Low engine temp/ poor mileage - Chevy and GMC Duramax Diesel Forum
11-16 LML Duramax Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission, Etc.

 
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post #1 of 7 (permalink) Old 09-15-2019, 01:44 PM Thread Starter
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Low engine temp/ poor mileage

My truck runs right around 165 degrees, it’s been doing it for months It use to run around 179/181. It also dropped 18-19 mpg down to 14-15mpg. I just checked for any thrown codes but didn’t have any. Any ideas why or where I should start looking to solve this? Thanks.

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post #2 of 7 (permalink) Old 09-15-2019, 01:50 PM
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Likely need change your thermostats. Low engine temp will cause poor mileage. I changed mine on my lmm this summer when I did the hsp billet thermostat housing. Temp is 185-190 and mileage came back up.


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post #3 of 7 (permalink) Old 09-15-2019, 02:13 PM
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Agree need tstats engine temp on my 12 is 183-187.

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post #4 of 7 (permalink) Old 09-17-2019, 01:55 PM
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Yep agree you need to change tstats I had the same thing going on this spring I changed thermostats and temp went back up and fuel mileage increased

2012 GMC Sierra 2500HD Diesel 2wd Crew Cab. Stock. 200 mile regen club
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post #5 of 7 (permalink) Old 09-17-2019, 03:14 PM
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My 2016 is acting similar. 90-100 outside temp here in Florida and it’s running 176-179, idling at 174. Not as low as yours. If I’m at highway speed it stays 181-188. If I get on it hard or in stop and go traffic will hang 188-190 but once I get moving cool off to 176-179.
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post #6 of 7 (permalink) Old 09-18-2019, 01:22 PM
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Quote:
Originally Posted by Bigdiesel1129 View Post
My 2016 is acting similar. 90-100 outside temp here in Florida and it’s running 176-179, idling at 174. Not as low as yours. If I’m at highway speed it stays 181-188. If I get on it hard or in stop and go traffic will hang 188-190 but once I get moving cool off to 176-179.
Those temps seem ok it's when you start getting down to 160 or lower

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post #7 of 7 (permalink) Old 09-18-2019, 06:45 PM
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Mine is doing the same thing. You need thermostats. There are two. They right next to each other just below the ac condenser on the front of the motor. Go with AC delco. Got mine from Amazon. Itís a good time to flush and replace coolant too.



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