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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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1,418 Posts
Discussion Starter #1
Most of what I have read says to run the engine at 1500 -2000 RPM's to charge the A/C system. I made up a cable and screw thing to get fine adjustment of the throttle linkage.

The problem that I am having is that if I set the RPM's much past 1000, the engine speed takes off and keeps climbing until I crank the screw back off.

What is going on here? Is this normal? Is there any way around this?

Thanks
 

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2001 Volvo V40
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2,954 Posts
That is normal. If the engine speed is above the idle range, it will run to the maximum speed unloaded. That is a feature of the governor of the injection pump.

There is no remedy for this, other than keeping the rpm in the idle range, i.e. not too much above 1000 rpm.

But why do you need this? do you spend a lot of time standing still and idling?
 

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1981 Mercedes-Benz 300TD
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871 Posts
I thought it was because the turbo begins producing a small amount of boost after 1000 RPM. My boot gauge confirms the small increase.

As boost increases, the ALDA supplies more fuel....which raises the RPM....increasing boost.....which increases fuel delivery.... The system feeds back on itself until about 3200RPM @ 2psi of boost.

Its a small change, but any small change to an unloaded engine will greatly alter its speed.

When I removed my ALDA a few weeks ago I noticed this cycle stops sooner, around 1600RPM, probably because fuel delivery is now a constant.

Its also impossible to get my friend's twin turbo gas race car to hold a steady fast idle; once you exceed a certain RPM, you build a little boost. Naturally aspirated versions like a 240D easily hold fast idle.
 

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2001 Volvo V40
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The governor of the IP is a min-max governor, i.e. it only governs the idle speed and maximum speed. Inbetween you control the rack directly by means of the pump lever (and accelerator pedal). Diesel engines tend to run away if the speed isn't governed. The runaway speed can differ between turbo and non-turbo engines, but that is not why there is a runaway to maximum speed.
 

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1981 Mercedes-Benz 300TD
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871 Posts
I thought the maximum unloaded speed was around 4800RPM? If I'm understanding you then the governor shouldn't kick in until close to that RPM. I can surpass 3200RPM, where it stops climbing, simply by pressing the petal more. I'm trying to understand the operation of the governor system better.
 

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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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1,418 Posts
Discussion Starter #6
But why do you need this? do you spend a lot of time standing still and idling?
I do spend too much time sitting in traffic. Hence my name "Commuter". Today for example it took 1.5 hours to go what usually takes 30 minutes due to an accident many miles up the road. Most of that time I didn't even touch the throttle. My newly charged A/C puts out only 60 deg F air at the vents at idle. I think that I have too much refrigerant. Yesterday I added a switch in parallel to the Anti-Icing switch (ETR). When closed the added switch disables the the ETR and the compressor is not turned off. The vent temp got to 15 deg F. when I was able to maintain freeway speeds. That is cold A/C.


By "need this" are you asking about the cable and screw thing I made up?

In this case I just wanted the higher engine speed that is used for charging the A/C system. However, I am thinking about using what is actually the Idle Speed Adjuster from a 300D non turbo. There are two reasons for this. I find that if I give slight throttle at cold start the engine is quieter when it fires up. Also I expect to get more cooling when stuck in traffic if I can get the idle to 1000 or so.
 

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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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1,418 Posts
Discussion Starter #7
I thought it was because the turbo begins producing a small amount of boost after 1000 RPM. My boot gauge confirms the small increase.

As boost increases, the ALDA supplies more fuel....which raises the RPM....increasing boost.....which increases fuel delivery.... The system feeds back on itself until about 3200RPM @ 2psi of boost.

Its a small change, but any small change to an unloaded engine will greatly alter its speed.

When I removed my ALDA a few weeks ago I noticed this cycle stops sooner, around 1600RPM, probably because fuel delivery is now a constant.

Its also impossible to get my friend's twin turbo gas race car to hold a steady fast idle; once you exceed a certain RPM, you build a little boost. Naturally aspirated versions like a 240D easily hold fast idle.
I pulled the hose off the ALDA but it did not make a difference.

I added an orifice to the front of the air cleaner to limit the amount air that could get into the engine but is smoked badly and ran rough.
 

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1981 Mercedes-Benz 300TD
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871 Posts
I too would like a high idle mechanism so I don't have to do it manually at start up, especially since it smooths out a miss I have at idle. I have also heard it's a bit easier on diesels.
 

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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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1,418 Posts
Discussion Starter #9
Using the cable and bracket from a 300D non-turbo can work. The problem is that there is not enough slack to push the emergency stop enough to stop the engine. The car that I got the parts from is gone so I can't go back to see how it worked. I am working on some ideas on how to make this work.

If there is enough interest I can go into mini production and make a kit for others to install on their car.
 
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