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Coolant Temperature Variations

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I thought I already posted this, but I can't find it now. Maybe I had a battery crash before it got posted.

2013 2500 Megacab Cummins 6.7 Aisin
I have been having trouble with code P20EE until dealer (After three trips) finally got FCA to authorize a Cat replacement. Before, it would reliably throw the code at highway speeds, usually within a couple hundred miles. Since the cat change I have driven two 700 mile trips with no code. Hope it is fixed.

But now I notice cyclic variations in the coolant temperature. To be fair, they may have been there before, but I don't think so. Also, I haven't done much driving at cold temperatures until this trip. What I am observing is the coolant temperature cycles between 190 and 197, usually 1 or 2 degrees at a time - 190, 192, 195, 197, 194, 192, 190, with more dwell at 190 than any of the other temperatures. The numbers are not always the same, but the min is always 190 and the max is either 195 or 197.

My best guess was this is normal low temperature operation with the thermostat cycling trying to maintain 190 degrees. If the temp goes too low, the thermostat closes and when the temperature gets to 195 or 197 it opens.

Is this normal or do I have something going on? Oil temperature and transmission temperature are rock solid.

Thanks,

Al
 
The 190° thermostat cracks at 190°±3° and isn't full open until 207° so what you are seeing is normal. The max allowable temp from Cummins on the 190° thermostat is 225°.

The 6.7 has a higher flow pump than the 5.9 which is why the 6.7 fluctuates a little more than the 5.9 with the same thermostat. There does come a point thou that the thermostat will start to cycle more and more and that's when it is ready for a replacement. My buddies 2010 goes down the road 200-202-206-208-210-208-206-204-202-200 and repeat. It's about a 10-20 second cycle each time and IMO the thermostat is ready to be replaced.
 
Cummins must have installed a lower temp T stat on the 2013's the 2012 I use to own would run 196-205. I will say the cab heat was much hotter than this 13 I've got now.
 
The 190° thermostat cracks at 190°±3° and isn't full open until 207° so what you are seeing is normal. The max allowable temp from Cummins on the 190° thermostat is 225°.

The 6.7 has a higher flow pump than the 5.9 which is why the 6.7 fluctuates a little more than the 5.9 with the same thermostat. There does come a point thou that the thermostat will start to cycle more and more and that's when it is ready for a replacement. My buddies 2010 goes down the road 200-202-206-208-210-208-206-204-202-200 and repeat. It's about a 10-20 second cycle each time and IMO the thermostat is ready to be replaced.

In the old cars that just had the dummy light. .... i wouldn't replace the thermostat until it stuck open or stuck closed. Learn something new every day on here.
 
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