I used an OBD to get data from one of my friend's car - Volkswagen Polo 1.5L Diesel. But the car was idling at around 900 rpm, while the throttle position I got was around 22%. How is it possible? As I am increasing the throttle its going to 88%, 90%.... But once I leave the throttle pedal, the load drops to 22%, which I think should be around 5%-7%

  • What year is the car? Could be a faulty TPS or sticking throttle cable/throttle plate. By load do you mean throttle position went back to 22% or that engine load went to 22%? TPS% should always be 10% or less at idle.
    – Ben
    Commented Jan 31, 2016 at 15:41
  • Diesel engines run very lean by design. That's why the throttle position is not very relevant. Esp. there is no need to get a "balanced" AFR. Engine power is almost exclusively adjusted by fuel amount injected, so basically, the throttle cannot be opened too much.
    – JimmyB
    Commented Jan 31, 2016 at 16:26

3 Answers 3


I don't know specifically about your car but in general it is possible. Some cars have a cam that opens the throttle plate when the engine is cold to increase the idle. My 99 Nissan Almera has such a cam.

The cooling system has two purposes. The obvious one is to disperse excess heat and prevent the engine from overheating. The less obvious one is to make sure a cold engine reaches optimum operating temperature as quickly as possible.

There are two ways this second task are accomplished. One is by the use of a thermostat. The thermostat prevent coolant from flowing from the engine to the radiator until the engine has reached a certain temperature ( usually around 85*C) at which point it starts to open and allow the circulation of coolant.

The second way is artificially increase the idle, which increase heat production, and brings engine up to temperature. This is usually done in one of two ways. Some cars use the IAC ( Idle Air Control ) and others use a special CAM connected directly to the throttle plate. As the engine warms up the amount of extra idle will slowly be backed off by the ECU.

Based on your comment, it sounds like you've got such a CAM on your throttle, and I suspect this may be more common on smaller engines. Also, if you're seeing that your friend's engine is not getting above a certain temperature, his thermostat might be stuck open, or a previous owner may have even completely removed the thermostat, which happened to me once.

  • Can you explain this a bit more please. His engine is cold. Coolant temp is around 50C, which I have seen should be around 85-90C
    – Soumya Sen
    Commented Jan 31, 2016 at 8:46
  • @SoumyaSen Please see my edits. Commented Jan 31, 2016 at 10:54
  • @RobertS.Barnes. The car is a diesel, it won't have a throttle valve or IAC.
    – HandyHowie
    Commented Jan 31, 2016 at 19:38

It is not actually. The OBD2 sensors probe the throttle positions in terms of percentage, not the actual mechanical position.

The idle of the vehicle Volkswagen polo was governed by ECM, and the rpm varies from 550 -900rpm and most standard is 600-800. It was based on the various factors as below.

  1. Ambient temperature
  2. Engine temperature
  3. Fuel Quality
  4. Engine condition which also include the filters like Air,Oil,and Fuel
  5. O2 sensors feedback

If the ambient temperature is too cold then the oxygen supply to the engine will be too high and if it is hot then too low, to achieve the balanced AFR (Air Fuel Ratio) the idle rpm of the engine will be increased or decreased by adjusting the throttle electronically. During high rpm obviously you will get the rise in the throttle percentage and vice versa.

We can explain for all the remaining conditions too.

So no need to bother more about the internals (Intelligent) part of the car. If a huge difference was found the system will give the warning. If you are curious you can get the status code on referring the ELM DataSheet and Command Set, or if you have any ready software available you can probe the error code and decode the errors with this site.

  • This doesn't answer the question. The main question was basically "why is the reading 22% when I would expect it to be less than 10%?" and you don't address that.
    – JPhi1618
    Commented Feb 1, 2016 at 14:30
  • The throttle position is variable and it can varies from 5%-30% depends on various situation as mentioned above. Commented Feb 1, 2016 at 18:30

The VW diesel engine controls that I'm familiar with (ALH and BEW mostly) are drive-by-wire and the throttle position will be at 0% at idle. You may have a different controller, but I wouldn't expect the throttle position to be much different.

The ECU is responsible for managing the idle speed and doesn't need throttle input to do that, hence the 0% (driver isn't asking for anything from the car) on the MkIV ECUs. I could see a different ECU / injection system doing it differently, but 22% seems very high.

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