fast idle after warm up

[ilovetrains]

Today it was 28 degrees. Started just fine (block heater was on all night) and after a few minutes idle, the idle speed went up to 1000 RPM. Let it idle a few minutes then started driving. About 45 minutes later, I am sitting a stop light for about 5 minutes (it actually got stuck, had to run the light, another story), truck is at full operating temp by this point, and the idle climbs up to 1000 RPM. I had just noticed that the truck seamed to be idleing a little rough, the shifter was vibrating more than usual in my hand, when it idled up this went away.



My question is, why did it idle up if it was already at full operating temp after 45 minutes of driving?

 

[SRadke]

All third gens will idle up to 1000 rpm after a period of 2 minutes if the temp is below 32° F. Why? It's somehow emissions related.



-Scott

 

[Mobius1]

why did it idle up if it was already at full operating temp after 45 minutes of driving

I was told by a diesel tech at a dealership I used to work for when the 3rd gen diesels first came out, that it would increase idle to 1000 rpm to "flush out" the emissions. He had an 03. My 05 never increased idle at all until

I had the fast idle feature enabled. Wierd. Maybe it has something to do with the cat??

 

[ilovetrains]

I was told by a diesel tech at a dealership I used to work for when the 3rd gen diesels first came out, that it would increase idle to 1000 rpm to "flush out" the emissions. He had an 03. My 05 never increased idle at all until

I had the fast idle feature enabled. Wierd. Maybe it has something to do with the cat??

Seriously? Of all the things I would have thought up on my own, I never would have thought it was emmisions related. Does it measure the temp of the incoming air is deciding when to fast idle?

 

[SRadke]

Does it measure the temp of the incoming air is deciding when to fast idle?

In short, yes.

 

[rbattelle]

Personally, I don't think it has the slightest to do with emissions. In my opinion, it's to help keep the cylinder temps up under cold conditions. This helps stave off "stacking" or "fuel washdown" due to incomplete combustion. Plus it helps keep water temperature up. Sitting at a stoplight for 2 minutes with my foot on the brake (which disables high idle), my coolant temperature can drop from 187 to 178 if the ambient temperature is below 25 or so. Any emissions benefit, then, would be purely coincidental.



And the bit about "flushing out the emissions system" doesn't sound even remotely plausible. Who ever heard of "flushing" your emissions system? What, precisely, would you be "flushing" out of it? :-laf



-Ryan

 

[jwduke]

Dealer I bought mine from, told me it was to maintain block temp, so the fuel doesn't was the cylinders. Diesels engines are known to do this, if the block temp. drops below a certain temp.

 

[Mobius1]

You guys are probably right Ryan and jwduke. Thats just what I was told by a tech that works there. But I wouldn't be suprised if it had something to do with emissions in one way or another seeing as how emissions are getting more and more strict.

 

[SRadke]

It all goes hand in hand. Low block temp combined with low IAT lead to incomplete fuel burn and washing cylinder walls. We all know incomplete fuel burn is what "emmissions" is all about.

 

[steved]

Today it was 28 degrees. Started just fine (block heater was on all night) and after a few minutes idle, the idle speed went up to 1000 RPM. Let it idle a few minutes then started driving. About 45 minutes later, I am sitting a stop light for about 5 minutes (it actually got stuck, had to run the light, another story), truck is at full operating temp by this point, and the idle climbs up to 1000 RPM. I had just noticed that the truck seamed to be idleing a little rough, the shifter was vibrating more than usual in my hand, when it idled up this went away.



My question is, why did it idle up if it was already at full operating temp after 45 minutes of driving?

Mine does it too... I always assumed it was based on temperature, not time.



Mine will sit for a while before it kicks up... way more than two minutes... If you just leave the truck to do it's thing, it will idle up and down as it needs, which again leads me to believe it is directly temperature related, not time.



steved

 

[betterthanstock]

the truck seamed to be idleing a little rough, the shifter was vibrating more than usual in my hand

I thought it says in the 2003 owner's manual that at low temperatures it will start idling on only two cylinders at a time to prevent condensation in the engine due to low temperature?? TOo lazy to look for it now...

 

[ilovetrains]

This is intersting as well, when it has idled up it will idle back down if you touch anything, put the clutch in, press the brake etc. not just touch the gas pedal.

 

[steved]

Mine only does it if you move or press the go pedal. . depressing the clutch or brake does nothing in mine... maybe different programming?



steved

 

[rbattelle]

It should disengage with a press of the brake or if vehicle speed is greater than 1mph or so.



-Ryan

 

[CGA97]

so what is the point of depressing the accelerator after a cold startup to reach 1000rpms? It just drops back down to normal idle, then waits the 2 mins before it goes to 1000 on its own. .

 

[rbattelle]

so what is the point of depressing the accelerator after a cold startup to reach 1000rpms? It just drops back down to normal idle, then waits the 2 mins before it goes to 1000 on its own. .

In my opinion, there is absolutely no point at all. I have always completely ignored that statement on the starting instructions sticker.



-Ryan

 

[CGA97]

I guess if the rpms stayed above 1000 after start, it would make sense to "force" them there before the 2 min period, but that doesn't happen.