1997 Honda Civic - Unless heater is on full blast, engine overheats?!

I’m not really a car guy, but I can’t afford a mechanic. So I’ve been trying to fix this myself, and any thoughts would be appreciated.

The car I drive is a 1997 Civic EX, and if I drive it for about five minutes, the temperature gauge starts creeping up real fast. It’ll go to the red if I let it, and the only thing that brings it down is turning on the heater and fan, at full blast. which is pretty miserable in, you know, JULY.

I looked under the hood and found the radiator fan isn’t starting up, no matter how hot it gets. So I checked the fuse - it looked good. Replaced it anyway just to be sure. No change. Then I replaced the relay switch. that didn’t change anything. And I also replaced the fan motor. Still nothing.

I have read elsewhere that I should check my thermostat, but I can’t see what it has to do with the fan motor not starting? also that the Engine Coolant Temperature sensor might be faulty but it is my understanding that that controls fuel-air mixtures and I just don’t know.

I’m stuck as to what I should do next.

a little background - everything seemed to start a month ago when my mother drove the car a good distance with the oil cap off, /somehow/. Everything under the hood got a pretty good coating of oil, and all the trouble started after that.

How did oil cap come loose? Did ma check/add oil or did you?

Do the fans run when you turn the AC on? If they don’t run with the AC on, the problem isn’t likely a temperature sensor because they are supposed to be on at that time.

Here’s a diagram of the possibly affected area.

Get a multimeter. With the car off, key out, set it to measure ohms and see if you have a solid connection from one side of each of the fans and to ground (like the engine block or battery’s - post). If you don’t see near zero ohms, G100 and G101 may be open. They could very well be located under the same bolt on the frame, which would explain it.
Still with the key out, also make sure you have 12V across the condenser’s fan with the AC on. You should have 12V on the common of the relays’ contacts and one side of each of relays’ coils. If you don’t, check the fuses and wires.
Then see what happens to the other side of the coils as you turn the car and the AC on. You should see one side of each of the coils go from 12V to 0V.
It takes a little detective work but the circuit isn’t super complicated.

Does it overheat at 60 mph? If it does, you have 2 problems, the fan not going on (bad sensor, connection) and plugged cooling system (try thermostat, if no help then I’d replace the radiator).

Cavell - Seems like she just forgot to put it back on. Usually I’m the one who checks the oil but that time it was her.

RemcoW - the fans don’t seem to be turning on, whether the AC is on or not. I’ll borrow a multimeter and check it tonight or tomorrow! Thank you.

texases - it takes longer to overheat at freeway speed but it still does. I’m checking the thermostat today (as soon as I can get the thing out, good grief), too.

I’d put on a new radiator cap with the thremostat, too, if you haven’t, a cheap fix if it helps. A new radiator is under $100 at rockauto.com, so don’t pay big $$ for one.

Run a jumper wire from the battery (+) to the fan connector. THAT should make the fan(s) run. If so, we are back to the temperature sensor which is located near the thermostat housing usually…

Welp! I half fixed it.

texases - you were right! the thermostat WAS shot (radiator cap was fine though). I tested it next to a new one to be sure and oh yeah. So that’s installed and now the engine is sitting right at the temperature it should be, even with driving up hills or in stop and go traffic, no heater required.

The fan still isn’t turning though, so I guess I still have that to take care of. I wonder if it’s been out for a long time and I just didn’t notice until the thermostat quit too…

Caddyman - I wanted to try and test/replace the sensor by the thermostat but I actually just COULD NOT get the thing to come off! And my tools were chewing up the brass so I’m letting it ride for now until I rule out the circuit being the problem or not.

If the sensor you’re talking about it the thermosensor on the above schematic, why not just short the contacts on its connector? It is just a switch. Shorting the contacts on the connector will simulate an overheating condition. If the fan turns on, you know the sensor is bad. If the fan does not turn on, the problem is likely elsewhere.
That way you don’t have to pull the sensor out and booger the thing up.

When you switch on the A/C, the fans should run regardless of coolant temperature…The fan relay is energized by the temperature sensor or A/C…

@Caddyman: On most cars, the a/c condenser is mounted in front of the radiator and the cooling fan runs to cool the a/c condenser. On the Civic, the a/c condenser is mounted next to the radiator and has its own fan (radiator and a/c condenser are each about 15" wide), which kicks on any time the a/c is turned on. The radiator fan does not turn on with the a/c, at least it doesn’t on my '96 Civic. If it helps any, on my Civic, the engine has to be sitting idling for probably 20 minutes before the radiator fan kicks on. I learned this today while charging the a/c system.

Some systems will turn the fan on if you disconnect the fan switch/sensor. That was how it worked on an 83 Camry. So if you disconnect the sensor turn the ignition on and the fan comes on, you’ll at least know that the fan works and the sensor may or may not be good.

The sensor you tried to remove may not be the one. Look for another one on the head. I don’t know if there is another one on your car. One is just a sensor and the other one could be the fan switch. Cars have lots of parts.