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Although I have had fuel pressure included in one of my FIS-Control MMI tables I only recently took an interest in the readings. The low pressure seems to vary around 5 bar, sometimes a bit below and sometimes a bit above. The high pressure seems to vary a lot. When I start the engine it is around 100 bar, but once I start driving at low speeds it drops to around 60 bar on average. When I lift off it can drop, and then just as I press the accelerator again it might drop to 40 bar before rising again. On light acceleration it rises to around 70-80 bar, and on medium acceleration it gets somewhere between 130 and 160 bar. Then a but of WOT makes it go about 180 bar and I saw it peak about 200 bar.

Audi specs show that the low pressure should be 4-5 bar and the high pressure 100-200 bar, for the CREC engine (the old engine was about 150 bar max). But since my car is running fine and can reach 200 bar, that must be a simplification. The pump is mechanical, driven by one of the cams, so clearly raw pressure is proportional to engine speed. But if pressure is 100 bar when I start the engine, then why does the pressure control allow it to be less? Can anyone explain?
 

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Dave, why you asking a technical question this close to Christmas!? I more used to the questions you get in Christmas crackers at this time of year! :D

That is exactly how mine behaves! When I had then power drop and the fault was 'Fuel pressure out of spec', I kept a close eye on the FIS readings for a while and of course it didn't give that fault again so didn't see that low pressure actually read 7.2bar which it stated on the VCDS fault read.

I think at low revs the HPFP isn't used, the LPFP is used for the port injection so the pressure drops off the HP, but it will pressurise pretty quickly at the stab of the throttle. I think the pressure control valve just alter the pressure according to demand and when you first start up, it probably uses a bit of the HP like giving it a bit of old school choke to richen the mixture!?

Maybe a mapper like Doug could explain?

I can send you the CREC workshop manual for some festive reading if you want!? :)
 

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Discussion Starter · #3 ·
Merry Christmas Dave. It's good to know that yours is the same. I tend to agree with you - dropping below 100 bar at low load must be by design.
 
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