How Long Is Too Long to Idle a Diesel Engine During Repairs?

[Mr. Beefy]

I understand the basics (I think) regarding why it's generally ill-advised to idle diesel engines for extended periods.
But how long is too long?
Is it a 'some damage is done no matter how long you do it, and cannot be reversed' type of thing?
Or is more that if you make a habit of doing so, you'll eventually create build-up that operating at load the rest of the time can't reverse?

I ask because I'm making small fixes to my air system that necessitates me running long enough to build up pressure a time or 3. Is there any harm in just letting it sit afterwards, or should I run it afterwards long enough to get up to temp? The latter would be a real PITA considering its in-construction status, but I want to do whatever it takes to take proper care of the powertrain.

/Really Stupid Question

 

[Rucker]

If you search this forum you may find previous conversations about periodic idling of diesels. Generally a no-no, I recall, because you can't really bring it up to full operating temperature, or something like that.

 

[Conversion Noob]

Had never heard of that before and I worked on a farm for about 4 yrs and those farmers always let them run anyhow glad I stopped by to check your thread out 'cause that got me looking for the answer.

Here's what Grok Ai found from 75 sources.

 

[Conversion Noob]

BTW, I accidentally forgot to type the word "not" the first Grok Ai search, so here's what it shared regarding why it's important to let your Diesel engine idle.

 

[Iceni John]

Do you have a Fast Idle switch? It will typically increase idle RPMs to prevent or reduce the worst effects of prolonged idling. With an electronically-controlled engine it may be as simple as adding some resistors to the ECM's accelerator pedal or PTO circuit to mimic a partially depressed pedal.

When I bought my bus its Fast Idle had been disconnected by the previous school district (why?!?), so when I reconstructed it I added in a second higher FI speed. I now can select 900 or 1225 RPM, the latter being useful for building air slightly quicker.

John

 

[fo4imtippin]

You can guarantee that when your bus was in service it idled 100 times the ratio you will let it idle. The bus drivers will get in and let them "warm up" then they sit and wait for the kids while idling. I have to move mine around the driveway many times a year which requires a few minutes to fill the tanks and idle. I also live pretty close to my highway on ramp, so i try to let the bus get up to temp a bit before i drive a half mile and then stomp on the pedal. I run 1/100 2 stroke oil in my fuel which supposedly helps the cylinder washing. I run my bus on the highway once a month now, so im not at all worried about the idling i do. I have had my oil tested and compression is good. Im not to worried about it. I think the summary is that you don't need to go start a diesel once a month to "exercise it" unless you are getting it up to temp to evaporate any condensation in the system, your not helping anything by starting it.

I recently bought a used truck with a 6.4 hemi. While reading about all the modern truck engines with lifter and camshaft issues, you arent supposed to idle anything anymore. You need high rpm to sling enough oil to the hydraulic lifters. Wont be using the remote start as much as i did on my other older SUV.

Edit:
Also, my bus actually had a factory shrader valve on the wet tank that i can fill my whole system with an external compressor. I used that while making changes to my air system. I also added a quick connect straight to my manifold.

 

[s2mikon]

First off, there is no such thing as a stupid question. I agree with everything with the Grok answer except the NOx statement. NOx forms under high combustion temperatures and pressures. You can't get that idling. With emission equipped diesels (EGR, SCR and regen systems) idling is the kiss of death. Over the road drivers used to (15 years ago) idle 8 hours for ac or heat and it didn't kill the engines. What you are doing I wouldn't worry about it. Finding the leaks is important.

 

[Mr. Beefy]

Thanks to everyone for all the great advice.
Yes, it does have a fast idle that kicks in after some amount of time. Maybe 5 minutes if I had to guess. I know it's in the software but I haven't checked out the values for that yet, nor determined whether that's a parameter I'm able to change.
I don't know why I didn't think of filling my tank w/ air from an external compressor. Thanks fo4! That's what I plan to do from here on out.
I did end up running it a bit for this purpose yesterday after hearing from y'all. Thanks for easing my worries and putting things into perspective.
Unfortunately our air door woes continue. But I think I've finally got the problem identified, and soon (as a part comes in) addressed.

 

[shorthair-SKO]

Used to idle and with the pac brake set the shutter valve would make the 3406 cat come up to temperature in under 5 minutes.

 

[Carrier6]

I've never heard that about idling. I work around heavy equipment and dump trucks all day. These things idle for hours at a time. **** they get off them and go to lunch and leave them running. Learn something new.

 

[nikitis]

My 2 cents on this is that the default idle rpm is not high enough to idle on most school buses. Modern diesel trucks can set idle higher and they run them at stops while they sleep. "SOME" buses, you can set the cruise control button to "On" and then hit the "throttleup/set" button to get to 1100 RPM. This is an okay idle rpm, it does burn more fuel but gets it up to operating temps.

On my bus, I couldn't originally do this, and the reason why is because ServiceMaxx on the ECM told me that I couldn't enable cruise control unless I was going at least 45 mph. It's a literal value in the ECM. Something like "Enable cruise Control at ___ MPH", and was set to 45. So I was able to change this with GodMode version of it from 45 mph to 0 mph, and now mine enables while idle.

I can't say if yours will work doing this but it did for my 3box. I can crank it up, turn it on, then hit the throttle up button until it reaches the desired RPM.

The downside to doing this on MOST of our older buses, especially the Internationals is the cooling is lacking without good running air flow, and if you IDLE, you're only getting what the fan can draw in which is some air, but often not enough for long idles. So you may not want to do this unless you've modified your Radiator size and fit it in somehow.

 

[#1FordFan]

Idle time damage 100% depends on which engine it is. The more modern the harder it is on the bus/certain engine parts. The older engines like the pure mechanical ones like say a 12V 5.9L
(Up to 1997) are quite happy to idle all day. The only thing you should look at is the coolant temp gauge If it drops below 160’F while idling, you can fix with grill cover or fix a faulty thermostat

 

[s2mikon]

@nikitis The fan NOT turning fast is a good thing when idling. You want low airflow to keep the coolant as hot as possible or close to t-stat temperature. I'm old enough to remember radiator curtains and radiator shutters that were air operated.

 

[LargeMarge-TC]

How does your oil test?
Oil testing helps identify problems after they are problems.
.
I have concerns about stationary conventional alternators getting enough air-flow to stay cool.
We partially-solved the cooling issue by installing a big-house alternator engineered as 'Continuous Duty' instead of intermittent cycling.

 

[Mr. Beefy]

@nikitis The fan NOT turning fast is a good thing when idling. You want low airflow to keep the coolant as hot as possible or close to t-stat temperature. I'm old enough to remember radiator curtains and radiator shutters that were air operated.

Yep. I could sit here in the middle of summer idling and I doubt it would get up to temp. Not that I'd do so (for all kinds of reasons, not the least of which being my internal temperature)... but I could.

 

[Mr. Beefy]

How does your oil test?
Oil testing helps identify problems after they are problems.

LOL. I love the way you put things.
Haven't tested the oil, only because I changed in recently (in miles, not time). Once we've put a couple months or so of actual travel in I plan to do so (so I'll know what's going to break just before it does).

 

[Mr P]

"Diesel" engine can mean several things. "Old school" mechanically injected or "new" computer controlled and emissions laden.

All engines are designed to "work best" at a certain temp, including piston/ring temp. Limiting extreme cold starts is more important (IMO) than limiting warm idle time.

 

[Omnibot2000]

I think the time you need to worry about doing damage while idling engines and damaging them is when it is frigid cold and not idling the engine high enough to put heat into the engine

Case in point, I remember 28 years ago when I got into the trade we stuck our oldest trucks onto the winter roads up north. Night time lows would be -45C, I think -50F for the lows without windchill. You can tell when the engines were not idled high enough as they would slobber at every joint on the exhaust system coming out of the cylinder head. That would be idling an engine for like 8-10-12 hrs at too low of an RPM and the engine sits idling way to cold in the operating temp. range.

Usually after a good hard drive we would burn that $hit off but if you do it long enough time after time then yes, accumulative damage/ wear can occur.

Idle the engine up, put a winter front on it to put heat into the engine and you should not have an issue. This all depends on ambient temps too. If you don't have extreme cold, you really should not have an issue.