Thermostat working?
 

On a freshly rebuilt 2.7 S, how long should it take for the engine to reach operating temp (say 180 degrees) if the thermostat is working properly? It seems that even after 15 minutes or more, the engine is still warming and never reached 180 on a 78 degree summer evening.

The temp sending units (there are two, one for the dash gauge and one for the ECU - its fuel injected now with ITBs) display within 5 degrees of each other.

Won't reach much temp until some load comes on the motor,a good working thermostat will bring the temp up reasonable fast from idle but not to the temps you are mentioning unless you drive it.
Seems they don't fail often but in my case it wasn't hard to test it and it worked as designed.
Maybe yours isn't working correctly as my calibrated gauge shows temp after 10mins atleast.

Thermostat tests.......
 

DB,

You need to physically test and observe how they slide under thermal tests. Do they open fully or partially? Not until you have physically observed how they work, you will be just guessing or hoping everything is OK. Unfortunately, you can not rely on LUCK all the time for your troubleshooting. Test and confirm. A set of test result or data is more reliable to have and make your conclusion.

Tony

20-30 minutes of soft driving warm up is not unusual to get to temp.

Sent from my SM-T580 using Tapatalk

To properly test, toss the thermostat in a pot of water and heat while watching. Use a meat thermometer to see the temperature.

But for your concern, it would have to be stuck open, rather than not opening. Should be easy to see if you remove it. With ITBs access ought to be a breeze.... on a 3.2, it sucks.