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  #16  
Old August 12th, 2017, 11:54
Tony the Liger Tony the Liger is offline
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Re: Temperature jumping from warm to hot and back again

Quote:
Originally Posted by RCP Phx View Post
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
What kind of damage? And was this damage a result of the cracked head, or is it just stuff that you discovered while swapping the head?
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  #17  
Old August 12th, 2017, 12:51
RCP Phx RCP Phx is online now
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Re: Temperature jumping from warm to hot and back again

Damage done by overheating due to the head, it took out the cam bearings also!
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  #18  
Old August 13th, 2017, 01:34
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Tbone289 Tbone289 is offline
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Re: Temperature jumping from warm to hot and back again

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Originally Posted by Tim_MN View Post
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:

Quote:
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
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  #19  
Old August 14th, 2017, 14:45
Tony the Liger Tony the Liger is offline
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Re: Temperature jumping from warm to hot and back again

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?
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  #20  
Old August 14th, 2017, 15:20
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old_man old_man is offline
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Re: Temperature jumping from warm to hot and back again

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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  #21  
Old August 14th, 2017, 17:44
Tony the Liger Tony the Liger is offline
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Re: Temperature jumping from warm to hot and back again

Quote:
Originally Posted by old_man View Post
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.
Yeah, I mentioned that above. When replacing the valve cover gasket, we disconnected and moved some hoses, so when I added coolant I did so through the hose in order to flush out any air. It stayed cool when I drove it after that, but I didn't drive far or long, nor have I driven it since.

My concern is with the coolant that was lost prior to it overheating: was it due to a leak, or a head crack? The oil isn't milky/frothy, best I can tell; I haven't seen any steam/white smoke coming from the exhaust, but I haven't viewed it from behind while it's been on the road, either - only while idling.

I haven't driven it since that first drive after topping off the coolant. Today I noticed a couple new (dried) wet spots on the cardboard I placed under the XJ, so there's definitely a leak, albeit a slow one. I also noticed one green drip on the bottom of the oil pan that was ready to drop. Anyone care to take a guess as to how it would've gotten there?

Not gonna get my hopes up, but I want to take that as a good sign. Coolant seeping through a crack in the head would either pass through the cylinders and into the exhaust, or mix with the oil in the motor, correct? Is there a way that coolant from a head crack could wind up on the outside and bottom of the oil pan?
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  #22  
Old August 14th, 2017, 19:44
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ehall ehall is offline
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Re: Temperature jumping from warm to hot and back again

wash the engine with a garden hose to get rid of all the drips from spillage, and to make it easier to trace the coolant leak if there is one
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  #23  
Old August 15th, 2017, 16:50
macgyver_7_6 macgyver_7_6 is offline
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Re: Temperature jumping from warm to hot and back again

My 99(early 99 with good head and one coil) just acted the same way a few weeks ago in the middle of Kansas when it was 102. I turned off the AC and the temp dropped back to 210. It spiked one more time going up a mountain in Colorado that night...threw it in neutral, tapped the throttle & it dropped right back down. I changed the thermostat the next morning at our cabin & it hasn't done it since. The thermostat neck is pitted...so I have a new one to put on with my new mechanical fan clutch, water pump & hoses.
I used rv sealant on the neck to put back on the hose when I changed the stat in CO. This got me around the rest of the vacation in CO and back to Missouri.
Check the neck for a leak. Hopefully it's as simple as that. Next, the water pump might be weaping...indicating failure to come. Hope it's not a head issue. Good luck!!!
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  #24  
Old August 18th, 2017, 13:06
Tony the Liger Tony the Liger is offline
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Re: Temperature jumping from warm to hot and back again

Update...

Had a compression and pressure test done on the XJ today. The compression test resulted in good news all around; looks like there's no issue with the head. The pressure test confirmed what I'd started to suspect over the last day or two: I need a new water pump. Gonna replace that (and the thermostat and radiator cap while I'm at it), then I'll hopefully be in the clear for a little while.

Thanks for all the great info and help along the way, everyone.
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  #25  
Old August 20th, 2017, 09:09
Heavyopp Heavyopp is offline
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Re: Temperature jumping from warm to hot and back again

Don't forget to take a good look at your freeze out plugs -- Most of them are under the exhaust manifold

Take a look with a mirror and brite light -- a leak may look like a wad of mud and oil stuck to the plug -- clean off that corrosion and you'll find the hole in the freeze out plug

There are 7 freeze out plugs in total -- 5 are on the manifold side, visible under the exhaust manifold -- One is on the back of the head, against the firewall -- the other is lower at the back of the block, You can't see it -- blocked by the flex plate -- Trans must come out
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  #26  
Old August 20th, 2017, 09:13
Tony the Liger Tony the Liger is offline
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Re: Temperature jumping from warm to hot and back again

Quote:
Originally Posted by Heavyopp View Post
Don't forget to take a good look at your freeze out plugs -- Most of them are under the exhaust manifold

Take a look with a mirror and brite light -- a leak may look like a wad of mud and oil stuck to the plug -- clean off that corrosion and you'll find the hole in the freeze out plug

There are 7 freeze out plugs in total -- 5 are on the manifold side, visible under the exhaust manifold -- One is on the back of the head, against the firewall -- the other is lower at the back of the block, You can't see it -- blocked by the flex plate -- Trans must come out
Hmm. Learning new stuff every day.

Would the pressure test not have revealed issues there?
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