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What's wrong with my '97 Jetta's Cooling System?

This summer, on a hot rainy day, my 97 Jetta’s temperature gauge climbed to ~300F and the warning light came on. So I let the car cool, topped off the coolant (which was low) and went on my merry way.

It ran trouble-free for 4 months, (though during that time, it has consumed about 1 gallon of coolant)…until this past weekend. On a 250 mile trip, returning from Thanksgiving in Northern New York State. The weather was sleet/freezing rain, about 32 degrees. I had been driving for about 30min at 60mph. Suddenly my temp gauge climbed to ~300 and the warning light came on. Fluid level was OK. I let the car cool, started driving again. Everything fine for ~10 miles, then same thing happened. I also noticed that the intensity of the interior heat dropped to luke-warm just before the coolant temperature needle began climbing.

For the rest of the trip, the car overheated four more times. Sometimes it would run fine for more than an hour, other times it would overheat after just 10 or 15 minutes. But the overheating was always accompanied by a loss of interior heat. I eventually figured that I could sometimes stave off a climbing temp gauge (and get a return to normal running temperature) if I coasted for a minute and then carefully finnessed the level of acceleration.

I have searched all over the web, but much of the info out there seems to conflict. Does this sound like a sticky thermostat? Water pump? Air trapped in the cooling system? Clogged coolant lines? Please help. I can answer any other questions you have that may help to diagnose this problem.

Thanks,

Frank

Good question about the thermostat. Change it, find out, and let us know. Oh, and fix that cooling system leak before you have another phenomenon to contend with.