Engine DTC "coolant temp lower than expected"

[fordbluefamily]

Our 04 Explorer has a check engine light as of this morning (86K miles). I had it read at Advance, and its "coolant temp lower than expected" Sure enough I notice on the drive home that the temp guage is going up and down between the middle and 1/4 up lines. About a year ago this happened under warranty (72K), and Ford changed the T-Stat, housing and sensor. Well now it looks like the same problem. Temp guage is normal at a stop until you hit the gas a bit, then it drops.

Are the T-Stats really this prone to failure in the 4.0? or did they miss something?

 

[fordbluefamily]

Followup:

I got into the Dash test and monitored the temp. It only got up to 78C driving and would only go above 80c at a stop. So I replaced the Tstat and housing gasket using a Stant 190. Then flushed and filled with Prestone "All colors" antifreeze. I could visibly see where the Ford Tstat was leaking through and not closing off, you can even see on the edges of the pellet housing where it was binding up. Junk. It was put in by Ford in December of 2006....

Now with the new stat it hits between 88-92 depending on cruising or stopped. I turned off the Service engine light this morning and it hasnt come back yet. Keeping my thumbs crossed.

 

[Melon]

The Dash Test? How did you do that? I know on my F150, there was a way to do it - holding the trip/reset button down while starting it up. Is it the same on the Explorer?

-Melon

 

[fordbluefamily]

Yep same on Explorer and full of neat information. Just scroll through until you see a two digit number with C after it. It worked great to monitor the real time Temp in Celsius. Its so quick you can see right when the thermostat opens. It really makes you aware that the dash guage is only accurate up to about 80C, then between there and 95C it will not move one bit. Probably to keep people from worrying about gauge sweeps.

Also saw my speedo is off by at least 3MPH from the dash to the needle...