94 Civic overheating when on Highway.

This problem has come on slowly over the last 6 months, but has also been getting hotter here in the last 6 months. This problem only occours when on the highway at around 70 to 80. It is worse when I have my kayak on top of car. The temp guage slowly works to top end of operating guage, when I kill AC turn heat on full blast, which will bring it right back down. Then 5-10 minutes later I repeat process. I just changed out thermostat, and that is not the problem. I don't think the radiator is clogged. Anyone out there have a similar problem? Any suggestions? I had the system flushed out a year ago. BTW, I use this car on the weekends to drive to riveres I kayak on. I have a Accord I drive other days. Thanks!
James

I have the exact same problem. This morning I noticed that my cooling fans aren't working, and I'm guessing that's what my problem is.

Have you checked the fan switch? After the thermostat, that would be the next thing I'd check.

The fan was the next thing I checked. It is running. If the fan switch is bad would it still run? Or is it something else?
Thanks again,
James

FIrst, that kayak is a huge parachute on a civic, meaning that it's alot of drag for the small motors to overcome. But if the fan is turning on then it's nothing to do with the fan system. I don't want to sounds like an ass, but have you checked the fluid level, not in the bottle but in the radiator? If that's not it, how old/how many miles are on the waterpump? Just to cover the really odd instances, like I had, you dont have an exhaust leak that blows massive amounts of hot gas on the radiator, right?

The civic has 138k miles on it. I'm not sure if my brother (previous owner, I've had it last 2 years) ever changed water pump when timing belt was replaced. If I'm pretty sure radiator is not clogged, is the water pump what it must be?
Thanks again!
James

It could very well be. Good honda mechanics strongly recommend that you change the pump with the belt just because of when the pumps tend to go bad. and when you have the belt off, it's only 4 more bolts to replace so very little labor cost. Whereas if you replace it now, you have to take off the timing belt again.

If everything else works fine in the cooling system, like radiator, fans, thermostat, then id bet it's the pump.