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99 Mercury Tracer - Overheating at Low Speeds

Hello and thank you in advance!

I have a 1999 Mercury Tracer with a 2.0 engine. 149,000 miles on the engine. Here is the trouble, it overheats at low driving speeds. My temp stays normal at idle in the driveway, normal at speeds of 35/45/55 or more. After about 10-15 minutes of driving around 25-30MPH though, the temp climbs steadily and approaches the red line. I checked my oil level and coolant level. Both of them appear to be fine. I have not noticed any leaks from the water pump area and cannot see any coolant in my driveway.

Could it be the thermostat is stuck closed? Air bubble in my coolant?

I have NOT pressure tested it yet, but I will be doing that this evening.

Any thoughts and advice are welcome!

Thank you!

Slow and stopped overheating normally indicates a failing (or failed) fan thermostat, or a bad cooling fan.

With engine and ignition off (and engine cool to avoid injury), try disconnecting the radiator fan electrical from the wiring harness. You may have to fabricate something to test it with, but try and apply battery power directly to the fan. If it powers up, then the problem is the fan thermostat.


I strolled to the parking lot here at my office and started the car. It ran and then the fan come on. So, the fan appears to be working. I am confident that the fan has been running while driving as well.

How long have you ever let it idle to see whether it would overheat or not? Sitting at idle it can take a really long time. Driving is a whole different story since everything is working harder.

I’m going to venture a guess on a partially clogged radiator. Basically, at higher speeds you’re ramming a lot more air through it just by the movement - much more than the fan can do. So even if flow thru the radiator is poor, the airflow still allows it to keep up.

You could also have a deteriorating water pump. You should have gotten a new one of those back at 120K with a timing belt. Did you?

It would be helpful to know things about the maintenance history of the cooling system, including the age of the thermostat & radiator cap.

Radiator cap is original to the car as well as the thermostat. Timing belt was replaced at 135k miles. Other than that, I have not done much with the cooling system.

"I have not done much with the cooling system. "

mmmm…that’s bad.

I’d check out the local yellow pages & see whether or not there is a local automotive “radiator shop.” Just bring it to them and ask them to do a full checkup & round of maintenance on the cooling system. (This should include a new thermostat & rad cap). The alternative is to let it go until you overheat it enough to wreck the head/block.

Then start following the maintenance schedule for the cooling system. Letting it get out of whack can easily be an engine killer.

I will have it checked out. I pressure tested and that came back fine. I also used an infrared thermometer to check the hose temps and they both came back at normal temp levels. When looking at my temp sensor though, I should have been noticing boiling in the radiator. I will have the radiator flushed, new cap, and maybe even replace the temp sensor. It may not be running as ‘hot’ as the gauge is reading. Thanks for the advice!

Yes people forget that radiators have a lifespan too…Especially when peeps use tap water in their coolant. I have replaced countless clogged radiators (due to calcifation sp…lol anyway the calcium or minerals in the water)…that once were fine and then all of a sudden…bam…overheating, unable to shed enough heat by themselves. Feel your radiator fins…it should be uniformly hot all over no cool or cold spots while u have a hot engine…thats another giveaway.

Keep in mind that radiators do go bad tho…I replaced too many to count so dont beat yourself up if you need to buy one…for that vehicle it should be round 120 for the rad.

ALSO if you did your T-belt w/o the water pump then you have a time bomb on your hands…IF your T-belt rides on the water pump…mite not…ususally does tho these days.