unexplained increase in RPM..

[Cancruiser]

It has been quite cold around here. -20C to -30C so, whenever I started the truck, it would idle at ~ 1200 RPM and drops gradually as it warmed up. But, I have noticed when the engine reached its normal operating temp, the RPM never really went down all the way as it used to do in summer/spring. It would stay around 900 compared to ~ 650 in summer. I am aware of the presence of the ACSD but it is now +5 outside and the RPM still goes down to 900 only.

This obviously affected my gas mileage and makes the truck wants to leap when stopped at traffic lights in "D" with my foot on the brakes.

Is that normal?? Do you think it will go back down as the weather warms more and more..?? How does the ACSD works anyway? does it sense the outside temperature and adjusts the RPM accordingly or does it use the engine temp instead??

Cheers,

 

[street tractor]

My 81 does the same. I think it will drop again once it gets warmer.

Have you done any longer drives, say 1 or 2 hrs? I find if I do a long highway commute and the truck has been up to temp for quite a while, the rpm's will drop closer to what you would be used to when it's warm out.

I can't say that I understand why this happens, just thought I'd let you know my experiences.

As far as the fuel consumption concerns, I'd bet that it's pretty insignifigant...

I'm sure someone will chime in with the technical explanation for ya!

 

[Radd Cruisers]

what model are we talking about here?


~John

 

[street tractor]

I was speaking of the HDJ81. I assumed he was as well from his sig.

 

[roscoFJ73]

Is that normal?? Do you think it will go back down as the weather warms more and more..?? How does the ACSD works anyway? does it sense the outside temperature and adjusts the RPM accordingly or does it use the engine temp instead??

Cheers,

It uses engine coolant to heat up wax which releases a spring which allows the idle and pump timing to go back to its warm setting.
(Im not sure how it goes back to its cold setting)

Its mainly an anti smog device. Most countries have emissions tests which include a cold start which makes diesels run dirty and gives a lower overall average score and the ACSD is there to clean it up.

 

[Cancruiser]

Thanks all for the replies..'
I have been doing ~30 kms commutes or so, mostly highway ~ 80km/hr. While this maybe a short drive, but I think it should be enough for the engine to reach its normal operating temp. But, the answer from "street tractor" makes me feel a bit comfortable as I am not alone..maybe wait a bit till it is warmer and see if it'll improve.
Thanks also for explaining about the ACSD, I knew of its existence but was not sure how it worked..
P.S. It is a 90' HDJ81 as in my signature..

 

[roscoFJ73]

Thanks all for the replies..'
I have been doing ~30 kms commutes or so, mostly highway ~ 80km/hr. While this maybe a short drive, but I think it should be enough for the engine to reach its normal operating temp. But, the answer from "street tractor" makes me feel a bit comfortable as I am not alone..maybe wait a bit till it is warmer and see if it'll improve.
Thanks also for explaining about the ACSD, I knew of its existence but was not sure how it worked..
P.S. It is a 90' HDJ81 as in my signature..

It shouldnt be on after 30klm regardless of the temp outside

 

[Ron R]

It shouldnt be on after 30klm regardless of the temp outside

Yeah, that's what I thought as well. But it does not.
Nevertheless, I was discussing this with a friend last week now that we had some lower temps (-5 to -10 C) over here.
His 80 as well as my 73 are displaying the same behaviour, a higher than normal idle, even after longer distances.
I can not explain why it is that way.
A few years ago I fiddled with the temp compensator to adjust, just to find out that when the overall temp rises the idle is too low....strange...

 

[roscoFJ73]

Yeah, that's what I thought as well. But it does not.
Nevertheless, I was discussing this with a friend last week now that we had some lower temps (-5 to -10 C) over here.
His 80 as well as my 73 are displaying the same behaviour, a higher than normal idle, even after longer distances.
I can not explain why it is that way.

Maybe the wax is no longer melting as well as it should or maybe the springs lose their tension.

A few years ago I fiddled with the temp compensator to adjust, just to find out that when the overall temp rises the idle is too low....strange...

In oz it is nearly always removed s soon as it becomes a problem.

 

[Cancruiser]

In oz it is nearly always removed s soon as it becomes a problem.

I may do just that when I get a chance.. Back to the good ol' manual choke then.. I have a couple of old British cars so I am quite used to using a choke (and seeing oil leaks too )