new question involving overheating

jeep has been runnin good no problems, i figured out the aux fan isnt turning on when it gets up to 190... it is when the a/c is turned on. so i looked at the diagnostics and realized the radiator switch also turns it on... when it gets hot, so i replaced that... still not turning on. what else could be the culprit? Also maybe im off on the measurement, does it have to be way hotter than that to turn on? I am going to replace the thermostat just in case, its cheap and easy to get to so it shouldnt be a problem, but i would like this damn fan to turn on.

I thought the aux fan didn't kick in until 217 degrees (F).

I´ve tested two thermal switches, one from an 87 and one from an 88, they closed at around 207-208. Both switches were originals, so they could be a little tired. All-data says 208. Haven´t found the info in my 88 FSM yet.
When my temp. gauge shows a little above 212 say 215-217 or so, when the aux fan comes on. The temp. at the gauge and the outlet of the radiator (by the thermal switch) are different.

whatis the gauge reading? it may be showing that it is running hot but the radiator may be cooler like it has poor circulation. then i would suspect a clogged radiator.

that is the thing, the gauge is showing like 200 it wont get any hotter than that. even after i drived it hard and let it sit and idle, it got up to like 210 for a tiny but and cooled right down.

This is a common stupid problem with XJ's. I don't know why Jeep designed them to have the fan come on at 218 degrees. When I first got mine, I ordered a universal fan cycling switch from Summit racing for about $22. It is adjustable from 180-220 degrees. You just wire it up to the fan, and with the twist of a knob you can determine what temp the fan kicks on at. With my 180 degree thermostat, I have my fan kick on at about 190 degrees. My Jeep never reaches 200 degrees, even in the dead of summer. A modify that must be done!

Jackhill442 said:

With my 180 degree thermostat, I have my fan kick on at about 190 degrees. My Jeep never reaches 200 degrees, even in the dead of summer. A modify that must be done!

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Considering that the factory designed the XJ to use a 195-degree thermostat and to run at around 210 degrees, coupled with the fact that I have over 245,000 miles on my '88 and have always run a factory 195-degree thermostat with no ill effects ... I respectfully disagree with the "must be done" part of your post.

dellstopjeep said:

jeep has been runnin good no problems, i figured out the aux fan isnt turning on when it gets up to 190... it is when the a/c is turned on. so i looked at the diagnostics and realized the radiator switch also turns it on... when it gets hot, so i replaced that... still not turning on. what else could be the culprit? Also maybe im off on the measurement, does it have to be way hotter than that to turn on? I am going to replace the thermostat just in case, its cheap and easy to get to so it shouldnt be a problem, but i would like this damn fan to turn on.

Click to expand...

The auxiliary electric fan doesn't switch on until the gauge shows ~220*F or past the 1/2 way mark. It switches off again at ~200*F (just above t'stat opening temp. where it should be). DC weren't stupid when they decided to install 195* t'stats in the Jeep engines. The engines are designed to run between 195-220*F to minimize emissions.
A lower temp. t'stat should not be used unless you've added a performance chip with a more aggressive timing curve. Making the engine run cooler reduces the risk of pinging and slightly improves performance, but you definitely don't want to go lower than 180*.
I have a 180* t'stat in my stroker and I spliced a 4.0kohm resistor across the two wires going to the coolant temp. sensor in the t'stat housing to trick the PCM into switching the electric fans on/off at lower temps. (on at 210, off at 185).