To fast idle or not to fast idle, that is the question

[cody]

Here is a good question of unknown interest, some say ALWAYS hit the fast idle, or else your risking eternal damage to your engine, others say only to use it if you want to air up quickly, I'd like to sit down with a bag of pop corn and watch the experts debate this into the ground, I really don't know what is the best approach and maybe it'll help the other mechanically challenged slackers in here understand another tidbit about the mysterious thing in that back compartment that makes all that noise.

[luvrbus]

Cody, straight from the DD bible " if you need to idle for long periods of time (over 5 minutes) use the high idle" they were all sent from the factory with some type high idle system, A/C,air up or batteries has nothing to do with it.

Always high idle at around 900 rpm because that is when the compression on a 2 stroke reaches the max, and that is the reason compression checks on a 2 stroke are preformed at 900 rpm.
Clear as mud LOL and you know what Paul Havery says


good luck

[cody]

I always use the high idle because I've heard so many differeing view points, I figured the worst i would do is run out some fuel, some even have said that low idle can cause a run away after the engine loads to a certain point, I know there are 2 shcools of thought on this.

[Adarian]

What about those of us that don't have DD? Same question.

[JackConrad]

I rarely idle except to build up air, so I use fast idle when I start the bus.  If stopping for fuel, etc., I shut down the engine.  Jack

[TomC]

On ANY Diesel engine, idle speed is the hardest on the engine.  Any vibration you feel from the engine is being transmitted directly through the bearings.  On my engines (I have both a Detroit 8V-71TATAAC and a Caterpillar 3406B jacket water aftercooled) after starting I wait for the oil pressure to come up then put on fast idle around 1,000rpm.  As proof, my Caterpillar has almost 1.2 million miles on it and has had just two in frames-never out of the truck.  Good Luck, TomC

[Melbo]

Tom

On my L10 there is no fast idle.  When I asked at Cummins about it they said it makes little or no difference if it is idling or being driven down the road. Of course that is the guy at the counter and he doesn't mind if I have to bring it in for service that is his business. 

Melbo

[belfert]

On my bus with Series 60 the fast idle switch is almost worthless.  It bumps the RPM from just over 700 RPM to just over 750 RPM.  If the fast idle switch is off the DDEC will kill the engine after five minutes idle time.  The cruise control when idling will control the RPMs so I kick the RPMs up to 1000 or so if I want the engine to idle.

When my friends drive they almost always leave the engine idling when fueling or stopping for any reason.  They sometimes get mad when I kill the engine, but I know the idling is bad for the engine and wastes fuel.  I will let it idle long enough to cool the turbo, but that doesn't take long.

[BG6]

The guy at the Cummins counter is either ignorant or is giving bad advice to get more business.  Either way, PASS HIM BY unless you are just buying parts.

Fast idle does several things for us, all of which make our engines live longer.

First, it helps the engine reach and maintain a safe temperature (use fast idle when cooling down after a hard run).

Second it gives the pumps enough power to prevent "eddies" in the cooling and oil systems (not a common problem, but one which generally cracks heads or blows gaskets if you get it).

Third, it provides enough power to drive the accessory pack (air, electrical, hydraulic, vacuum and HVAC) at the proper speeds for longevity and to meet needs.

At low idle, your engine will get hot and cold spots, water and oil circulation are reduced in efficiency and your air compressor runs a long time without a break to cool down.

[Adarian]

The fast idle on my L10 has two speeds. When the fast idle is initially turned on it raises up to one speed, after 1 minute it drops down to a slower speed but still higher than base idle. If you turn it off and back on after 1 minute it will just raise to the second lower speed.
The fast idle won't come on until I have at least 80 lbs of air pressure built up.

[fe2_o3]

I don't think mine ever had a fast idle...Am I doomed?...Cable

[RichardEntrekin]

Interesting that on one has commented on DD's logic on idling. Their comments in the Series 60 manual say idling produces excessive unburned diesel in the cylinder which washes the lubrication off the cylinder wall which leads to cylinder wear. How to quantify this I do not know. I don't know if idling at any speed has impact on the above logic, or purely running the engine without a load is more of the culprit.

More fuel for the drive it like you stole it school of thought.

[DaveG]

I think there should be two discussions here...idling for two strokes and idling for four strokes...while there are some things in commons (as have been shared) the two stroke has other considerations.

When you get to the four stroke part, don't forget keystone compression rings.

[trucktramp]

I was always told that at idle excess fuel would wash off the lubrication on the cylinder walls.  This was true for both 2 and 4 stroke engines.  Also, the oil pressure on a 2 stroke Detroit is not very high and when you bump off idle the pressure jumps up ensuring that there is plenty of oil flow to all cylinders.  Somebody once told me that the back 2 cylinders on a 2 stroke barely got enough oil at idle.  Can't say as I know this for fact though.  I'm sure that someone can confirm or deny this.

[BG6]

I was always told that at idle excess fuel would wash off the lubrication on the cylinder walls. 

I've heard that too, but nobody has ever been able to explain how this would happen, why it doesn't happen to ag diesels (which spend a LOT of time at idle) or why it wouldn't happen at 2000 RPM.  However, fast idle would seem to solve that problem too.