Any of the smart guys here know how long the fuel system should maintain pressure after the engine is shut down? At idle I'm seeing 3.6 bar on Motec (3.8-0.2 vacuum) and after the engine is shut down fuel pressure is 3.4bar and it slowly bleeds down to about 0.3bar after 1 hour.

I was pretty sure bleed down pressure specification was buried in the service manual but could not find it. Any help will be appreciated... Thx..

 

Completely normal. I wouldn't worry.

 

I didn't think it was abnormal but was wondering what the official specification was. I'm sure there there has to be one published by Porsche, just can't find it.

 

Engine on - 53psi is the OEM pressure afaik, that equals 3.65bar and OEM FPR is 3.8bar

 

Hey John, what pressure are you reading when you crack the throttle or are out of vacuum?

 

Right at 3.8 when out of vacuum Sam. When I blip the throttle it goes up to 3.8-3.9bar. At idle it's 3.5-3.6bar. I think that's a tad high as I figured there was about 0.5 bar of vacuum but I may be wrong. I haven't driven the car yet to look at the rest of the behavior and how it does under load. We have a bunch of conditional alerts set in Motec that would alert us if fuel pressure drop under various conditions.

We are also adding a Motec fuel controller which will alleviate some of the fuel bypass by slowing the pump at low engine loads and especially at idle. This will significantly reduce the huge amount of fuel bypass that you get running a large fuel pump(s) thus reducing fuel heating. We will employ a similar system in our Spec Boxsters which have issues with fuel boiling during long endurance races in the summer. We run 6, 8, 12, and even 24hr races and this becomes a big issue under those conditions.

I was just curious what the bleed down limit was. Since fuel pressure bleeds down after shut down it must do so through the FPR or the check valve just aft of the fuel pump. Again, it takes about an hour for the fuel pressure to bleed down fully after the engine is shut down.

 

I just dug it up in the service manual, idle fuel pressure with OEM 3.8 fpr is supposed to be 45-50.75psi (3.3 +/- 0.2bar)

 

Yeah that makes sense. Another observation I had his big injector car stumble on start up depending on how long they've been sitting it's never the same. Quirk