I feel like this is probably a beaten to death topic, and I've found several good resources on how to go through the coolant system components to determine and replace the culprits(s). But since I can't work on it in the dark, I figured I'd use the time to see if anyone has had an identical issue and maybe skip straight to the problem.

History: radiator replacement a few hundred miles ago, along with new radiator cap. Total system flush in the process, 50/50 mix to refill, following all appropriate procedures to do the refill. Has been running between 210 and 220 consistently since, according to the dashboard gauge, without any issues at all, and no leaks.

Today: ambient temp 60ish. Drove on highway for ~25 min., temps at normal range. I have been able to hear the electric fan kicking on, and today was not noticeably different. After picking up my toddler I turned the ac on, which I haven't ran for any significant time since the rad swap, and drove for another 20 ish minutes at less than highway speeds, ac running continuously.

Symptoms: temp gauge suddenly to 250, "Check Gauges" dummy light on while sitting at a red light after the above 20 minutes with ac running. Continued driving to get to safe place to pull over, ac off, blasted heat, and temp gauge dropped from 250 to 220 quickly. Pulled over, no other signs of overheating, so continued onward to get as close to home as I could. Went a mile or so, and then same., but this time the ac wasn't one. Two more times like this, temp 250, then back down to normal, in the couple of miles back home. No steam, no leaking, nothing except the gauge. Then as soon as I put it in park in the driveway and turn it off, it starts pissing coolant from the overflow, and a small amount of spatter from under the rad cap. Nothing from anywhere else that I could see.

Has anyone experienced anything like this? It's a daily driver and I have to work so if I can avoid having to guess and fix every part of the system over the next few days it would be super helpful. Thanks in advance.

 

Is your mechanical fan working properly?

 

Working, yes, in the sense of it being attached to the serp belt. Working properly in the sense of it engaging and disengaging as needed, I do not know. Would a bad fan clutch cause the problem?

 

Yes it could.
Also do you have the shroud in place?
There is a YouTube video on how to check if a fan clutch is operating properly.
Might want to check it out.

 

Ill double check on the shroud after work, I'm almost positive I put it back on but can't say for sure right now. I saw that video and once I get the coolant topped off again I'll run the test and see if that diagnoses it. I've also seen where running a 40/60 or 30/70 coolant to water mix helps cool more efficiently. I may top off with distilled water instead of more 50/50, since I'm running straight 50/50 currently.

 

Have you burp'd it? I.e. run it until it warms up without the cap on?

 

Not since I filled the radiator when I replaced it. I'll do that when I top it off.

 

First question..what year jeep...2 nd question...Waco...south texas?...to test fan clutch Have someone turn off engine with you watching fan, you can't do both by yourself. The fan should stop spinning in less than 1 second from key off. If not clutch is bad

 

Sorry, 98 Sport 4x4, 4.0

And no, Asheville, NC. The Waco Kid was Gene Wilders character from Blazing Saddles 😆

 

Well raise my rent!

 

Ok just making sure it wasn't a crappy 0331 head with a big crack in it..98 head is fairly robust. In addition how old is the t-stat and im sure its a 195...too hot for an old motor in traffic with ac in a moderately warm climate. In NY and Vt skiing , i use a 195 in winter and a 180 for summerish. Normal driving on hwy it runs about 185 but in traffic and slow driving sits about 195-200 When I had AC it would run 210-220+ in traffic with the aux fan cranking away. I use the 195 and 180 hi flows from the flowkooler water pump people..very happy with their performance and accuracy...and yes...well acquainted with the Waco Kid

 

Update: Topped up the overflow tank, as the radiator was still full, and ran it up to temp. Blasted the AC, and as it should the electric fan came on with the AC. But when I would throttle up the engine the electric fan would switch off, and then kick back on with a clicking sound from somewhere. I have a video of what it was doing, so once I figure out how to post videos then I'll let you take a look to see if it's normal or not. After running at around 210 for 10 minutes or so I killed it and lo and behold, the mechanical fan kept spinning for what seemed like forever. So, I think that's the problem. Replaced the fan clutch just now and I'll find out next week when it's in the 80s if that was the solution.

 

Here's what a properly functioning mechanical fan should sound like with a hot engine on a warm day.

 

thoughts from a 20-yr owner of a Radiator shop. Thermostat 180 or 195 or 160 standard thermostats from Stant, there engineering design and tested stats "begin" opening at +1 deg of the stat, and are fully open at +24 degrees, soooo a 180 begins at 181 (to open) and at 204 is fully open. an accurate thermometer and water prep to boil will show you... What does water boil at? ....... (212). .... So, by watching the stat in water getting to the boiling point you'll see it begin to open and at 204 it will be completely open. (prior to water bubbling at 212)

Now in the case of the 195 stat, it's like this.... begins to open at 196 and is fully open (+24) at 219, BUT only 'under pressure' will this happen. A BAD CAP will not pressurize the system (leaks or loss of coolant) therefore the Stat will not open fully (overheating situation).
Pressurized numbers are (+3 degrees per lb. of press cap) so a 10 lb. cap gives the system an additional 30 degrees of temp.
A 195 stat most commonly uses a 16 lb. cap ... hum, that's 48 degrees added to the system. that says your water system will get hot up to 265 degrees, (instant STEAM) ALL under pressure. All done under the pretents of EMISSION controls........?

Clutch fans might have a thermo-bimetal-spring which opens internally a reservoir of hi temp liquid which is the clutching effect to spin up the fan and provide the necessary air flow to cool the system.

Your A/C turns off and on with the cycling of the Pump and or in case of overheating it will come on.

Hope this will help a little.