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post #1 of 7 Old 07-28-2019, 01:02 PM Thread Starter
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Hemi thermostat

Those of you who did a hemi swap on a JK what degree thermostat did you use? I used a 2012 Ram truck hemi engine and I think the thermostat is 203 degree? I was wonder if I could use a lower degree thermostat. I know the newer engines like the higher temperature but I think 203 is a little high.
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post #2 of 7 Old 07-28-2019, 01:16 PM
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After buying a 180 and 195, I decided to stick with the 203. Without a proper reprogram and tune for the lower temp, you'll risk burning out your cats (if you're running them). Probably other potential problems... If you're concerned about engine bay temps, venting is the way to go.

OTOH, the 195 t-stat was advertised as doing a gradual open so that the coolant temperature wouldn't suddenly drop and hunt around the set temp. I used to watch my 3.8 do exactly that with the stock t-stat.

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post #3 of 7 Old 07-29-2019, 05:00 AM
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Originally Posted by rockystock View Post
After buying a 180 and 195, I decided to stick with the 203. Without a proper reprogram and tune for the lower temp, you'll risk burning out your cats (if you're running them). Probably other potential problems... If you're concerned about engine bay temps, venting is the way to go.

OTOH, the 195 t-stat was advertised as doing a gradual open so that the coolant temperature wouldn't suddenly drop and hunt around the set temp. I used to watch my 3.8 do exactly that with the stock t-stat.

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you may not be speaking about recalling seeing this fluctuation on your prior v6 3.8l
when your jk had that "powerplant" in her ( 's'cool you got a hemi in yer jk) on the JKs stock temp gauge ,right? heh
I can't see anything useful on that factory temp gauge at all so wondered what you meant.




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post #4 of 7 Old 07-29-2019, 06:34 AM
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you may not be speaking about recalling seeing this fluctuation on your prior v6 3.8l

when your jk had that "powerplant" in her ( 's'cool you got a hemi in yer jk) on the JKs stock temp gauge ,right? heh

I can't see anything useful on that factory temp gauge at all so wondered what you meant.





Naw, I was watching it on a Bluedriver scan tool. On some days, the temp would hit about 200, and then dip by around 20 degrees and then oscillate for 20 minutes or more. The trend below was a warm start, but the trend is the same for cold starts too. Compare to my '03 Highlander which is very stable. Looks like the OEM 3.8 JK t-stat only goes full open / full shut.



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Last edited by rockystock; 07-29-2019 at 06:36 AM.
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post #5 of 7 Old 07-29-2019, 06:50 AM
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Robbie from Motech or ronjenx or White13JKUR could probably describe it better or confirm my suspicion about you fluctuating data graph from the 3.8l cooling cycles in lower image ; I am tempted to suggest that the 20second / 20° up & down pattern may be by design considering that the cooling was being controlled by a double-wrapped ,voltage divider thersistor controller of the fan.
in essence , you have a stepdown resistance in effect based upon temp that repeatedly cycles based upon fan cooling....

I think that would be a 3.8l cooling data graph for any of them.

I maybe wrong .





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post #6 of 7 Old 07-29-2019, 11:19 AM
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Robbie from Motech or ronjenx or White13JKUR could probably describe it better or confirm my suspicion about you fluctuating data graph from the 3.8l cooling cycles in lower image ; I am tempted to suggest that the 20second / 20° up & down pattern may be by design considering that the cooling was being controlled by a double-wrapped ,voltage divider thersistor controller of the fan.
in essence , you have a stepdown resistance in effect based upon temp that repeatedly cycles based upon fan cooling....

I think that would be a 3.8l cooling data graph for any of them.

I maybe wrong .



Pretty sure the cycling is only due to the thermostat opening / shutting. I had the fan low speed set to come on at 215. AC was not running; so the fan wouldn't have been on for that reason either.
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post #7 of 7 Old 07-30-2019, 05:29 AM
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Pretty sure the cycling is only due to the thermostat opening / shutting. I had the fan low speed set to come on at 215. AC was not running; so the fan wouldn't have been on for that reason either.



makes sense. I just think the fan circuit & design in these early JKs is weird & was suspecting incorrectly; you're right thts the thermo opening/closing.




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