OK, the throttle body's main function would 'seem' to be to act as an Anti Shudder Valve when stopping the engine. But that could be accomplished by a simple solenoid connected to the Stop button to open and close the flap whereas it would seem that the throttle body has at least two multi pin electrical connections plus a sensor.

So what else does it do?

Last edited by Simon J on 17th Sep 2019 9:11 pm. Edited 1 time in total

A quick Google threw up this:

Volume

Throttle bodies have electrical components called mass airflow sensors mounted inside that sense the volume of air entering the engine. These sensors send information on the amount of air detected to the vehicle computer, which adjusts the amount of fuel added to the incoming air to maintain the proper fuel-to-air ratio for proper combustion in the engine cylinders.

Throttle Position

Throttle bodies contain a sensor that monitors the position of the valve that controls airflow and the speed at which it opens. This allows the computer to note the position of the valve and helps the computer to determine the proper amount of fuel to add to the incoming air. The throttle position sensor also notes when the valve is opened quickly, allowing the computer to add more fuel to the incoming air, which improves throttle response and acceleration. Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

Many thanks for the speedy reply and perhaps I should have said my query was about the Diesel engine. Are these explanations not for a petrol engine? As I understand it, the throttle flap/butterfly on a diesel is either open or shut as the airflow is largely irrelevant in a diesel as it’s the fuel flow alone that determines revs and power.

P.S. I’ve changed the title to make it clear that it’s a diesel I’m enquiring about.

The throttle valve on our Diesel engines is there to stop run on and in extreme cases, a run away engine that won’t stop as it is using oil for fuel, has air from intake and turbo and combustion takes place due to compression rather than ignition electrically as in a petrol engine. We’ve got sensors to monitor the position of the butterfly valve and a motor to drive it to the required position.

Thanks, but surely if the sole purpose of the butterfly valve is to stop the engine running on by cutting off the air supply, this could be done simply by a solenoid linked to the stop button? Hit the stop button, close the valve, engine stops. So why the need for a sensor and the complex wiring?

It’s all very well having a simple butterfly valve (compared to the Venturi slide and needle in basic carburettors), but the ecu doesn’t know where the valve is without the sensor. A motor drives the valve to the required position and the ecu knows that the valve is in that position so stops driving the valve. Open or closed, it’s a feedback loop.

But why does the ECU need to know where the valve is? It doesn’t control airflow like a petrol, spark ignition engine. It only needs to be open when the engine is running, and closed to make the engine stop. So what purpose is served by intermediate positions? I’ve read that the engine will run perfectly well without the throttle body but it won’t stop as well.

The throttle body works in conjunction with the egr system. The ecu can alter the flap position under light engine loads to restrict airflow thus forcing more exhaust gases into the engine to be reburnt thus reducing emissions , it is not a simple on and off system and as also stated it helps the engine shut down smoothly. Years ago cars used to have anti run on valves fitted and these were very hit and miss as the engines used to shudder to a stop.

Thanks for this. New to diesels, I’m slowly starting to get the hang of things.

In addition to the above, I can say with certainty that if the throttle body on a FL2 fails/malfunctions, then it *can* cause hell with the car's electrics.

In fact, a dead throttle body produced a phantom fault with the EGR system on my car. The car thought the entire EGR system failed and produced an engine management light alongside "Engine System Fault" on the dash display. Once the throttle body was replaced these faults disappeared entirely. It's just a shame I only started to suspect the throttle body was causing these problems after getting through a few cans of EGR cleaner (which produced no soot out of the exhaust from the get-go), and many hours swearing at the diagnostic system :<

Now, engine shutdown doesn't shudder the whole car, and throttle response seems to be a bit quicker (though that might be placebo). Either way, its clearly well worth making sure your FL2 has a working throttle body.

EDIT: Oops- old thread.