engine running too cool

I have a 2000 ford escort zx2 with the dual overhead cam engine. About a month ago the temp. gauge started showing cold when running down the road above 30mph. Since then I have changed antifreeze, burped the cooling
system, changed the thermostat and checked the new stat in hot water-it is working perfect. Also the fan is not running. And the engine is still running too cool. When sitting at an idle the engine will come up to temperature but quickly cools down when going down the road-the heater also starts blowing cooler air. What else should I be looking at.
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Kegge wrote:

Try another thermostat.
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FanJet wrote:

i have replaced multiple thermostats before finding a good one. BUT after driving it for a WEEK or so is the antifreeze level constant?.... Do you have to add any? if so could be a leak letting air in cooling system causing no heat.(would have to be quite low though)
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