[Carolinarider]

1996 Ford Explore over 200,000 miles, original engine and other major parts (don't know about the radiator).

Had an issue a few years ago with the engine temperature never getting very high. Figured it was a thermostat failure but never worried about it (yes I know I should have).

I had the Explorer serviced sometime last year for a transmission leak and some brake issues and well, they did a lot of work including fixing the thermostat issue.

All things were normal unit today when was driving it into town and noticed the temperature gauge was reading sort of low (yes it was a cold day but I had driven the Explorer about 10 miles or so). I kept an eye on it and noted that it finally climbed to the "normal" center location on the gauge.

I kept a further eye on the gauge and it looked real low. So, I watched it really close and noticed it would drop down to just above the low spot and then, after a few seconds slowly begin to climb back to and then slightly above "normal" reading. After reaching that point it would fall back down to the low position.

See the pictures below.

I checked the fluid in the radiator at a stop after I was sure it was cool and it was just right at the top of the radiator opening. I checked the Coolant Reservoir and it was empty. I purchased some fluid while on the road and filled the Coolant Reservoir up to the "normal" level.

When I got home and let it cool again, I checked the radiator and the level was below the top opening. The Coolant Reservoir was about the same.

All this seems very, very strange. Any suggestions on what might be happening. One thing that could be happening is a bad sensor.

Pictures below are about 10 seconds apart.

 

[DILLARD000]

Possibly the thermoStat sticking\failing.

 

[Carolinarider]

Sorry, I missed the response above. Here is what I did. I replaced the sensor and no change. I replaced the thermostat and no change.

Thermostat looked just as clean and new as the new one.

I am at a loss.

 

[DILLARD000]

Possibly air trapped in the CoolantSystem.
Park vehicle with front end elevated on ramps\jacks or a curb by a few inches; set the EBrake.
With engine off & cold, remove radiator cap, start engine, let it idle & warm up.
As engine warms up, ThermoStat should begin to open & allow flow into\thru the radiator;
when you see coolant begin to rise out of the cap opening, shutdown engine & reinstall cap.
Allow engine to cool for an hour+, then top off radiator with coolant as needed.
May need to do this 2 or 3 times to completely bleed all air from CoolantSystem.