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Throttle valve on an E60 Diesel |
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richd
Forum Member Joined: 07 Jul 2015 Location: Axminster Status: Offline Points: 9 |
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Posted: 08 Aug 2015 at 5:37pm |
Can anyone enlighten me - as to what this valve actually does? (the valve in front of the EGR valve)
Diesels normally don't have a throttle - the engine speed is controlled solely by the amount of fuel injected, so what does it do on a 530D? Mine apparently is showing a fault on a diagnostic check, and I have been told it needs replacing, but apart from poor mpg - only mid 20's it runs fine. Would a fault cause low mpg? The mpg was OK, until a few months back, I had a new starter motor fitted, which entailed taking off the inlet manifold - ever since then the mpg has been well down on what it was before. I thought the pressure converter may have got damaged that operates the EGR valve, as it is plastic, but that has been replaced, and it made no difference to the mpg afterwards - so something else is at fault I don't want to fit a new valve- which isn't cheap - only to find it is still the same!! Rich
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Mike Fishwick
Forum Member Joined: 04 Aug 2006 Location: France Status: Offline Points: 2742 |
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As you say, diesels do not have a throttle valve! The valve in question - like the EGR valve - is common to all modern automotive diesel engines. My experience is of the unit-injector Volkswagen TDI engine, but they share basic similarities with the more usual common-rail systems.
If you look closely you will see that the valve is an air shut-off valve, and only closes the intake tract when the key is turned off, and then opens a few seconds later, after the engine has stopped. Its purpose is to prevent the partial reverse rotation on stopping the engine, which could otherwise occur as it rocks back against compression, and which could damage the cam chain/belt and high pressure fuel pump etc. By closing the inlet tract there is no intake air, so no compression, and the engine therefore runs smoothly to a halt in its normal direction. As your low mpg only began after the inlet manifold was removed, perhaps the seals have been damaged (or not replaced) and you are losing charge air (ie boost pressure) past some of them. Also, is the air shut-off valve opening fully when the engine is running? You can check this by looking at the 'running' position of the valve, and seeing is the operating arm can be moved any further towards the vacuum cylinder which operates it. Perhaps that the fault code really means low boost pressure, relative to the engine speed and throttle opening signals - these codes usually point at something, which then still needs diagnostic work to find the real fault. In spite of BMW Propaganda, computer diagnosis is more of an art then a science! Incidentally - what do you mean by a 'pressure converter?' On older engines there is a spherical chamber which acts as a vacuum reservoir for the solenoid valves which control the operation of the shut-off valve and turbocharger nozzle control. (The turbocharger nozzle ring is controlled by an electric motor and a gearbox on later engines - more complexity for no real gain!) BMW tend to give misleading names to persuade people that their diesels are something special, and not similar to those of other manufacturers - for example, the press release announcing the 320d engine claimed that at its heart was a 'Radial-piston injection pump,' which was sheer nonsense. Like all other automotive common-rail engines there is a camshaft-driven high-pressure fuel pump, which produces the high pressure to supply the injectors - but is not an injection pump in the accepted sense. It does however have its piston elements arranged in a radial configuration within a rotor . . . just like on a Ford Transit! Perhaps in BMWSpeak this pump - which increases the fuel pressure from the lift pump to the massive level feeding the injectors - is a 'Pressure Converter' . . . Edited by Mike Fishwick - 11 Aug 2015 at 12:25pm |
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