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Old 09-03-2017, 01:10 AM   #1
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How Long can I idle

I have a1999 Cummins 5.9 Thomas bus. I want to camp out in it one night and run the on board a/c. Will it hurt to run the engine all night? it has a high idle switch also.
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Old 09-03-2017, 01:21 AM   #2
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I drive big trucks for a living and generally leave mine idle all the time. Of course I'm not running the 5.9 engine either. I remember reading something about using the high idle to avoid excessive low combustion chamber temps (I forget the details of it now), it's particularly relevant in winter temps. On a regular basis it will increase wear and tear on the engine and such, but for one night every so often it should be fine.

You'll want audible warnings and shut-downs enabled. You won't want a ruined engine if you snap a belt in the wee hours and the engine overheats, you'll want it to shut down instead.
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Old 09-03-2017, 02:50 AM   #3
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Idling a bus, even on fast idle to run the A/C system, is not a very good idea.

It will tend to not get the engine warm enough which will tend to create a lot of moisture that will condense inside the crankcase and exhaust system which will cause premature wear and failure.

Also, with the engine running fast with virtually no load the engine will not be running very efficiently. It will tend to not burn all of the fuel that is entering the combustion chamber. That excess fuel will tend to leak past the rings into the crankcase where it will dilute the oil. With the oil diluted it will tend to cause more wear in areas like cylinders, bearings, rockers, etc. The no load will also tend to make the exhaust dirtier and it will tend to gunk up the valves, the top of the piston, the top ring, and the exhaust system.

If you need A/C either plug in or get a genset to run a window shaker or roof top RV A/C.
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Old 09-03-2017, 07:16 AM   #4
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Howdy Bud, my 1998 Detroit Diesel 60 manual says the following, not certain this is applicable to your motor but I suspect it is:

"Avoid Unnecessary Engine Idling. During long engine idling periods, the engine coolant temperature will fall below the normal operating range. The incomplete combustion of fuel in a cold engine will cause crankcase dilution, formation of lacquer or gummy deposits on the valves, pistons and rings and rapid accumulation of sludge in the engine. NOTE: When prolonged engine idling is necessary, maintain at least 850 r/min spring/summer and 1200 rpm fall/winter."
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Old 09-03-2017, 08:40 AM   #5
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Buy a $300 dollar generator and $100 dollar window AC and idle that all night and not your $5000 plus engine and drivetrain. I agree with others not good for the engine at all or fuel you will 0 MPG
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Old 09-03-2017, 09:30 AM   #6
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the high idle switch was put in at the factory for extended idle periods. if you cant maintain engine temp at high idle then you have thermostat issues that need to be addressed. one nite a week is not a problem
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Old 09-03-2017, 10:07 AM   #7
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Quote:
Originally Posted by mmoore6856 View Post
the high idle switch was put in at the factory for extended idle periods. if you cant maintain engine temp at high idle then you have thermostat issues that need to be addressed. one nite a week is not a problem
I don't know for certain, but in my opinion the high idle switch was put in place for winter idling, usually to warm things up a bit, or for short idling situations. Most engine manufacturers consider long idle times as "Severe Engine Service". If you're going to be doing it on any regular basis you need to follow the severe engine service maintenance schedule, which is expensive.
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Old 09-03-2017, 10:11 AM   #8
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It was put in for summer ac use also for us truckers sleeping on a asphalt parking lot in the day if you are on a nite drive schedule
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Old 09-03-2017, 10:53 AM   #9
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I will agree with comments made by others regarding lowered coolant temperatures. It should remain at operating temperature the whole time, if it doesn't, it should be checked (mine does). I will also agree about the various incomplete combustion and oil dilution comments. It often does put an engine into "Severe Service" rating which, for the most part, includes much more frequent oil changes which addresses the oil dilution concern.

With all that said, I would (and do) leave it idle and let the A/C run. Keep a check on the oil, change more often as needed, and the most important thing is to leave auto-shutdowns enabled. You don't want to wake up to an overheated and blown engine because of a snapped fan belt.
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Old 09-03-2017, 12:21 PM   #10
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Idle

This is one of the reasons we are moving from our stock roof mounted AC units to RV style. SO we don't have to idle to be cool.
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