A note about thermostats, thermo senders, and other tiny dictators and terrorists.
If you are going to replace any of them--test the new one. The thermo sender is easy to test in situ on the engine. Most of them are standard termistors that you can test with an ohm meter. They will read somewhere between 0-40 ohms when red hot, 200-240 ohms when the engine is cold, and someplace in between at normal temperatures. Anything close is usually good, when they go wrong they tend to go VERY wrong.
You can also remove the sender, place it in a pot on the range with a kitchen thermometer, and check the temperature versus resistance there. You unplug the sender wire, attach an ohmmeter where it was connected, and then measure the resistance to the block or ground or exterior of the sender.
Similarly, even a new thermostat should be checked in a pot of water
, sometimes they are DOA. They usually work when a tiny hydraulic cylinder filled with thermal wax expands/contracts to power the whole works, and that wax can leak, or be mis-filled when new.
So, first check the little critters that love to drive humans crazy, and only THEN assume the temp readings are right. ~140F is right for a raw water
cooled engine, ~180F for a closed-water (radiator) engine. When the engine is working right, and the cooling
system working right, tmeperature should not change much, if at all, from idle to load.
I think we've had some other threads about how you really need to check the whole cooling
system, from intake to exhaust, for all the "I shoulda seen that" things that can cause problems like this. Hosings, fittings, filters, intake clogs, lost