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Discussion Starter #1
I have got a 2012 dodge 2500 6.7 it has 104,000 miles on it the truck has been running good and now it has started running hot for some reason the first time it ran hot I replaced the thermostat it ran fine a couple days done it again replaced again ran hot took to the shop they put a mopar tstat in it and changed antifreeze and said had an air pocket in it stil wants to run hot I have been in 20-50 degree weather for a little bit now and soon as you crank it up after sitting all night it’ll warm up to about 140 and the heater is hot that’s warmed up for 30 minutes when I drive down the road heater starts blowing cold temp starts going down to 130 or so but will slowly work it’s way up to 170-190 and heater works good you can stop at a red light temp goes down to 160 or so heater gets cold leave the red light warms back up the only time it will run hot now is if you really gas on it to pass someone or something then the heater will get ice cold and the temp will shoot up to 220 you can cut the truck off and let the air off the radiator cap and it’ll go back down and heater will get hot again been driving the truck a month like this just drove it 3 straight hours doing 70-75 and ran perfect as long as you just cruise in it the mechanics said it might have a slight crack in the head but there’s not smoke or water in radiator and I’ve done the combustion gas test on it and it never changed colors so I thought I’d get on here and see y’all’s opinions?
 

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The truck should run around 200F if the correct T stat was installed, and by " correct " , I mean the T stat for a 2010 - 2012 model Cummins. The Gates part # 34212, and others sold by Mopar and Cummins are correct for these trucks. Any other T stat, even though it will go in place, will not work correctly because it will not let the system and consequently the heater stay warm at idle, if I am saying this right. You didn't mention how hot the coolant temp was reaching, or whether your truck is stock. More info might help diagnose the problem.
 

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Discussion Starter #3
The truck should run around 200F if the correct T stat was installed, and by " correct " , I mean the T stat for a 2010 - 2012 model Cummins. The Gates part # 34212, and others sold by Mopar and Cummins are correct for these trucks. Any other T stat, even though it will go in place, will not work correctly because it will not let the system and consequently the heater stay warm at idle, if I am saying this right. You didn't mention how hot the coolant temp was reaching, or whether your truck is stock. More info might help diagnose the problem.
It has something done to it I don’t know much about tuning but it was like that when I got it it has a 5 switch nob on it and it has gotten to 235 before but when u let the pressure off the radiator cap it runs good again
 

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Define "let the pressure off."
Like there's not enough coolant to spew out and burn you if you open the cap engine hot? Or there's still residual pressure after the engine cools down?

If the first case, it's way low on coolant.
If the second, you have a head gasket leak that is over pressurizing the cooling system.

(I think. At least that's where I'd start looking)
 

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Define "let the pressure off."
Like there's not enough coolant to spew out and burn you if you open the cap engine hot? Or there's still residual pressure after the engine cools down?

If the first case, it's way low on coolant.
If the second, you have a head gasket leak that is over pressurizing the cooling system.

(I think. At least that's where I'd start looking)
When u open the cap it does spew out
 

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Discussion Starter #7
I just crack it and let some air out without just pulling it straight off
And could be a head gask bc coolant it’s low the coolant doesn’t leak and there’s not water in the oil and the combustion gas test says there’s not gases in the water
 

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In my limited opinion, the test for combustion gases in coolant is useless in a diesel. Are there bubbles coming up through the overflow reservoir when the engine is up to or above normal operating temp, or coolant pushing out the cap on the reservoir? Does the truck sound like it is percolating when the engine is turned off after warming to operation temp? When the HG failed in my 2011 Cummins coolant was puking out the overflow onto the ground and it sounded like a coffee percolator when turned off. After my truck sat overnight if I removed the rad cap it would still have pressure and blow a little coolant out. The culprit for failed HGs in these engines, in my opinion, is bad tuning. If indeed the HG is out, if you or someone else installs a new gasket I would advise to make sure the head is not warped or cracked. The head on my Cummins was warped but was machined flat by a competent machine shop before the new gasket was installed.
 

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Discussion Starter #9
In my limited opinion, the test for combustion gases in coolant is useless in a diesel. Are there bubbles coming up through the overflow reservoir when the engine is up to or above normal operating temp, or coolant pushing out the cap on the reservoir? Does the truck sound like it is percolating when the engine is turned off after warming to operation temp? When the HG failed in my 2011 Cummins coolant was puking out the overflow onto the ground and it sounded like a coffee percolator when turned off. After my truck sat overnight if I removed the rad cap it would still have pressure and blow a little coolant out. The culprit for failed HGs in these engines, in my opinion, is bad tuning. If indeed the HG is out, if you or someone else installs a new gasket I would advise to make sure the head is not warped or cracked. The head on my Cummins was warped but was machined flat by a competent machine shop before the new gasket was installed.
It doesn’t sound like like coffee percolators and it isn’t bubbling but it will push water out only while driving it won’t do it at idle but what gets me is last night I just got back in it after 2 days 50 something degrees I didn’t have time to warm it up just got in it and got on the interstate it was first at 100 degrees heater was hot got to about 180 heater got cold when all the way to 235 I slowed down because it was on the verge of overheating let out heater got out ram good for the next 3 hours straight doing 75-80
 
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