Overheating while on Freeway

I had the catalytic converter checked today. They said it is fine.

It feels like it has lost some power and gas mileage is worse than normal. Also, when it does start to get hot the a/c starts blowing warm air.

Also, the coolant level has not dropped since doing the head gasket so there is no leak.

The muffler shop said the converter is not plugged. I will check the timing and post here after I do it.

Thanks

2001 Honda Civic LX 1.7L (not VTEC), MT, 90500 miles. This car has developed a mysterious problem this fall and winter of increasing in temperature on the freeway (i.E. Driven at 2700 to 3000 rpm at speeds of 60-70 mph). The problem is most noticeable when placing a load on the motor, e.G. Driving uphill in rolling Missouri terrain. Turning the heater on helps alleviate the problem and sends the gauge back to 2/5th's where the engine has always run steadily the previous 90000 miles, including uphill in the summer at speed with the A/C on. I do this when the temperature approaches 3/4 on the guage, or near the top of the temp "icon" on the guage. The ambient temperatures encountered with this problem have been 30-60 deg F so far. I have already replaced the thermostat as a cheap precaution. A radiator flush and fill was performed by a nationwide brand name auto care center at 77000 miles. The current coolant mix has protection down to 20 deg F, meaning with more water it should provide better heat transfer (I think; the relatively high freezing point is a separate problem and will also need to be addressed). Lower and upper hoses are both hot. No sweet smelll or white smoke in exhaust. No coolant loss has been noted, and no appearance of oil or bubbles in coolant when run with the engine warm. Any help/advice would be greatly appreciated, as I am worried about this car overheating in hotter summer months.

I'm so sorry - I triple-checked my post and thought I had included all of the pertinent information. After installing a new thermostat and rubber seal (gasket), I idled the car in my garage on blocks at 1500 rpm to open the thermostat and purge air from the system in accordance with my Chilton's manual, and the fans kicked on twice in this process as described in the manual with the temperature at a normal fully warm setting (although without the A/C on). Original timing belt and water pump, but not req'd until 105000 miles according to service manual. I am thinking this might possibly be a plugged radiator somehow (?).

I tried a new radiator cap today after work and drove it on the highway again. No luck, same symptoms - the car will run very hot at high speed (70 mph, especially uphill) and it drops back to normal (2/5 of gage) as soon as the heater is turned on. I also tested the collapsing hose tip; after returning home I gunned the throttle body with my thumb and saw no sign of hose collapse. Starting to really think this is a bad radiator somehow, but not ready to jump to conclusions. Great site; great advice!

I have a 2001 Honda Civic, I recently have had overheating problems. I have changed out the thermistat, the radiator cap, flushed the radiator, I drove it for two days after the repairs with no overheating issues, until today, after running the vehicle for approx. 20 minutes, once I came to a stop, the Temp. Began to rise again. There are no visible leaks either from hoses or water pump, I would appreciate some help with this issue.

Yes, that was a good tip! I finally had a chance to swap radiators this weekend, used a Koyo model and got it done in one afternoon. Took it out for a test drive this evening and it runs like new, no overheating problems, even uphill accelerating on the highway w/ the AC on! Thank you all for the great website and advice.

Some advice of my own for other DIY mechanics - the engine drain plug is a bear to get at, had to use a 1/2" drive ratchet w/ 2" extension and universal joint to get it out. Sometimes the owner's manual is a good source too - I flipped through it after finishing and discovered I was supposed to put non-hardening sealant and a new washer on the plug; Chilton's never told me that! We'll see how long it holds for without leaking.