an oem one and its been good.

If then fan turns on with the paper clip trick leave the paper clip in the connector for a day and see if your car still over heat. Note the fan will be running all the time.

2:32

Here is an example what the switch looks like.

 
Rep Power: 517

^Yes this, and the fan switch is on the thermostat housing on the front of the engine.

Note: If you get the system filled and burped, that alone should also stop the overheating.