To idle or shut-off: Which is best?

CT_Tree_Guy · Nov 13, 2005

This is a toughie for me. I hate to listen to my tractor just sitting there with the engine running, but I don't really want to keep starting and stopping it either. I'll pretty much shut it off if it's going to be idling for more than about 5 minutes, and then try to get back to using it before the engine has had much of a chance to cool down. When the cold weather hits I'll probably leave her idling a bit longer than I do now because the engine will cool down so much more quickly.

I guess I just try think like an engine, and avoid as many heating/cooling cycles as possible. Like many of you, I also like to idle (and warm up) at 1200-1500 rpm, depending on my mood.

As far as engine problems caused by excessive idling, I have a feeling a slow idle would cause more of them than a faster idle due to lower oil pressure and less efficient combustion. I used to think that it did no significant harm to a diesel to have it idle for hours on end based on what I'd seen at truck stops, but I keep hearing that it's not a smart thing to do. It would be interesting to know if owner/operators idle their trucks as much as company drivers do, especially for "creature comforts" like heat and a/c.

There, I guess that's about 3 cents worth /forums/images/graemlins/smile.gif

Toolguy · Nov 13, 2005

Idleing for a few minutes at a time won't hurt anything but it is using fuel. If it is warm out and I need to get off of the tractor for a minute or two I will shut it off. If it is very cold I will let it idle if I plan on using it again within ten minutes or so. I have a Cummins B engine in my work truck and I can tell a difference in my weekly fuel usage when I let it idle in the summertime to run the air conditioning vs. spring and fall when it is not needed. I drive about the same miles each week so it is easy to tell. I doubt if it will hurt the engine whether you let it idle or not. The starter might wear out a little sooner if you shut it off everytime but I would bet that the fuel savings over the years will pay for a new starter if you do a LOT of idleing. Especially at $3 bucks a gallon. /forums/images/graemlins/tongue.gif

Dmace · Nov 13, 2005

( As far as engine problems caused by excessive idling, I have a feeling a slow idle would cause more of them than a faster idle due to lower oil pressure and less efficient combustion.)

The oil pressure will be the same at low and high rpms once the engine is running. The flow is a little faster but always the same pressure. Plus or minus but a very unmeasurable amount.

I don't quite understand the less efficient combustion thing...
If anything, faster idle will yield a "less eficient combustion" due to the possibility of the injectors skipping or being off just a fraction of a second. Plus it has a lot less time to complete a full combustion cycle.

Starting a diesel engine is very tough. If you think about the process of how it works. On start-up (say a 3 cylinder) you will have one cylinder near TDC and two near BDC +/-. So one piston is trying to compress air and combust it. At the same time it is turning that crankshaft by itself with no help from the other two. Worst case scenario. This is very hard on an engine, plus the oil isn't at full pressure yet.

40 +/- degrees and up, I only leave a diesel idling for no more than 5-10 minutes.
32 and lower, I see no problem in leaving it idling for 10 - 30 minutes.

That's my opinion,

Berniep · Nov 13, 2005

The reason the combustion is less efficient is that combustion temerature is lower and the fuel has more time to condense on the cylinder wall and can cause fuel washing of the oil film that is on the cylinder wall and increase wear.

ccsial · Nov 14, 2005

My brother-in-law is a diesel truck mechanic. He told me slow idle (running cool) causes a build up of sludge in the engine which can eventually plug oil passages. The big rigs all have a high idle setting feature for extended idle. But then these guys let them run all night.

I idle mine at 1500 because it makes me feel better but 10 or 15 minutes probably doesn't hurt anything.

There is an old D9 Cat a few miles from here that they left running all weekend. It is out in the field puffing right now.

guyrj33 · Nov 14, 2005

My understanding is with smaller diesel engines it's hard to inject a small enough amount of fuel at idle. To say it another way there is too much fuel injected at idle. This leads to the problems discussed and also explains why idleing is apparently less damaging to larger diesels.

CT_Tree_Guy · Nov 14, 2005

( The oil pressure will be the same at low and high rpms once the engine is running. The flow is a little faster but always the same pressure. Plus or minus but a very unmeasurable amount.)

According to my Kubota manual, engine oil pressure factory specs for the Grand L's are 14 psi at idle speed, and 43 to 64 psi at rated speeds. Minimum allowable pressures are 7 and 36 psi respectively.

This data corresponds to my experiences with every vehicle that I have every operated that had an oil pressure gauge - as the engine speed increases, so does the oil pressure.

( I don't quite understand the less efficient combustion thing...
If anything, faster idle will yield a "less eficient combustion" due to the possibility of the injectors skipping or being off just a fraction of a second. Plus it has a lot less time to complete a full combustion cycle. )

This web page ("Idling Gets You Nowhere") from an agency of the Canadian government concerning the downfalls of excessive idling strikes me as having the "ring of truth" to it and explains it better than I could. It also bears out what Berniep and ccsial said, practically word for word.

"Idling Gets You Nowhere"

ArkCivEngr · Nov 18, 2005

Something that big (and a 9 is really, really big) is an opportunity that doesn't happen very often. I'd be awfully tempted to give it a spin if no one was looking. /forums/images/graemlins/grin.gif

bugstruck · Nov 20, 2005

4 to 7 minutes. We're talking 40 to 100 hour per year CUTS here. Leave it running. Many are all worked up about cool cylinders, condensing, and oil plugging.....engine death. That article referenced puts engine life at 600 hours but 150 if you idle too much?? If it's a turbo, idle it for 5 minutes to cool it with little boost, but make sure you shut it down right away of it's natural. Nuts I say.

I let mine idle up to 10 or 15 minutes at anything over say 1,200 rpm. I'll call you when the engine dies, in my next life.

Lockport · Nov 21, 2005

The reason large trucks idle at high RPM's is to keep warm and rid the engine of moisture and burn off the diesel at those hotter temps. Cooler running (less that 180) results in sulpher residue that mixes with water moisture and comiles to form a nice sulphuric acid bath for your crank down in the oil pan. This isn't good, obviously, for the crank, connecting rods, etc. When I idle my tractor I keep it warm at a slightly higher rev. The age old adage that the oil will seep down the cylinders when you shut it off and run metal on metal when starting also complicates the issue and would seem to worsen the longer it sat. 10 minutes compared to 5, say. /forums/images/graemlins/confused.gif
What's a would-be farmer to do? I would say if you are going to idle, idle warm. Also, as someone mentioned, turbos do need a cool down period from hot running at either a fast or slow idle. Slow idle is fine as it has been 'working' and is already hot.
Just my $0.02 /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif