How does an engine get its air when you take your foot off the throttle but leave it in gear (engine braking). I am assuming that the engine is not actually firing and is creating a near total vacuum inside as the idle bypass could not possibly provide enough air flow to keep 5k revs happy.
Please enlighten me!
Many thanks,
Dan
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Exactly! That's the reason for high vacuum in the inlet plenum/manifold when engine braking.
Watching a vacuum guage in my old Mini some years ago taught me a lot.
The ECU will crank the air bypass or throttle valve open slightly to allow some air through in an effort to reduce NOx emissions - this is the reason for the slightly delayed return to idle some modern engines demonstrate.
MG-Rover Questions? forums.mg-rover.org/
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Thought so, thanks for confirming.
On a related note, if you hold a throttle body to the sky and you can see a hairline crack of light between the butterfly and the body when closed, does it mean it's shot and will give a rough/high idle or is this normal?
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It really depends upon the application really.
If it were a plastic MG Rover one, then yes, I'd condemn it.
MG-Rover Questions? forums.mg-rover.org/
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On some Fords you used to disconnect the supply to the idle sppeed control valve-adjust the throttle plate to give 800rpm.(thats why a tiny sliver of light could be seen at edge of plate)reconnect the valve which would bring idle speed back to 900rpm..
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Its a renault engine TB and the butterfly appears to be made of something durable and metal-like (er metal?). The sliver of light looks more like a manufacturing tolerance issue rather than intentional (uneven) This gap is very slight though, the TB will hold water when closed!
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....hairline crack of light between the butterfly and the body when closed.....
If you are looking in the direction of air flow then this will be the minimum setting of the butterfly. At idle the butterfly only needs to be open a tiny sliver - the engine will suck just enough air through the tiny opening for idle purposes when very little power is required, just enough to overcome internal friction and electrical load (plus pas & aircon too if fitted). Many butterflys are set so that they never actually fully close, but (on modern cars) the exact position will be constantly adjusted by a stepper motor to maintain idle speed (on an older carb equiped car, the butterfly position will be static hence engines used to slow at idle when an electric load was put on).
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And not a idle bypass valve?
I'm also sure this engine is a simple cable that mechanically opens the butterfly - there appears to be no other mechanism to operate it independently of the loud pedal.
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