COOLANT TEMPERATURE GAUGE FLUATUTESCKT

[RFA1]

MY 1991 EXPLORER 4.0L HAS A COOLANT TEMP PROBLEM,THE GAUGE ON MY DASHBOARD SWINGS FROM 175 TO 195 DEGREE [ F] EVERY 1.30 SECONDS I HAD INSTALLED A MECHANICAL GAUGE WHERE THE TEMP. GAUGE WAS SCREWED IN ,AND STILLS SWINGS FROM 175 TO 195 . MY COOLANT IS NEW, RADIATOR IS GOOD ,THERMOSTAT IS NEW,WATER SEEMS TO BE O.K ,NO SIGN OF A BLOWN HEAD GASKET , FAN CLUT CH IS OK .THERES NO LOSS IN ENGINE PERFORMANCE ,AND ENGINE NEVER OVERHEATS.JUST WONDERING WHAT WOULD FIX MY LITTLE PROBLEM .OPEN FOR SUGGESTION.:

 

[mrboyle]

For starters, reinstall your origional sending unit. It not only feeds the guage, it also sends engine temp info to the computer. Without it the computer thinks the engine is running cold all the time and will cause the vehicle to run rich. Over time this will kill the gas mileage, plugs and eventually the cat.
As for the fluctuating (sp?) guage, this is most likely due to the thermostat (I know you said it was new, but that doesn't mean it isn't bad).

 

[TPLYNCH]

You can also try replacing the radiator cap. They weaken with age....

 

[Ross-N-Chicago]

I second the cap but replacing my water pump has virtually locked my needle in one place (at the o in NORMAL). It used to swing up and down, up and down, but now it stays like a good dog.