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I remember when driving a passenger car, when I took my foot off the accelerator pedal completely, but remained in gear, the car would normally continue to move at some low speed, different for each gear, more or less constant for each gear (provided the road is flat).

Now I drive a van, and I am surprised it doesn't behave like that. No matter what speed I drive, even if the road is flat, no matter which gear I am in, if I take the foot off the accelerator pedal completely, the van soon comes to a stop, the engine shuts down, and that's it.

Is there something wrong with this van? The van in question is a:

  • Mercedes-Benz 308D year 1992, model 602.316, curb weight 2035 kg (4486 lb)
  • OM601.940 4-cylinder 58 kW (79 hp) 2299 dm3 diesel engine

Is it normal for such vans to behave like this? Can something be regulated to make it behave like the petrol/gas passenger car I drove before, which continued moving at a constant slow speed when I took the foot off the accelerator pedal?

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2 Answers 2

up vote 4 down vote accepted

I think the listed stats make it pretty clear: you are driving a heavy van with a fairly low power engine. More importantly, it's likely that you have a low torque engine which is critical to keep the vehicle rolling.

What you're used to in the passenger car is the same as what happens in the van, just to a lesser degree. In your passenger car, you were witnessing the engine idler trying to keep the engine from dropping below a target RPM value. In your passenger car, there was sufficient low end torque to overcome the aerodynamic and rolling resistances of the vehicle.

Clearly, in the van, you have less power available and, likely, a much heavier and higher drag vehicle. The engine idler is not able to keep the vehicle moving without your input.

Here are three things to try in order to ensure that this isn't a different problem:

  1. Does the van idle smoothly with the transmission disengaged? If it stalled immediately even out of gear, you might have a problem.

  2. What happens if you try idling down a shallow hill in the lowest possible gear? If you take some of the rolling resistance out of the equation, your engine idler should be able to pull you along.

  3. Are your tire pressures to spec? If your tires are significantly low, that greatly increases your rolling resistance.

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When the transmission is disengaged (gear stick in neutral), it idles smoothly. I check the tire pressure every time I am bored near a filling station, so I keep it within spec. I'll try the hill thing when I'll be driving over one. What I wanted to know is maybe something can be regulated to have the engine deliver a little bit more torque when idling, maybe cleaning/changing air filters will help, or something else needs to be done to increase the torque while idle? But maybe I add another question about that. –  miernik Apr 14 '11 at 10:09
    
@miernik, okay, it sounds as if the van is behaving roughly as it is supposed to - tires are good and it idles without load. Some power improvements are definitely going to fix inefficiencies (e.g., an air filter). However, adding more power may interfere with your fuel efficiency goals in other questions. –  Bob Cross Apr 15 '11 at 1:25

The engine is calibrated to idle at a low speed when the accelerator pedal is released. Otherwise, you would need to keep a foot on the gas while stopped.

When in gear, most cars will continue to feed in a little gas, comparable to the amount needed to idle. This results in the forward creep.

If the force stopping the car exceeds the force accelerating the car, it will slow down. If the clutch is engaged, the engine speed will slow proportionally with the car speed. If the engine speed is too slow, it will stall, often with a sudden jerk.

It sounds like you have stalled the van.

An automatic transmission should never allow a stall. A manual transmission is very easy to stall, and you would probably know this already as it's part of learning to drive stick.

So, most likely, you have a faulty automatic transmission.

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That is not applicable here, as both the passenger car and the van where manual transmissions. Its in Europe. I didn't ever even sit in the driver's seat of a vehicle with an automatic transmission. Yet in the passenger car with manual transmission it was possible (when releasing the accelerator pedal slowly) to have it keep moving with the foot completely off the accelerator pedal and off the clutch. –  miernik Apr 13 '11 at 18:29
    
@miernik: Well, that doesn't change much. This answer covers manuals, but as I said, I assumed you would recognize a stall. I'd be surprised if you never stalled the passenger car, when you were learning. –  Potatoswatter Apr 13 '11 at 18:54

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