The line leading from the cap for overflow would be spraying fluid all over the inside of the car if I were boiling over. . .and again, not experiencing any fluid loss. To ensure my gauge is reading right (and to burp the system, just in case) I will likely replace the gauge at my nearest convenience. Then we will see whats up.

 

I have an 88 XJ and was chasing overheating issues and found out it was the result of a bad fan clutch and several bad temp sensors. Just some advice a lot of people post how the closed system was so terrible but properly functioning mine never goes over 210. Most of them don't understand how to diagonose the problem and spend hundreds of dollars converting their system when in the end it was probably an inexpensive part. It really is a good system and people who switch say they never have overheating issues but fail to mention that they replaced another component of the cooling system. Any cooling system will not function properly without having every component working correctly. Where your temp gauge is reading is too hot you should always be just around 210. I would try little things first like the fan shroud and fan clutch and then move to thinkking of things like head gasket and what not.

 

Is the little reader that sits on top of the cts securely on it? If it's lose and bobbing about it can cause erratic readings. Mine was loose once and would only read in 'd' because the motor had a load and would bump a little. Just keeping an eye out for the simple things

 

Unsure I will be checking this tonight most likely. I figure to burp the system I will park nose pointed down a slope. Pull off the sensor and run that way for a few minutes being ready with coolant. . .

Then I will put a new temp gauge on a check that out. Temp gauge isn't too expensive from what I hear. . .

 

If your 1/2 way converted cooling system is full up to the top with coolant, then u should see coolant spray all over the motor without a properly plumbed coolant recovery bottle. U did say u don't have a bottle, right? U've got to have a recovery bottle with either the "closed" system or the "open" system. Sounds like your current system has relieved itself (somewhere) of coolant during the "heat cycle" and nothing but air has returned to the cooling system during the "cool cycle", thus u r low on coolant.....low enough that nothing but air escapes when it heats up. If u don't replace air with coolant AND prevent further coolant loss/air return, u'll continue to see the gauge cycle up/down until u properly finish the coolant recovery system.

 

You know, they must have talked to GM when they made these gauges. To the left of 210, the span is 110 degrees (4 ticks). To the right of 210, the span is only 50 degrees (also 4 ticks). Since there are the same number of major and minor tick marks on each side, there is no way to make this work unless the scales are different on the right than on the left. It's the same as the volt meter. My Equus gauges don't have this crazy scale. Each scale division is the same on both sides of centre.

If you ignore the centre tick at 210, the gauges could make sense if they were 100-260 degrees (160 degree span). That would make the centre tick 180 and each tick would be 20 degrees. So your gauge could be read 100, 120, 140, 160, 180(centre), 200, 220, 240, 260. My $0.02.

 

I don't think any aftermarket gauges have split/different scales on the 2 halves of their display. Aftermarket gauges basically "tell it like it is". 2 different scales seems to be unique to the goofy factory gauges. Most factory gauges now days have a "C" at one end and a "H" at the other.....the driver will never see coolant temp fluctuations throughout a days worth of various driving conditions. As long as coolant temps remain with-in factory operating temp parameters, the factory gauge will always point +/- the middle.

 

Surprised nobody's mentioned this yet lol.

In Renix and OBD-1 Jeeps the dadh gauge is a separate sensor from the one the ECU/PCM uses to control the fan. The sensor in the thermostat housing is the ECU/PCM one, and the electric fan is triggered at approximately 218 degrees (+/- a couple depending on year etc). The dash gauge uses one at the rear of the head, so not only can one fail and not affect the other, but you can legitimately get different readings.

Your symptoms are the hallmark of a poor mechanical fan. Usually a worn fan clutch that doesn't increase in resistance when hot, but your first post mentions a busted shroud. This will cause exactly what you are describing.