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Help with rail pressure pre-start-up.

ROH ECHT

K04 PLAY
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PDX OR
Car(s)
2007 MKV GTI
Just out of curiosity so this information is here to help in determining if fuel pressure has anything to do with stumbling cold starts.

If some of you can post what your rail pressure is before you start the engine. Please include ambient temp or VCDS MB 004 which includes coolant and IAT. https://forums.vwvortex.com/showthr...stumbling-at-start-up&highlight=rail+pressure

I initially thought it would be closer to shut-off pressure of near 50 bar...a bit lower of course because of heat...but I went out into the garage and checked one morning. It was 65°F on the MFD and the rail pressure was 7 bar. This may be normal, but I would like to know what others see at near the same temps, just to store the information in here for any future need to check if the rail is holding fuel as it should. For example...if someone is thinking "are my injectors leaking? Is the FPRV leaking prematurely?"

I have new injectors and its 145 bar FPRV appears to be OK; as it holds pressure up to 155 bar after shutdown when heat raises the rail pressure.
 
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ROH ECHT

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2007 MKV GTI
Still hoping someone will add to this......(y)
 

vwengineer

Ready to race!
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Switzerland
I did not check right now, but I remember seeing it more or less equal to the LPFP pressure also with ignition ON, car not running vs 50 bars requested I think. Not sure mine is a good exemple though with its mileage, nevertheless pressure also holds and then rise when the car is shut down.
 

ROH ECHT

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2007 MKV GTI
Right...and thanks for participating. I had vaguely recalled seeing posts stating the rail pressure was low when cold. This is why I thought it might be helpful if people would post theirs when it is known all is good but also for people with starting issues to post their cold pre start-up pressures.

I expect it to drop significantly after it heats up and expands from 50 bar at idle to the FPRV limit(155 bar in my case) and then cools. I suspect it is good to see 7 bar when cold...it means it is holding 100 psi in the rail given all parts are new. A prematurely leaking FPRV is only visible when its pressure rises from heat expansion following shutdown and then cools...unless its ball valve is damaged/worn.

A leaking injector though...again; here I expect fuel to also leak at a given pressure rather than all of the time. Unless its tube is split or its needle is damaged/worn.

As I said...curiosity is behind this post. I was thinking some folks with starting issues could also share to perhaps determine if cold pressures, or lack of pressure, could be responsible.
 

ROH ECHT

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2007 MKV GTI
I just did a search and found a Ross-Tech forum post I made in 2018 asking about the rail pressure before start-up. No one responded, but it looks like I was trying to sort out if I possibly had a leaking injector. I said then my rail pressure was 6.4 bar before starting it...and I also posted someone told me theirs was 25 to 30 bar when cold. I imagine whoever told me this was checking in a warmer climate...perhaps it was you. This information I shared on Ross-Tech's forum may have come from a thread I made on Vortex.

This is the title I used in Ross-Tech...but not finding the Vortex with a search; "What should the rail pressure be? MK5 GTI 2.0t BPY". This must've been when I was having the issue with the fuel pressure specified at 110 bar with my GIAC H.O. K04 tune.
 

ROH ECHT

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2007 MKV GTI
Thanks GTI's for finding that. I see your posts and remember now you were seeing pressures higher than mine and that of another. And recall temps made the difference. I believe this is why I posted in my first post; here it was 65° on the MFD. I should always check coolant and IAT temps with VCDS as you did though. Yours was finally showing sub 10 bar at cooler temps. All of that I was going through back then was likely the result of my LPFP deficiency...which is finally sorted now. Much appreciated again.
 
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ROH ECHT

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Car(s)
2007 MKV GTI
GTI's? What is your experience with the ones people are reporting on having those seemingly common rough or stumbling starts? You may recall mine did it in the past, but it was something that only happened one time here or there out of the blue. I would turn the key and it immediately stumbled and wouldn't continue to run. It wasn't until I turned the key a second time that it would sound normal turning over and ignite as usual. Never found out why this happened from time to time.
 

vwengineer

Ready to race!
Location
Switzerland
It's that a problem with a faulty LPFP which drop pressure when not running? To diagnose this problem you would need to put a gauge in the fuel line, turn ignition ON, watch pressure, turn OFF, see if pressure drops or stays.
 

ROH ECHT

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Car(s)
2007 MKV GTI
Which problem? I am only trying to see rail pressures before starting...and hopefully from those experiencing rough starts. I cannot find where anyone who experiences rough cranking at start-up has looked at their rail pressure before turning the key. The LPFP yes, one would need to not have power going to the LPFP after it has primed in order to see if the filter regulator or LPFP is allowing fuel to return to the tank...if fuel isn't lost the other direction.
 

GTI's

Drag Race Newbie
Location
MD


Coming in through the passenger door and not opening the drivers door and looking at pressures & temp.
 

ROH ECHT

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Car(s)
2007 MKV GTI
Do you see it? The passenger door entry still shows the LPFP at 5.9%....does that mean it still primed?
 

GTI's

Drag Race Newbie
Location
MD
Idk but that %age did not really change from the other scans I posted on vortex and all those were also taken by coming in through the passenger door.
 

ROH ECHT

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Car(s)
2007 MKV GTI
I recall someone recently saying it would show a percentage unless the power to the LPFP controller was disconnected....say at the fuse IIRC.

Mine just now...through the driver door. Still, it buzzes when the key is turned so VCDS will connect.
Look at the voltage...and with a newish battery. Just drove it two days ago. Going to check for a draw soon. It takes two hours for it to go to sleep and a 0.04A/40mA or less draw is acceptable, correct?

prime.png
 
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