Temperature spike to 260F on the highway? D:

Tl;Dr : I hit an unexpected temp spike to 260f, everything seems fine, Wtf.

Hey gang! I have a 96 se automatic with 140k miles that I've always been flirting with overheating issues with. When I bought it, it had a new radiator, a new water pump, and I'm pretty sure a new fan clutch as well.

Turns out the issue was a cracked head, replaced that. I popped in an Ultraguage, and my highway temperatures are generally 195/200, going up to "normal" temperatures of 215 in town, and I've hit 230 in traffic.

Today I was on the highway after some in town traffic and I glance down and see 260 on the Ultraguage, with intake temperate at 130 (normal). I shit my pants, and turned on the heater. Heater brought it down to 245. I got off the highway and let the rpms stay low while I kept it moving, got down to 230 in the couple of miles to my destination. I jumped out and checked under the hood, all of the fans seemed fine, the top (return) radiator hose was hot, but kind of soft. Overflow tank was higher than normal, but not steaming or or overflowing.

I'm kind of dumbfounded, no idea what went wrong. Maybe the water pump failed? I don't believe it was the sensor, I have a mechanical guage plugged in to the rear sensor port, it's not wildly accurate (it reads under on temperature whole moving - says 160 when the UG says 195) and it was reporting 210.

Any help guys? I'm going to let it cool down and see what happens.


It's been 10k miles since the head swap, mostly issue free. Radiator was flushed then. I did a little searching and read about how the computer is programmed to jump the temperature up, but I don't think that is the issue, as it was a steady decline in temperature, and I don't think a 96 is that smart.

A thermostat that stuck closed could cause this.

md21722 said:

A thermostat that stuck closed could cause this.

Click to expand...

I could believe it, it would explain the lack of pressure in my top hose. I can't remember if the heater core is before or after the the thermostat though

The thermostat goes into the head. The heater core connects to the thermostat housing. The heater core is after the thermostat.

Also, overheating at speed can also be caused by an inop. fan clutch.

An inop fan clutch would more typically allow overheat at low speeds such as 4x4 trails or stop and go traffic.

At the minimum, I would install a genuine Jeep thermostat and a top quality radiator cap. I would also closely inspect the radiator and consider installing a new one.

Another factor not ordinarily considered is a dirty A/C condenser.
A friend's 1997 XJ had a similar problem, the cooling system was all new yet it would overheat at high and low speeds.
Inspecting the A/C contender, we found it was dirty and corroded, allow very little light to pass though.
Several attempts to clean it failed, not being able to remove the corrosion. The overheating continues until the condenser was replaced
This eliminated the problem.

a new thermostat is a easy swap and can eliminate it from your trouble shooting.
water pumps dont generally just go bad, but they can wear out and loose efficiency over time.

md21722 said:

The thermostat goes into the head. The heater core connects to the thermostat housing. The heater core is after the thermostat.

Click to expand...

On the Renix heads the heater core bypasses the T-Stat, and I suspect the HO is the same.

Even if the T-stat was not the original problem, it is now a problem. Hitting 260 F will damage the T-stat. The wax plugs that make the T-stat work leak out of the sealed area at that temperature and will no longer operate the T-stat at the right temp if at all.

I am having good luck with T-stats from Murry that are made in Israel, otherwise go OEM-Jeep, the rest are junk.

Ecomike said:

On the Renix heads the heater core bypasses the T-Stat, and I suspect the HO is the same.

Click to expand...

Same as my 1994 and 1996 and all others I have seen.

Turning on the in cabin heart take the water off before the thermostat and return it into the water pump. It is an effect emergency radiator that will get you home slowly or get you to the next exit i.e. if there is sufficient coolant in good condition in the system.

Ecomike said:

Even if the T-stat was not the original problem, it is now a problem. Hitting 260 F will damage the T-stat. The wax plugs that make the T-stat work leak out of the sealed area at that temperature and will no longer operate the T-stat at the right temp if at all.

I am having good luck with T-stats from Murry that are made in Israel, otherwise go OEM-Jeep, the rest are junk.

Click to expand...

Can confirm this - I blew an expansion plug on the highway and briefly pegged the temp gauge due to total coolant loss before I noticed/smelled the burning coolant. When I rebuilt the thing the thermostat wasn't stuck open and looked fine so I re-used it. It worked for awhile, but recently it was preventing me from getting to temp. I replaced it with an OEM Mopar stat (which was a Motorad, incidentally also made in Israel). Temp is perfect now.