Engine temp. very unstable

[anhtony96]

The truck runs fine just gets bad mileage. It's a 1996 XLT 4.0 V6 4WD.
I thought it was due to the thermostat. It never held a steady temperature, the gauge goes up and down, So today in the freezing cold I replaced the stat with a new 195 unit.
After filling and burping it does the same thing, only this time it does not get as hot and when it drops it drops below the normal line.
I am seriuosly considering bringing it to the dealer buti hope one of you can stop me.
Can anyone help me get this temperature stable?
I used a Robertson Shaw stat from Autozone.

 

[TPLYNCH]

Try replacing the radiator cap too....

 

[prybylo]

I had similar problems. The temperature would rise to normal, and then drop to cold and then keep cycling. It is still under warranty so I took it to the dealer three times. They couldn't fix it -said it was normal. Once they changed the thermostat with no success.

I changed the thermostat myself with one form NAPA and also broke the little plug out of the breather hole. This fixed it - great heat now and stable engine temperature. It does take longer to heat up though. So I don't really know if it was the new thermostat or breaking the plug out, but that worked for me.

 

[Alec]

whenever you replace the tstat you should also replace the radiator cap.

 

[RiverRat]

There was a TSB on 96 ranger 4.0s having erratic coolant temp.