2000, coolent temp spikes out of no-where!

This is my 2000;

It will, for no particular reason, spike the coolant temp while sitting ata stoplight or drive through.

I'm not loosing coolant. In fact when this happens, I don't notice any change in how it runs. Just the gauge will jump from 215 to 250 in a mili-second and as soon as I start moving, it drops just as fast to normal.

I think its the coolant temp sender.

Both the e-fan and clutch fans are new, new radiator two years ago, new hoses, new water pump two years ago. I just put a
T-stat in last summer.

Thoughts?

You say yours runs at 215? I own an '00 and mine may hit 210 but will drop to about 195. Combination of the e-fan turning on, sometimes, or just the thermostat opening even when idling for longer periods of time. Is the e-fan turning on? Do you have the fan shroud over the mechanical fan? Might be a defective thermostat or waterpump. Could also be a temp sensor. Think there is just one in an '00. In the thermostat housing. Gotta be careful with the '00's cause of the 0331 heads.

The only reasons for the temp gauge to jump from normal to maximum is loss of cooling system pressurization, or the need to replace the radiator and/or water pump. Assuming there are no other cooling system leaks, and no cooling system deficiencies such as a clogged radiator core, or rusted away water pump fins, a new genuine Jeep radiator cap will likely fix it.

Have you actually checked the coolant level in the radiator?

I agree sounds like the radiator cap opening to dump coolant, so either a bad cap or or coolant boiling making the cap open. I always pressure test new caps before I leave the store, too many are junk off the store shelf.

Tim_MN said:

The only reasons for the temp gauge to jump from normal to maximum is loss of cooling system pressurization, or the need to replace the radiator and/or water pump. Assuming there are no other cooling system leaks, and no cooling system deficiencies such as a clogged radiator core, or rusted away water pump fins, a new genuine Jeep radiator cap will likely fix it.

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I will replace the cap.

I did replace the temp sender today, let it idle for two hours, it never overheated.

The radiator is full, the radiator is 2 years old. The head was new in 2007. The t-stat, water pump, hoses were replaced a year ago.

I'm not loosing coolant

I have a P0303 misfire code, it's always had that code at times. But nothing related to the coolant system.

What is causing the miss fire code?

I would take a good look at the #3 spark plug, look for contamination from coolant or oil residue on the plug, and look for coolant dye on the top of the piston. Also have a muffler shop test the Cat for excessive back pressure.

Does that year have the coil on each plug?

If you can get one of those little "mirror on a stick" doohickies from the local VatoZone, use it and take a good look at the condition of the freeze plug on the back side of your cylinder head.

I recently replaced a blown head gasket due to severe engine overheating after the temp gauge had been doing essentially the same thing you're describing.

The source of the problem was that a small hole had rusted through the freeze plug, likely due to previous owners running straight water in the cooling system as well as living on the beach!

Ecomike said:

What is causing the miss fire code?

I would take a good look at the #3 spark plug, look for contamination from coolant or oil residue on the plug, and look for coolant dye on the top of the piston. Also have a muffler shop test the Cat for excessive back pressure.

Does that year have the coil on each plug?

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Its COP, its due for a tune-up, been a couple of years. i will look at the plugs

UncleBuckwheat said:

If you can get one of those little "mirror on a stick" doohickies from the local VatoZone, use it and take a good look at the condition of the freeze plug on the back side of your cylinder head.

I recently replaced a blown head gasket due to severe engine overheating after the temp gauge had been doing essentially the same thing you're describing.

The source of the problem was that a small hole had rusted through the freeze plug, likely due to previous owners running straight water in the cooling system as well as living on the beach!

Click to expand...

I'm not loosing coolant. The thing is, when it spiked, it went from 215 to 250 in a split second. It will stay that way for 30seconds or so, and drop back just as fast.

I doubt the coolant is getting that hot that quickly or cooling off that fast.

If the temp changes that quick, it has to be wiring or sensor related. There is no way you could see that much change in less than a couple of seconds.

The temperature sending unit on 2000+ does not work the same as earlier models. It shows the warm up cycle to 210, then has some "logic" built in that retards movement far away from the center. At 246F the PCM puts the gauge into the red and at 257F it buries the gauge.

I have seen bad connection at the sensor to cause some erratic swinging of the gauge, although in my case it would flop and forth to a lower temperature.

In other cases the thermostat can stick but I would expect a boil over condition to occur.

A properly running 200 should show half a tick mark above/below center. The fans should kick on at 226F IIRC which is about half tick above center.

Do you hear the fans come on when this happens?

xjtrailrider said:

The thing is, when it spiked, it went from 215 to 250 in a split second. It will stay that way for 30seconds or so, and drop back just as fast.

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That is exactly what one should expect to see if the radiator cap is weak. The cooling system looses pressure, a hot spot flashes the coolant to steam, and the steam bubble passes by the Coolant Temp Sensor.

Tim_MN said:

That is exactly what one should expect to see if the radiator cap is weak. The cooling system looses pressure, a hot spot flashes the coolant to steam, and the steam bubble passes by the Coolant Temp Sensor.

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I agree, I use to have that problem with my 1976, 400-4barrel lean burn beast, Dodge Charger SE. I had a 20 lb cap on that beast finally, and still had idle overheating issues in the summer with the AC on. The learn burn was a beast, but the control of the mixture was too lean or the cooling summer on hot humid days in the sun at idle. But back to the cap, I could see the cap open for about a second from about 210 to 240 F as the temp jumped about 30 F and then ran right back down to about 220 F.

But I also agree, it sounds like a bad electrical connection. Had that same problem with my sons 96 Ford Taurus. Bad sensor-connector connection. I would go from 220 to pegged red zone in milliseconds. And at odd times go back to normal, while the thermal IR gauge said nothing had changed. A wiggle of the sensor wire would trigger it back and forth.

Thanks all!

I think its fixed. Yes the e-fan is cutting on at the first tick past 200. the clutch fan is tight and is working properly.

I changed the sender and the cap today. Drove it for 2 hurs up and down mountains n this warm early spring day (78 here).

The fan ran almost constantly but it never overheated nor did the gauge spike.

I did order a new e-fan from Oriellys today under warranty because I think this one is sub par and its vibrating badly.

AAANND it did it again today.....

I'm going to put another t-stat in it tomorrow and if that doesn't fix it, I don't know where to go after that.

I suspect a wiring/electrical issue. I have checked/cleaned all engine grounds, cleaned all electrical connections between the CTS and ECU.

Its not loosing fluid
its not even boiling over
its spiking temp then dropping it back quickly, too fast for actual cool down

New radiator
new water pump
new CTS


Im pulling my hair out.

If it was me, I would test the cap with a pressure tester, test the cooling system with a pressure tester.

Then if both are OK, I would add 1-2 pellets of Bars Leaks stop leak pellets (dry) to make sure the head gasket (and other areas) stay sealed if it or they are getting marginal.

A sudden gas bubble, pressure spike that opens the cap for just a second will/can cause the gauge to spike and drop back down. The cap or cap area might not be sealing properly, or there may be a tiny intermittent exhaust gas leak into the coolant.

i don't have any answers, but i have experienced what you describe before. exact same scenario, one on a 98 and another on my 01. it is like the temp will creep up to about 215 or so but once it gets any hotter the gauge will spike. once the rad gets enough airflow to bring the temp down under 215 again the gauge will go back down.

on the 98 i saw it a lot because it ran a little hotter than it should so always would get to 215 or so running temps. on the 01 it only happened when driving in california in 110+ temps on long uphill climbs(otherwise it always ran around 200 normally), but exact same symptoms.

on both vehicles i triple checked the cooling systems. on the 98 i did a lot more as i was trying to solve the running hot issue anyways. no coolant loss, no leaking cap, changed sending units, did a leakdown test, pressure test. i verified actual temp with a laser temp sensor and it was always just above 215 or so.

it seems that this issue is rare because most 97+ xjs will not normally run that hot on a normal basis, and every time i did find someone describing this exact same thing it was always shrugged off as either a hot air pocket or a bad sender.

i'm convinced it is a 97+ specific issue because i can recognize it any time i see it in person or read about it. it may have other factors, but it isn't a generic cooling problem. either way i still have no answers, but i don't freak out as much when i see it happen because know it isn't actually that hot. before summer hits i think i am gonna install an aftermarket temp sensor and gauge just to see what is really happening.

Ecomike said:

If it was me, I would test the cap with a pressure tester, test the cooling system with a pressure tester.

Then if both are OK, I would add 1-2 pellets of Bars Leaks stop leak pellets (dry) to make sure the head gasket (and other areas) stay sealed if it or they are getting marginal.

A sudden gas bubble, pressure spike that opens the cap for just a second will/can cause the gauge to spike and drop back down. The cap or cap area might not be sealing properly, or there may be a tiny intermittent exhaust gas leak into the coolant.

Click to expand...

I replaced the cap, I will try sealant.

When I bought the Jeep from a reputable Jeep dealer 7 years ago, it had a dealer installed replacement head. I actually still have the shop ticket.

I really do not think its even getting hot. I need to add a stand alone temp sensor to find out.

I would not try the stop leak hack job repair.

As far as the talk about air pockets, if the coolant system is underfilled, you can overheat the engine and the dash gauge will not show it.

A properly working XJ coolant system is adequate to drive across the desert in California or Arizona in the daytime, 100F+ ambient, with the A/C, on long uphill grades on I-8/ I-17. I don't drive those roads every day but I have done it repeatedly in my 2001.

Over time engine components get old and lose their performance and then the coolant system loses its ability in extreme temperatures. Fan clutches get weak, radiators/condensers get clogged with dust and leaves, catalytic converters don't flow as freely, thermostats may not open properly, ...

I would look at the misfire code as well. A weak head gasket can cause some funny problems.

And consider testing the coolant sensor through its entire operating range or replacing it.