Recently, I have started to notice that the temperature gauge on my 2000 Hyundai Elantra was starting to run a bit hot. She would get to mid-range in only a minute or so, and she would get up torwards the red if she idled for too long. A cursory examination revealed that the left radiator fan was non-funtional. A quick check with a test light revealed that it was the fan, and not the temperature sensor. Since I had to remove the upper hose to replace the fan, I decided to replace the hoses and thermostat while I was at it. After all, at 160K miles, it was probably about time. I had no problems at all replacing any of the parts, although I did get coolant all over the transmission when replacing the t'stat. Also, the t'stat was difficult to seat, because it goes in sideways. When everything was back together, I took her for a test drive this afternoon. Everything was fine for about half an hour. The temperature gauge was moving considerably slower than it had before, and it took nearly five minutes to get up to about 1/3 of the way up. I stopped for about half an hour, and when I started again, I did notice that I was running a bit hot. Less than a mile later, I was almost in the red. This was not at highway speeds, but on twisty-windy mountain roads. I pulled over as soon as I could, but couldn't see and problems under the hood. The fan was running, and I had plenty of coolant in the overflow tank. I could smell antifreese, but I figured that it was just what I had spilled earlier boiling off. It was either call a tow (for more than $100.00), or limp home keeping a close eye on the temperature gauge. I was way out in the boonies on a Sunday afternoon and I didn't have a cellphone, so I decided to risk driving home. Experimentation proved that turning on both the heat and the A/C seemed to help regulate the temperature. I could understand the heat helping, because airflow over the heater core acts as a second radiator. At first I didn't understand why the A/C helped, until I remembered that the second radiator fan only came on with the air conditioning. I was able to determine that speeds of about 50 mph definitely helped keep the temperature gauge at about the middle- halfway between the C and H marks (actual numbers on the gauge would help). As soon as I slowed down, however, the temperature shot up to H, then just as quickly went down. Downshifting also made things worse. As I live in the mountains, driving at a constant speed of 50mph is somewhat difficult. After all, Sterling Moss might be able to take a hairpin curve at speed, but I ain't him, and my Hyundai ain't a Mini. When I was on moutain roads again, driving slowly (uphill, unfortunately), the temperature was hovering around H, although it never did get into the red. After I made it back home, I looked under the hood again. The fan was running for all it was worth, and I still had plenty of coolant in the overflow tank. I also noticed that I had a small leak around the thermostat housing. As the coolant dripped out onto the transmission, it was actually sizzling and steaming. However, there were still puddles of coolant from the earlier spill, so I don't think my tranny was that hot. I used a Fel-Pro thermostat gasket rather than the Hyundai gasket, so that might be the problem there. Also, the lowermost bolt is a pain to get to, so it could still be a bit loose. There are four things that I think could be the problem: 1) Thermostat. Perhaps it was seated wrong, or the non-Hyundai gasket might be interfering. The new thermostat was from the dealer, so I doubt if it was bad. The upper radiator hose was hot, but not unberably so. 2) Plugged radiator. The coolant that I drained out looked clean, but that may not mean anything. Less than a gallon came out into the drain-pan, and less than a gallon when back in. I don't know how much spilled out through the thermostat housing. I haven't opened the radiator yet, as she is still hot, and I am alergic to high pressure steam. Again, the overflow tank is at the 'full' level. I used a non-hyundai radiator cap, but I don't think that should cause any problems. The pressure valve on the cap did not trip. 3) Water Pump. I still have the original at 160K. I've had the timing belt swapped out twice at the dealer, and both times the mechanic said that the pump was still in good shape. I was under the impression that when a water pump takes a vacation, it starts leaking before it is actually dead. My water pump is not leaking, to my knowledge. I have never been low on coolant. 4) Transmission running hot. This is my third transmission. My first one went out at 150K miles. I went to a franchise transmission shop to have it rebuilt, and it failed after 8K miles. The shop refused to honor their warranty, because they only guaranteed 6K miles. I was able to get a junkyard tranny with only 40K miles. It is still under warranty, so I'm in luck if it is the transmission, although I am not sure if the transmission would cause her to overheat that much. Hopefully, she wasn't running so hot as to warp the heads. A head gasket job on a DOHC engine does not come cheap, and it would probably take me two months to do it on my own in my spare time. I am going to limp in to my mechanic tomorow. He should be able to let me know if it is the tranny or not at least. Does anybody out there have any ideas? Fortunately, my old Chevy truck is still running, otherwise I'd be driving a rental for the past month.