My 1993 YJ w/ teh 25.l 4-banger runs at about 100-110 degrees in the morning, and about 140-155 in the afternoon if I let it idle. The T-stat is opening, and the fluid is hot, and I just replaced the CTS. I also have an exhaust leak where the hanger rotted off of the front pipe.
Any ideas as to what reasons it would be doing this?
The exhaust is hot, and so is the coolant, it seems. I'm stumped.
It's near the firewall on the back of the engine, and you can get to it from the driver's side. It's screwed into the engine with one wire coming out of it, and it's not the easiest thing to find when you don't know exactly where it is. When buying the replacement, ask for the temperature sending unit for the gauge, not the computer.
Use an open ended wrench when installing the new one. You may think you can use a deep set socket and do it faster, but you'll probably break the plastic on the sending unit and end up buying another one.
also, from what i've read and heard, those fail safe t'stats aren't very reliable....high failure rate....it might have failed in the open position as it's designed to do....which kinda defeats the purpose....
Sounds like your therm is stuck open to me. Any of your friends have an infrared thermometer? If so you can check the temp at the thermostat. Otherwise, you don't know if the coolant is "hot" or not, cause 140 feels pretty warm, but is a low operating temp.
The temp sensor should climb, plateau, then drop slightly once the therm opens.
An old 'shade tree' way to tell is to feel the therm, if you can on a 4 banger (IDK where it is on that engine), and feel for a difference from one side to the other shortly after start up (before the therm would be open, but after running 2-3 min). No difference = open thermostat.
Also, if you have any heat while temp is still going up on the gauge, the therm is open.
I'm not sure or not if sending units are known to fail on those, could be that just as well. I don't mean to discourage that diagnosis, only add another possibility for ya.
Hope this helps. GL.
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