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Old 10-07-2012, 03:08 PM   #1
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Running engine temperature

Cummings 400hp diesel. What temperature should it run at at around 1500-1700 rpm. Around 85 degrees outdoor temp. I run about 195 degrees ferh. Is that too hot? A few times this summer it was hitting 202 and the beeper was just starting to beep. This was uphill. If its too hot should I change the coolant and thermostat. The coolant was last changed 25000 (2008) miles ago but not the thermostat. Thanks

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Old 10-07-2012, 03:33 PM   #2
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I normally run a shade under 200 in all temps.

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Old 10-07-2012, 04:23 PM   #3
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That is a perfect temp. I would worry and shut down around 220, dont let it run any warmer than that.
We had a fleet of large diesel pusher 84 passenger school buses here that ran the 5.9 Cummins, way too small for that size of bus and they tended to run real hot in the summer.
You also dont want a diesel that runs too cool either, it causes incomplete combustion and the unburned fuel can washdown cylinder walls. I say this if you have run that runs less then 160 degrees.
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Old 10-07-2012, 04:32 PM   #4
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Main cure for getting hot on long uphill grades is to turn off the aircon, manually drop back a gear to raise the revs to 1900 to 2000 and slow down.

Apart from that, it sounds as if there may be nothing at all wrong and in any case there are a couple of things that need to be checked before spending money replacing parts. Cleaning the radiator for one.
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Old 10-07-2012, 07:48 PM   #5
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We have Thousands of the 400 Cummins at work,,although in big trucks,, the engines are basically the same as yours. 195-200 degrees is perfectly normal...
Monkey, pilot of a Great Dane hauler,
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Old 10-07-2012, 08:19 PM   #6
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Compared to my 40' MH, 400 ISL,EPA 07,yours is running about 5~10 degrees high.
Have you checked / cleaned the radiator?
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Old 10-08-2012, 06:11 AM   #7
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when you say clean the radiator the inside or outside?
Outside via power washer?
Inside via change the coolant and flush out ?
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Old 10-08-2012, 06:28 AM   #8
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Siggy is probably referring to the exterior of the radiator and CAC. Oil vapor from the engine and road grime get pulled into these items and clog the vanes. Spraying with a cleaner like Simple Green Extreme (purple in color) and flushing with a garden hose will do the trick. There are threads explaining this in detail on the forum.

Do not use a pressure washer as that is too powerful and may damage the fins. Steam cleaning can work too, but it is usually just done from the outside and may not be as effective.

Check the engine breather tube to be sure it extends well aft of the fan housing to minimize clogging the radiator.
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Old 10-08-2012, 02:40 PM   #9
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I have the 400 ISL in my '09 Winn Tour. When we got it I felt that the temp was running too high and took it to a Cummins repair station. They discovered that the fan was programed to start at 212 degrees and cut off at 195. Contacted the factory and were told that the engine was designed to run hotter than earlier years and that it could go as high as 220 before any alarms were sounded and even at that 230 would not damage the engine. There are several threads about the 400 ISL running temps in the Cummins section, one in particular written by a former Cummins engineer. Do a search there and you will feel a little better about the high temps. I did!

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