I know there have been thousands of threads about the cooling fan not coming on but mine seems intermittent and I couldn't find any specific threads about that; only ones about it not working at all.

First the car is a 1987 Honda Civic 1.5L carb'd.

It overheated one day and I saw the fan wasn't coming on. I did the test of jumping the fan wires and the fan came on so I replaced the fan switch in the radiator. Still the fan doesn't come on. So I swapped the relay with the other relay that is the same (defroster, maybe?), let the car come up to temp and the fan comes on. Ok, so I'm driving to work and the car over heats again, still no fan. So I pull over, turn on the AC and the fan comes on for 5 seconds and turns off, but the temp gauge is pegged so it should be on constant. It does this turn on the 5 seconds, off for 10 seconds things for a while. I wait for it to cool down, drive back home and by the time I get there the temp gauge is pegged again, but as soon as I pull in the driveway the fan comes on and stays on.

So I'm at a loss. The fan seems to come on ok when it wants to, but I can't figure out the conditions of when it wants too, it seems rather random and to never happen when I'm actually driving the car.

I'm going to try bleeding the coolant system again, and just connecting the two fan wires so it should stay on all the time. Then if it still goes on an off randomly is has to be the fan. Could that be what happens, maybe the fan motor is overheating and it is turning itself off? I don't think this is it because when I turn on and off the AC I can hear a relay in the dash clicking and the fan comes on and off appropriately, it just will not come on when I'm actually driving.

 

I think you have other cooling problems than just the fan. Car shouldn't overheat when driving with air flowing thru the radiator

 

I've had the same thing happening to my 99 Accord. It's got the Vtec 4 with 290,000 miles and began overheating two months ago. It was losing small amounts of coolant over a one to two week period. Didn't think it was the head gasket or a crack because there were no contaminants in the oil and no loss of compression.
I swapped out the thermostat, relay, switch and eventually put in another used radiator. The problem only got worse. My mechanic couldn't figure out what was wrong either. Then last weekend, I decided to flush the cooling system, replace the thermostat again and put the original radiator back in. It was then that I noticed that the cooling fans only come on if the defrost is on (condensor activated) and they do the same thing. On for 10 seconds, off for 20 and so on. The good news is that the used radiator was obviously part of the problem but the car is clearly going to overheat in warmer weather, even if that's a few months away.
Rather than tracing down the wiring issue, I'm considering using an aftermarket relay system from Advance Auto Parts and wiring it up directly to the fans. It's only $18 and would give me peace of mind. Any thoughts about this approach would be appreciated.

 

Good info

http://techauto.awardspace.com/overheating.html

 

Check your relay wires you could have a false negative to your relay meaning not a good connection oor a broken wire.

 

Is your car automatic?