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Temperature jumping from warm to hot and back again

Tony the Liger

NAXJA Forum User
Location
USA
Just completed the following on my new-to-me '00 XJ: replaced the valve cover gasket, a pinion seal, steering stabilizer shock, two axle U-joints, one wheel cylinder, and one set of brake shoes. On my way home from the garage, the XJ started to overheat a bit.

This is the first time it has overheated on me. I've only had it a few weeks and haven't driven it much, but I bought it in a town that's around three hours away from me, so I drove it around town and on the interstate for well over three consecutive hours on the day I bought it and didn't experience any overheating whatsoever. After warming up today, the temp sat just a hair past 210, then gradually climbed to the first notch past 210. I made it home by that point and parked it for a couple hours. After letting it sit, I took it up the road and back, and in that time the temp climbed to the same point, sat for a few minutes, then out of nowhere it spiked to the notch below 260. It sat there for a few second, then dropped back to just above 210. Sat for a few more seconds, then did it all over again.

Hoping the thermostat's sticking and I can get by with a $12 or so fix. Any thoughts?
 
Check operation of both fans. You should easily hear the main fan after the clutch has engaged at full operating temp (around 210). When you pull away from a stop, you should hear it over the sound of the engine. Your aux (electric) fan should turn on once 223 degrees is reached.

Were you driving around when this happend, or was the Jeep sitting and idling? If the cooling system is working properly, you really should never get over 210 while driving around, but it will creep up while idling.

Your description of the problem is not indicative of a sticking thermostat...
 
Replace the radiator cap. Buy a premium Stant or genuine Jeep radiator cap. The symptoms described are what happens when the stock 16lbs radiator cap starts to fail. The cap cannot hold the normal cooling system pressure and the pressure loss allows the boiling point of the coolant to drop, and the coolant momentarily flashes to steam. As the steam bubble passes the Coolant Temp Sensor the gauge jumps to 260. When normal or near normal cooling system pressure returns, the steam no longer forms.
 
Check operation of both fans. You should easily hear the main fan after the clutch has engaged at full operating temp (around 210). When you pull away from a stop, you should hear it over the sound of the engine. Your aux (electric) fan should turn on once 223 degrees is reached.

Were you driving around when this happend, or was the Jeep sitting and idling? If the cooling system is working properly, you really should never get over 210 while driving around, but it will creep up while idling.

Your description of the problem is not indicative of a sticking thermostat...

I'll double check, but I'm almost positive both fans are working. The problem occurred while I was driving. When it happened the first time, I got home, parked, and let it idle for a few minutes. It stayed at the notch above 210 the entire time.
 
Replace the radiator cap. Buy a premium Stant or genuine Jeep radiator cap. The symptoms described are what happens when the stock 16lbs radiator cap starts to fail. The cap cannot hold the normal cooling system pressure and the pressure loss allows the boiling point of the coolant to drop, and the coolant momentarily flashes to steam. As the steam bubble passes the Coolant Temp Sensor the gauge jumps to 260. When normal or near normal cooling system pressure returns, the steam no longer forms.

I'll give it a shot. Makes sense. I appreciate the info.

Just got home with a new thermostat. You guys recommend taking it back, or go ahead and change it?
 
Get an infrared thermometer to check the thermostat housing. The gauge setup isn't the most reliable.

Not sure if this affects your (or anyone else's) prognosis, but there was steam coming from the engine. I don't think it was an inaccuracy in the gauge.
 
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Just completed the following on my new-to-me '00 XJ: replaced the valve cover gasket, a pinion seal, steering stabilizer shock, two axle U-joints, one wheel cylinder, and one set of brake shoes. On my way home from the garage, the XJ started to overheat a bit.

This is the first time it has overheated on me. I've only had it a few weeks and haven't driven it much, but I bought it in a town that's around three hours away from me, so I drove it around town and on the interstate for well over three consecutive hours on the day I bought it and didn't experience any overheating whatsoever. After warming up today, the temp sat just a hair past 210, then gradually climbed to the first notch past 210. I made it home by that point and parked it for a couple hours. After letting it sit, I took it up the road and back, and in that time the temp climbed to the same point, sat for a few minutes, then out of nowhere it spiked to the notch below 260. It sat there for a few second, then dropped back to just above 210. Sat for a few more seconds, then did it all over again.

Hoping the thermostat's sticking and I can get by with a $12 or so fix. Any thoughts?

After extensive testing with my overheating XJ, this is pretty standard. If you look at my thread on the very same issue you can see the video I posted.

What is happening is that the stock gauge in the dash is not very accurate. Your engine is probably actually running about 240 all the time like mine is. When it finally hits 260 the check engine light comes on and the gauge shoots up almost simultaneously. When the thermostat then opens and pushes the cool coolant from the radiator back into the block it is just enough to send the gauge back down out of the red. My gauge basically doesn't read anywhere in the upper 2/3's of the gauge. It runs just to the right of center and then shoots over to just below the max temperature.

I finally figured this all out by doing what RCP Phx told me. He said to order a cheap meat thermometer off of Amazon with dual inputs. The inputs are very thin and can easily fit in the hoses with the wiring leading out under the clamps back to my interior. I put the probes in every hose that I could and drove around and checked out the temps. My Jeep runs hot no matter what I do.

I just orderd the fancy Hesco Water Pump and the Griffon Radiator. Hopefully that solves my issues. I have replaced everything else.
 
Steam would indicate a coolant leak ?? Leakage would allow loss of system pressure.

The thermostat and the radiator cap are both routine maintenance items that should replaced every few years, and as a bonus they are inexpensive. Be sure to buy genuine Jeep replacement parts, or the best quality parts the store has. Cheap parts are cheap for a reason. Another thing to look for is leaky frost plugs on the side of the block and the rear of the cylinder head.

The dashboard temperature gauge is not a certified measuring device, it is more correctly described as an indicator. Normal indication is about 210*. If you drive for a whole year and it indicates in the same location every day, everything is normal and OK. If it shows much hotter than normal, you need to pull over until it cools down, and then do some diagnostics. The PCM does buffer temp gauge readings, and temps will vary more and quicker than the gauge indicates. Part of the PCM programming is also to alert to a impending overheat by flinging the needle all the way to the right and lighting the Check Engine Light.

The stock 195* thermostat, unless defective, is already fully open at about 195*.
 
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Thought it might be possible that when removing hoses to changing the valve cover gasket, air might've gotten into the system, messed with the pressure, etc. So, this morning I added coolant via the hose, resulting in short-term good news but possible long-term bad news.

My XJ has definitely leaked coolant at some point. It accepted 1.5 gallons of coolant before it was fully topped off. This remedied the overheating issue for the time being, but I'm left to wonder where that 1.5 gallons of coolant went in the first place. I haven't found any coolant on the ground around the truck since I've owned it, nor can I find any fresh coolant where it shouldn't be in the engine bay.

This being a '00, my fear is that I've got the head crack and coolant is flowing out the exhaust. Going to look into having it pressure tested this week.
 
Thought it might be possible that when removing hoses to changing the valve cover gasket, air might've gotten into the system, messed with the pressure, etc. So, this morning I added coolant via the hose, resulting in short-term good news but possible long-term bad news.

My XJ has definitely leaked coolant at some point. It accepted 1.5 gallons of coolant before it was fully topped off. This remedied the overheating issue for the time being, but I'm left to wonder where that 1.5 gallons of coolant went in the first place. I haven't found any coolant on the ground around the truck since I've owned it, nor can I find any fresh coolant where it shouldn't be in the engine bay.

This being a '00, my fear is that I've got the head crack and coolant is flowing out the exhaust. Going to look into having it pressure tested this week.

That's how it all started on my 2k!
 
Steam would indicate a coolant leak ?? Leakage would allow loss of system pressure.

The thermostat and the radiator cap are both routine maintenance items that should replaced every few years, and as a bonus they are inexpensive. Be sure to buy genuine Jeep replacement parts, or the best quality parts the store has. Cheap parts are cheap for a reason. Another thing to look for is leaky frost plugs on the side of the block and the rear of the cylinder head.

The dashboard temperature gauge is not a certified measuring device, it is more correctly described as an indicator. Normal indication is about 210*. If you drive for a whole year and it indicates in the same location every day, everything is normal and OK. If it shows much hotter than normal, you need to pull over until it cools down, and then do some diagnostics. The PCM does buffer temp gauge readings, and temps will vary more and quicker than the gauge indicates. Part of the PCM programming is also to alert to a impending overheat by flinging the needle all the way to the right and lighting the Check Engine Light.

The stock 195* thermostat, unless defective, is already fully open at about 195*.

Maybe, maybe not. In my case, if the thermostat is already fully open how do you explain the engine cooling off so quickly and the guage returning back to the middle? These are confirmed temperature trends using two real-time probes. Anyhow, I am completely befuddled by my overheating :banghead:. Hoping that the new Hesco Water Pump and Griffon Radiator fix the problem; otherwise I will have to light the thing on fire.
 
That's a kick to the nuts. I'm guessing you replaced the head?

Did the head crack lead to any other issues that I should expect, or did a replacement fix everything?

When I bought a new head last year, it just revealed that other damage had occurred, I got another 6 months out of it before I couldn't stand it any longer and decided a stroker was in order!
http://naxja.org/forum/showthread.php?t=1140839
 
Damage done by overheating due to the head, it took out the cam bearings also!
 
The stock 195* thermostat, unless defective, is already fully open at about 195*.

I've never bought a Mopar thermostat, but that's not how any aftermarket thermostat I've ever tested or run in my rig works. They all *start* opening at 195 degrees and are fully open at about 210.

From the Stant website:

Thermostat Temperatures


  • Thermostats have a “rated” temperature such as 180F or 195F
  • This is the temperature the thermostat will start to open, give or take 3 degrees
  • The thermostat fully open about 15-20 degrees above its rated temperature
 
Haven't driven the XJ since the one trip down the road after I added the gallon and a half of coolant. Put some cardboard underneath it, and there appears to be a pretty slow drip of antifreeze. I found one fresh drop on the bottom of the oil pan; guessing it's hitting somewhere on the top, then slowly running down the side.

Any thoughts?
 
A quick change in temps is virtually impossible on an engine due to the shear thermal mass. What is possible is a quick change in temp of the sensor caused by air in the system. When the sensor is covered with coolant, all is fine, but get an air bubble and presto the sensor reads way hot.

Now comes the question, why is there an air bubble? Have you replaced any hoses or the like where you drained coolant? If not, you may have a head gasket leak or a cracked head letting exhaust gas into the coolant. The first quick and dirty step is to let the engine cool and then pull the radiator cap. Start the engine and allow it to warm up. Do you see bubbles in the radiator? You will see the level surge as the thermostat opens, but look for a bunch of bubbles.

There is a kit that allows you to "sniff" the radiator coolant for hydrocarbons to detect a leak of this type.
 
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