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Old 01-04-2006, 06:31 PM   #15
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2000 25' Excella
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Idling Diesels

You mustn't equate idle of gasoline engines with diesels. In the gasoline engine the air fuel ratio is about 16:l by weight, whether it is idling or at full load. Further more speed is controled by a throttle plate that is almost closed off at idle, restricting the flow of intake air.

On a diesel the air fuel ratio is about 16:1 at full load, but at idle it will be much higher, perhaps as high as 50:1. Speed is controlled by the amount of fuel injected only. There is no throttle plate to restrict air intake, so the diesel takes in a far greater volume of intake air to cool the turbo quicker. Unless a diesel engine's cooling system is overheated it doesn't hurt it to shut down at normal operating temperatures. Similarly, because of its high air fuel ratio, it doesn't heat back up quickly at idle speed or the light loads of starting a load and up shifting through the lower gears.

Many thousands of line haul diesels have their electronic fuel systems set up to shut down after 3 minutes of idle time. This is done by large fleets who take into consideration all the owning and operating costs of their engines and they have the records to prove the wisdom of it.


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Old 01-04-2006, 08:57 PM   #16
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The diesel ignites more completely at higher temps about 203 degrees (better milage) than when the manufacturers oem water temperature thermostat opens around 195. If you idle your diesel for long periods, do like Porky suggests and install an AIC (Automatic Idle Controller) to prevent wet stacking. Shutting down a hot engine (after towing) will definitely shorten the life of your engine and turbo bearings do to coking. One thing to remember is the parasitic heat after you shut off the engine. If you do not cool your engine sufficiently after a hard tow that parasitic heat can do a lot of damage over time. As far as burning a little diesel to cool down your engine consider the high cost of rebuilding a diesel long block, plus replacing injectors, fuel pump, turbo, oil pump and the myriad of sensors, hoses and electrical components, plus the loss of use of your tow vehicle. Consider the stress the block and heads have taken over time with the excessive heat stressing the metal along with micro warping. The old block will never be quite the same even though you spent the cost of a new engine getting the old one rebuilt. What’s that, five to six thousand dollars on a 6.0 or 7.3 liter. That would be a lot of diesel. A lot less than an engine rebuild, and while your at it, you might as well have that transmission redone because it has suffered every bit as much as your engine.

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Old 01-04-2006, 09:21 PM   #17
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I also own a 1999 F-350 7.3 Liter Powerstroke Truck. Whether it is right or wrong, I often leave my engine at idle when I do short task (fill up my tank, run into the store, dropping off something, etc...). This is often for less than 5-10 minutes at a time. I understand that starting is hard on engines, but I also have heard that the fuel loss at start up (or the amount of fuel used to start the engine) is greater than the diesel used to idle for a few moments. Now, I know at some point (after just a few minutes...) this would no longer be true, but is the above statement accurate?

So, let's consider a couple of situations...

-To leave your truck (not a 'big-rig') at idle for 20 minutes or more is BAD? Right?

-Is it better to leave your engine at idle when you run into a store for less than 5 minutes, or should you shut your engine off?

-What about fuel burn at start up vs. fuel burn during idle?

Maybe someone can help try to answer some of these questions. As always, thanks for everyone's help on this forum...


-John Foshee
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Old 01-05-2006, 12:13 AM   #18
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I am referring to towing, hard driving on hot days, long uphill pulls, a hot engine from really working hard. A cruse 10 or more miles to go to town to get some brew it is better to idle an not shut off the engine mainly because the engine really isn’t hot. But an idle for 20 minutes you should have a Automatic Idle Controller. When I go to the Post Office to get my mail 16 miles away, I turn off my engine. When I pull the Airstream I let it cool. There is really no right or wrong. It’s your decision. It’s your diesel. By the way, you should always turn off your diesel when your fueling. Common sense.

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