High Idle

[oscar]

Greetings esteemed knowledge body.

Yes, I did a search. Lots of talk about regens, and don't see the correlation (yet)

So, after idling for a while, 5-10 minutes, the truck goes to high idle for about 5 or so minutes.

This happens in ambient temps of 40-90ºF

Anyone know why? Thanks.

(Yes I know that Cummins advises against prolonged idling and that's not what I'm doing. I'm hooking up a trailer and or pulling a boat out of the water, not spending the night at a truck stop with the AC running.)

 

[techman]

Remote start? Do you have your comfort setting set to auto for "remote" start or "all" starts? If yes to any of these, that is normal operation to speed up heat/cooling of the cabin.

It's all in your owners manual.

 

[H3LZSN1P3R]

Remote start? Do you have your comfort setting set to auto for "remote" start or "all" starts? If yes to any of these, that is normal operation to speed up heat/cooling of the cabin.

It's all in your owners manual.

It wont idle up to cool the cabin in my experience

 

[Brutal_HO]

It will idle up and/or engage EB to maintain engine temps, remote start or not, doesn't matter.

 

[H3LZSN1P3R]

It will idle up and/or engage EB to maintain engine temps, remote start or not, doesn't matter.

At 90°F?

 

[Brutal_HO]

At 90°F?

Unlikely, but I never said that. Read what I said.

Now that I think about it, it's also possible it's idling up to maintain oil pressure. Given the "pressure" is computed by the PCM based on several factors, there could be something going on there.

 

[Will_T]

So, after idling for a while, 5-10 minutes, the truck goes to high idle for about 5 or so minutes.

This happens in ambient temps of 40-90ºF

Mine does not do that but I will often use the cruise button to "high idle" after starting so may bypass it. Sometimes when hooking up or parking the TT, I may sort of idle for 10 or 15 minutes, but it still does not do what you describe.

The only thing that seems off about idle on my truck is that, at the very least, I really think the computer counts high idle that as idle time. When I look at the ratio of idle time to non-idle time, there is no way it is correct for my truck. It is obviously counting something other than regular engine idling in the total number. Probably 95% of the miles on my truck are towing the TT. Usually hook up and then drive for 1 to 5 hours on highways so very little idle in traffic. My sense is that probably 20% at most of my engine time is idling, maybe less. But the ratio shown on the evic says my idle % time is almost 1/2 of the total hours! I usually lock out 6th gear when towing, so even on level road the RPMs are over 15 or 1600. So can't imagine what the computer is seeing to get almost 1/2 of my engine hours listed as idle.

 

[H3LZSN1P3R]

Unlikely, but I never said that. Read what I said.

Now that I think about it, it's also possible it's idling up to maintain oil pressure. Given the "pressure" is computed by the PCM based on several factors, there could be something going on there.

I thought you were also referencing the OPs comment at the same time

 

[tchur1]

my truck goes into high idle whenever i start it and its below 40 F or if it’s at idle for a few mins in those temps. helps warm the motor and cabin quicker but I’ve never had it happen in the summer to cool anything.

 

[mbarber84]

Three possibilities come to mind here. In cold temperatures this would be considered normal, but if it’s occurring in warm temperatures and well after the truck has reached operating temperature, there could be some variation as to what you are seeing. When it automatically idles up, what RPM will it hold during those events? That may be a clue to help figure it out. If you’re seeing a constant 1,000 rpm then that idle-up is related to engine temperature and is being triggered by readings taken from the IAT (intake air temperature) sensor and the ECT (engine coolant temperature) sensor. If it’s below 40° and coolant is below 180° you’ll get an auto idle up to 1,000rpm. If that’s happening when it’s 90°F outside and the truck is at operating temp and has been running, I would tend to think your IAT is getting a false reading.

If you are seeing a constant 900rpm during these events, then that event is being triggered due to emissions related programming in the ecm. What you’re seeing could be HCD (hydrocarbon desorption) mode and the truck is raising its engine rpm in order to more efficiently burn the diesel fuel. Idle speed is notorious for not fully combusting the fuel and allowing fuel to “wash” down cylinder walls, contaminate engine oil, and flow downstream into the DPF / SCR system. This tuning prevents this so that you don’t have issues further down the road. It keeps the DPF from filling with soot from extended idling or low speed / low load operation.

The last possibility I can think of is electrical in nature. You didn’t mention specifically what truck you have, so this one is more of a stretch, but some trucks could be ordered with an electrical load monitoring system that will automatically increase engine RPM to help maintain system charge. Not sure if you have that or not.

Take a look at the two pages I attached to this post. They’re pulled directly from my 2022 Ram HD owners manual.

 

[mbarber84]

Mine does not do that but I will often use the cruise button to "high idle" after starting so may bypass it. Sometimes when hooking up or parking the TT, I may sort of idle for 10 or 15 minutes, but it still does not do what you describe.

The only thing that seems off about idle on my truck is that, at the very least, I really think the computer counts high idle that as idle time. When I look at the ratio of idle time to non-idle time, there is no way it is correct for my truck. It is obviously counting something other than regular engine idling in the total number. Probably 95% of the miles on my truck are towing the TT. Usually hook up and then drive for 1 to 5 hours on highways so very little idle in traffic. My sense is that probably 20% at most of my engine time is idling, maybe less. But the ratio shown on the evic says my idle % time is almost 1/2 of the total hours! I usually lock out 6th gear when towing, so even on level road the RPMs are over 15 or 1600. So can't imagine what the computer is seeing to get almost 1/2 of my engine hours listed as idle.

I’ve been trying to find out specifically how our trucks calculate “idle” time. Reading various different factors but nothing specifically concrete. Sounds to me, based on what users report, that “idle” doesn’t necessarily mean sitting stationary and operating at 850rpm. My gut tells me they’re measuring idle time either by RPM thresholds, or % Load on the engine, or a combination of those. Think of it this way, if you put your truck in drive and let it creep forward on its own, it’s now physically moving, but the engine is still technically operating at idle speed.

I’m going to keep digging on this one until I find concrete facts. I too have noticed my idle hours are higher than what I expect. (Still low compared to drive hours and total hours) but definitely something to figure out, if for no other reason than knowledge.

 

[oscar]

Thank you for the posts. To clarify:

* Yes I get high idle on (very) cold start. This I know is normal.
* The phenomenon I describe is well after warm up and driving. Ex: Yesterday it happened after I drove a few miles from the house to the car wash. Ambient was about 40º. This summer it happened after hours of moving trailers around a site with a (low speed) one mile run between storage and the MX facility. Ambient was 85-90ºF. This scenario would fit the "emissions operating mode" described in the book. It may apply to the first scenario too. I will check the RPM next time to confirm its value.
* None of the scenarios included a high electrical demand. (I don't believe AC requires more than the juice for a fan and the compressor clutch.)

 

[mbarber84]

Thank you for the posts. To clarify:

* Yes I get high idle on (very) cold start. This I know is normal.
* The phenomenon I describe is well after warm up and driving. Ex: Yesterday it happened after I drove a few miles from the house to the car wash. Ambient was about 40º. This summer it happened after hours of moving trailers around a site with a (low speed) one mile run between storage and the MX facility. Ambient was 85-90ºF. This scenario would fit the "emissions operating mode" described in the book. It may apply to the first scenario too. I will check the RPM next time to confirm its value.
* None of the scenarios included a high electrical demand. (I don't believe AC requires more than the juice for a fan and the compressor clutch.)

I’m willing to bet you’re experiencing the emissions related high idle. If you’re just doing a lot of slow, low rpm leisurely driving you’ll definitely rack up enough to warrant that mode activating. Also wouldn’t hurt to check to make sure your sensors are functioning as intended.

 

[oscar]

I’m willing to bet you’re experiencing the emissions related high idle. If you’re just doing a lot of slow, low rpm leisurely driving you’ll definitely rack up enough to warrant that mode activating. Also wouldn’t hurt to check to make sure your sensors are functioning as intended.

Yes, starting to look like that. I had a 6.7 in my motor home that never did that, but it was driven differently.

 

[mbarber84]

Yes, starting to look like that. I had a 6.7 in my motor home that never did that, but it was driven differently.

Out of curiosity, what does your DPF gauge read currently?

 

[AH64ID]

Temp related fast idle (technically high idle is rated rpm no-load) should kick in after 2 minutes of stationary time, unless it’s really cold at initial startup and then to goes to 1,000 instantly. This can happen anytime the ECT is below 180°.

On my previous trucks the ambient temp had to
be below 32°F. I haven’t looked at the tables yet for the ‘22 to see when it kicks in, but it seems to act the same as my ‘05 and ‘18 so far… I’ve seen temps as low as -5°F.

Anything more than 10 minutes per hour of operation is considered excessive idling. That’s only 16.6%. Excessive idling is why the hour meter on these trucks now shows idle time.

 

[oscar]

Out of curiosity, what does your DPF gauge read currently?

I will have to check, but I've never seen it above 25-30% of scale......(Not sure what the units are, not in my truck)

 

[oscar]

Temp related fast idle (technically high idle is rated rpm no-load) should kick in after 2 minutes of stationary time, unless it’s really cold at initial startup and then to goes to 1,000 instantly. This can happen anytime the ECT is below 180°.

On my previous trucks the ambient temp had to
be below 32°F. I haven’t looked at the tables yet for the ‘22 to see when it kicks in, but it seems to act the same as my ‘05 and ‘18 so far… I’ve seen temps as low as -5°F.

Anything more than 10 minutes per hour of operation is considered excessive idling. That’s only 16.6%. Excessive idling is why the hour meter on these trucks now shows idle time.

Normally I hardly idle at all. It's the two weeks a year of working an event moving trailers etc that I do quite a bit more. I could just start/shut down all the time, but that kinda goes against my grain.

 

[mbarber84]

Normally I hardly idle at all. It's the two weeks a year of working an event moving trailers etc that I do quite a bit more. I could just start/shut down all the time, but that kinda goes against my grain.

You’re far better off to start and stop than to let it idle. Idle is hard on the engine and the emissions system. In my humble opinion it’s just not worth it.

 

[H3LZSN1P3R]

You’re far better off to start and stop than to let it idle. Idle is hard on the engine and the emissions system. In my humble opinion it’s just not worth it.

No you are not. If its not prolonged its fine if its idling for a while just put it in high idle. No need to wear out the starter switching out trailers