My car is overheating at high speeds

I own a 1997 Mazda Protege, manual transmission. It has over 290,000 miles on it (I know what you are thinking), but runs really great. 85% of the miles are interstate miles going 75 mhp with little braking or “ware and tare”. Earlier this summer as the temperature began to get into the 90’s I noticed the temperature would get up near the “hot” range on the temp gauge. It does this in the heat of the day when I am driving on the interstate @ 75 mph, and the A.C. is running. When the car temp gets up near the top of the gauge, the on board computer cuts off the A.C. compressor, and the car temp will cool down. The A.C. compressor will then kick back on, and the cycle starts again. For example, on a 95 degree afternoon, I can start the car up cold. I can drive it on the interstate for about 45-60 minutes and the thermostat runs right in the middle of the temp gauge (with A.C. running). After 45-60 minutes, the thermostat starts gradually creeping towards “H” on the gauge. Finally, it will reach a temp such that the computer shuts off the A.C. compressor, and the thermostat will cool down until the A.C. can re-engage. Once this cycle starts, the A.C. can run 3-4 minutes, then it will kick off for about 2 minutes until the car temp cools. This pattern will cycle over and over again until the outside air temp cools off after the sun goes down. Here’s a couple of kickers that throw me for a loop… Driving around town at lower speeds (&RPM’s), in “stop and go traffic” , the problem never shows itself!! The car temp runs right in the middle of the gauge. NO OVERHEATING at all??? Also, I have determined that when I am on the interstate and running hot, I can get off at an exit ramp and down shift to slow down. When I do this, you can literally watch the temp gauge drop for “H” into a normal operating temperature within seconds. When this happens, all I can conclude is that I have maximum coolant circulation and minimum heat production from the engine (throttle is not being applied). If I just kick the transmission into neutral while going 75 mph and coast, the engine will cool off but it takes a minute or longer, and… it does not cool off nearly as drastically as when you down shift while going off an exit ramp.

I had my mechanic look at the car. He first determined that the auxillary A.C. fan on the radiator was not working. It was replaced. However, the problem persisted. Next, we replaced the radiator, but the problem still persists. The fans are blowing in the right direction… Is this a thermostat or water pump problem?? These symptoms that I am having are not typical of a pump or thermostat…are they??

Please advise. I don’t want to do any more guessing on the car’s overheating…

Yes it could be the thermostate and they are easy to check and cheap to replace, but many are difficult to get to. My guess is the water pump. If it is the original water pump I would just replace it.

With that many miles on it, plan on replacing the water pump and T-stat. Also check the condition of all hoses including those little (1/2", 5/8" or 3/4") heater hoses. If they’re originals, replace them all with new. Before you dismantle anything, drain about 1-1/2 quarts of coolant. Don’t do this with a hot engine. Buy and pour in a can/bottle of cooling system cleaner. This will get most, if not all, of the contaminants that inevitably build up in cooling systems. Follow the directions. After engine cools, drain the cooling system, flush thoroughly with clear water, then change out the hoses, water pump and T-stat. Now is a good time to do a second clear water flushing. Get as much junk out of the system that you possibly can. Dispose of properly. Empty plastic 1 gallon milk jugs work well for disposal purposes. Thoroughly clean all old gasket surfaces. A rag stuffed into the T-stat hole will help keep old scraped gasket materials out of the T-stat port. Just carefully pull the rag up out of the hole. A wet rag will help keep the gasket material stuck to the rag. Carefully blow out the water pump housing with low-pressure compressed air. Use the new gaskets and replace the T-stat and water pump. Use new clamps when replacing hoses. Get as close to recommended torque specs as you can. DO NOT OVERTIGHTEN. Also check for obstructions on the radiators. (Leaves, twigs, bugs, etc.). Clean as necessary. Use at least a 50/50 anti-freeze/coolant mix. Most vehicles can use up to a 60/40 mix. (60% anti-freeze/coolant). Don’t forget to re-fill the coolant recovery bottle to the “Full when cold” level. Start 'er up. Check for leaks. Run it up to operating temperature. Shut 'er down. If coolant level in the plastic recovery bottle is low, add fluid to the “Full when hot” level. You should be good to go. Follow your vehicle manufacturer’s recommendations for coolant/anti-freeze miles or time intervals for replacing coolant/anti-freeze. Don’t let coolant stand in a puddle or leave it in an open container. Dogs and cats love that stuff but it’s fatal to them.

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Sorry about the shouting, but this little, inexpensive part is overlooked so often. These are the classic symptoms for a radiator cap that no longer holds pressure. It’s only $12 or less.

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Thanks for you r input. I’m trying a new cap. A friend at work recommended this too. I hope this is all the problem is. I will let you know if it works

Many cars that have a small frontal area use an airdam to aid in cooling. This is a small plastic strip several feet long and usually 3 inches or so wide. It is usually mounted up underneath the lower radiator support and its job is to force air upwards so airflow through the radiator will increase.

I do not know if your car is (was?) equipped with one or not, but you might look underneath and see if one may have been present by any remaining scraps or empty holes where it was mounted.
Curbs and critters knock these off all of the time; usually without the car owner even being aware of it.

The symptoms of a missing airdam are overheating at highway speeds, with or without A/C on, but overheating will not occur with in-town or slow speed driving.
Just a possibility anyway; hope it helps.

Can you plz update what was the conclusion, my car is getting heat up same way

It’s been 11 years. People die. Thermostat, bugs covering front of radiator, radiator no longer effective, or air dam issue, would be my guess…

Since the OP’s last post was about replacing the radiator cap, I suggest let’s assume that is what fixed it. Pressure testing the cooling system (and the cap) is where to start on overheats at high speeds, presuming a visual inspection shows nothing obvious, and the engine timing and fuel trims are ok.

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Considering OP had 290,000 miles on a 1997 Mazda Protege back in 2007 . . . I will assume he doesn’t even have the car anymore. It’s probably scrap metal by now

OP has not been active on this website since August 20, 2007, so your only shot is to click on OP’s avatar. Then you will have the opportunity of sending him a message, which will be sent to the email that was associated with his cartalk account in 2007. If he has a different email address, you’re out of luck.

Nice catch, db.
I suspect the problem is solved by now… perhaps just like the car’s in your photo!. :smile:

You know, a few times over the years we’ve donated cars . . . to our local npr-affiliated radio station, for the tax write-off

These particular cars were old, physically battered, running okay, but were worth peanuts, so little that we felt it wasn’t even worth listing them on craigslist for $500.

I suspect they look like that hunk of metal in the picture now

tl;dr I’m having same problem and the only thing I haven’t replaced in the cooling system is the radiator cap. I WILL report back on if this fixes it. Or I’ll choose the comedy option and leave you all hanging for another 11 years.

I’m having a similar issue with my wife’s car. It has a relatively new radiator, and I just did the thermostat, then water pump, did the timing belt and associated stuff in-between. The car was never equipped with an auxiliary fan, but does have an AC fan, which is working. I also, replaced the fan clutch, which helped because before the car would heat up just tooling around town, and now it just heats up on 90’s and hotter days or 80’s days, when we happen to be using the AC.

The one thing I have not replaced related to the cooling system is the cap. And I know it’s not right as one of the two gaskets came away from its seat slightly so it can’t be functioning correctly.