# 2.0t A4 Quattro dies after 20 minutes - fuel pressure falls to 5 bar and won't restart



## DarinDux (Jan 30, 2018)

*2.0t A4 Quattro dies after 20 minutes - fuel pressure falls to 5 bar and won't restart - SOLVED*

08 Audi A4 (B7) 2.0t

Low Fuel Pressure: Fuel Pressure Outside Specification
CEL on
Car runs well with intermittent minor hesitation, then will die and not restart after 20 or so minutes of driving.
Car will restart after 2 hours of resting, then run and drive OK until it dies again. 100% repeatable.
Car will restart IMMEDIATELY if fuel return line is opened at engine and fuel/pressure is bled off. How much pressure should be in this line?

Scanner readings:
Low Pressure Fuel Pump runs at 50% at idle, up to 60% at full load.
High Pressure Fuel Pump rail pressure runs at 60 bar at idle up to 120 bar at full load. 
Fuel rail pressure drops to 3~6 bar when car dies.

Have replaced fuel filter, low pressure fuel pump, fuel pump control module, high pressure fuel pump, fuel pump cam follower. 

Key seems to be relieving the fuel return line pressure - car will immediately restart and run and drive for between 5 and 20 minutes after this line is bled off.

I cannot locate a fuel system diagram anywhere to help determine what might be happening.

Thank you for any assistance!


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## DarinDux (Jan 30, 2018)

*Update: LPFP duty cycle rises to 90%*

I took a higher-frequency reading and see the LPFP does go to 90% at WOT. Still stays at 50% at idle. 

Car will just suddenly fall off and die and not restart. Multiple key-on cycles runs the LPFP but the rail pressure never gets above 5 bar. Switching out the LPFP tonight again.


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## Thy_Harrowing (Dec 7, 2014)

Curious symptoms, has an interesting twist on the usual fueling issues I'm used to seeing. Off the top of my head you've pretty much replaced and checked damn near everything known to be problematic except for the fuel pressure regulator valve. There are some threads I recall commenting on awhile back with fueling issues that turned out being due to this part although they symptoms weren't entirely the same. Similar though.


You've checked the DC on the LPFP (which seems fine) but what about the fuel pressure it's supplying. Should be around 5-6 bar max. I'd assume the LP side is fine at this point though considering you replaced the whole pump/filter and control module.


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## quietA3guy (Oct 12, 2007)

DarinDux said:


> I cannot locate a fuel system diagram anywhere to help determine what might be happening.


Search "golfmkv Course Number 821503" for a pdf self study guide on the fsi 2.0t.

I agree with above that the fprv (fuel pressure RELIEF valve) is the most likely cause. It gets stuck in the full open position and won't close until pressure is relieved.

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## quietA3guy (Oct 12, 2007)

http://forums.vwvortex.com/showthread.php?t=7113461
DIY

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## Thy_Harrowing (Dec 7, 2014)

^ yeah what he said. "relief" NOT "regulator". Got the names mixed up. Regulator valve is the one on the HPFP. Relief valve is the one we're talking about here. And that DIY is definitely the best for doing it without removing manifold but it is still a huge PITA so just be ready for that...


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## quietA3guy (Oct 12, 2007)

quietA3guy said:


> Search "golfmkv Course Number 821503" for a pdf self study guide on the fsi 2.0t.


LOL. I just reviewed that pdf and discovered that they used diagrams of the non turbocharged fsi 2.0 for the fueling diagrams.

Search "ssp_337" for a more accurate diagram.

And they technically refer to the valve as a pressure limiting valve.

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## ROH ECHT (Dec 9, 2010)

Here you go; http://www.volkspage.net/technik/ssp/ssp/SSP_334_d1.pdf

Did you/they do the adaptation(steps pasted below) when you replaced the LPFP's control unit? Any time you replace the control module or unplug it...do the adaptation right down to the very last step = "switch the ignition off" for at least 60 seconds.
Is your engine a B(belt)xx? 
C(chain)xxx that are front chain driven do not have a Group 103 - Basic Setting.

* Fuel Pump*

Prerequisites:

Ignition ON
Engine ON
System voltage at least 11.0 V.

[Select]
[01 - Engine]
[Basic Settings - 04]
Group 103
[Go!]
Activate the Basic Setting.
[ON/OFF/Next]
Wait until Field 4 shows "ADP. O.K.".
[Done, Go Back]
[Close Controller, Go Back - 06]

Switch Ignition OFF.


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## DarinDux (Jan 30, 2018)

*Module Adaptation*

No, I forgot to do the adaptation for the replacement module. I will do that today. Although this issue existed both before and after the module was replaced, the adaptation can't hurt.

Thanks for the references on the tech specs for the fuel system. Very helpful, although I'm still trying to figure out where the check valve is that keeps pressure in the return line after the engine is shut off - perhaps the restrictor shown just after the fuel filter is actually a check valve, and exists in the outlet of the filter.

The possibility exists that the Pressure Limiting Valve sticks open on some occasions, and is somehow held open by the return line pressure. I don't know how this could physically happen due to the nature of how the valve works, but for the sake of argument let's say it does. That would explain why when I crack the return line to relieve pressure the car immediately starts and runs fine. Also, this would explain why when I don't relieve the return line pressure and cycle the key on and off the low pressure fuel pump in the tank does not generate more than maybe 4 bar in the fuel rail - which is not enough to start the engine.

It also would explain why everything runs great with up to 120 bar fuel rail pressure, until the rail pressure drops to 5 or less bar and the car falls on its face and dies, then won't restart. Just no way to build pressure in the rail until the return is cracked allowing pressure to bleed off and the PLV to close again. 

I'm replacing the PLV tonight, although I only have a 116 bar valve from an '06 to try, in place of the stock 126 bar on the '08. At least will allow me to evaluate almost every condition except WOT.

After that, I'm maybe going to swap Motronic modules with a known good one.


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## ROH ECHT (Dec 9, 2010)

DarinDux said:


> No, I forgot to do the adaptation for the replacement module. I will do that today. Although this issue existed both before and after the module was replaced, the adaptation can't hurt.


Right, the issue may or not have been with the control module, but needs to be done now that you have a new one.



DarinDux said:


> Thanks for the references on the tech specs for the fuel system. Very helpful, although I'm still trying to figure out where the check valve is that keeps pressure in the return line after the engine is shut off - perhaps the restrictor shown just after the fuel filter is actually a check valve, and exists in the outlet of the filter.


The restrictor is basically a diverter/rerouter. Any fuel released by the FPRV or pressure limiting valve will go back into the hpfp feed line rather than heading back toward the lpfp...path of least resistance. The lpfp or electric pump always maintains low pressure as soon as you open the door. The fuel likely remains in the lines as there is no chance of draining without a vent up above...it's "the fluid in a drinking straw with your thumb plugging the top opening" theory.



DarinDux said:


> The possibility exists that the Pressure Limiting Valve sticks open on some occasions, and is somehow held open by the return line pressure. I don't know how this could physically happen due to the nature of how the valve works, but for the sake of argument let's say it does. That would explain why when I crack the return line to relieve pressure the car immediately starts and runs fine. Also, this would explain why when I don't relieve the return line pressure and cycle the key on and off the low pressure fuel pump in the tank does not generate more than maybe 4 bar in the fuel rail - which is not enough to start the engine.


The way it sticks open is a a damage spring inside the valve. High pressure opens the valve but the spring rate keeps it closed when there is not excessive pressure in the high pressure lines. The pressure limiting(fprv) valve spring can break or get stuck allowing the ball seal to stay open regardless the pressure inside the high system...or the spring rate may weaken allowing the fprv to open at lower pressures in the high system.



DarinDux said:


> It also would explain why everything runs great with up to 120 bar fuel rail pressure, until the rail pressure drops to 5 or less bar and the car falls on its face and dies, then won't restart. Just no way to build pressure in the rail until the return is cracked allowing pressure to bleed off and the PLV to close again.


If your high pressure is dropping to 5 bar...you may very well have a damaged fprv. This matches my description above.



DarinDux said:


> I'm replacing the PLV tonight, although I only have a 116 bar valve from an '06 to try, in place of the stock 126 bar on the '08. At least will allow me to evaluate almost every condition except WOT.


Hope you see good results when done.



DarinDux said:


> After that, I'm maybe going to swap Motronic modules with a known good one.


Not familiar with that one, but will look into what you speak of later. 

Good luck with it all.


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## DarinDux (Jan 30, 2018)

Set about last night pulling the pressure limiting valve from my 2006 2.0t and installing into the '08. Took some time but wasn't too terrible - except for working in the dark at 10°F. The part numbers on these were identical: 06d 130 757c. Got everything put back together and did some runs. There is definitely a difference in the limited pressure - the '06 only allowed 110 bar, while the '08 allowed 120 bar. Other than that, the performance of the car was (unfortunately) identical. Still stalled out after maybe 15 minutes of driving. Wouldn't restart until I cracked the fuel return line.

I have additional issues that may also be indicators of the problem: 
1. the rpms hunt 50-75 rpm at any given cruising speed, especially noticeable between 1500 and 3000 rpm. I see this on the needle and feel it in my butt dyno.
2. (and perhaps most interesting) when the engine dies my VCDS stops reading for a short time. I hadn't noticed this before because I was usually trying to find a good place to pull over on the road but last night i was looking right at it when the engine died and the readings froze and went away. So am I intermittently loosing power to the engine ECU (Motronic unit)? The VCDS readings come back during restart attempts to give me pressures and rpms, but the car won't restart.

Now, that doesn't explain why the car won't restart until I bleed off the return fuel line pressure, but perhaps there's a limp mode setting or some other thing that won't reset an ECU setting until this goes away? I did read in one forum where someone had some similar seemingly fuel delivery issues that turned out to be a bad battery, manifesting itself as low fuel pressure errors. So the fuel pressure faults that I'm getting may be effects instead of the causes.

Or the action of getting out of the car and/or opening the hood simply resets something that allows the car to restart. I have to keep testing as it got late last night, but so far the only consistent solution is cracking the fuel return line. 

Eventually the problem may sort itself as I crack the fuel return and pour gas onto the hot engine. :banghead:


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## ROH ECHT (Dec 9, 2010)

There's a secondary coolant temp sensor that is for reporting to the ecu for fuel pressure adjustments...look into what it does when failing. I do not have time right now, but I will do research on it when I can. Obviously the fprv is working, so then "why is the high pressure dropping to 5 bar?" 

Did you do all of the adaptations?


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## DarinDux (Jan 30, 2018)

Yup. Did all the adaptations and they all completed OK.

So I took the car out and carefully logged the pressure in the high pressure rail. When the car would stall the pressure dropped to 3.5 bar and nothing I did would raise it. This is obviously not enough for the engine to run. 

Then I watched the rail pressure as I bled off the return line, and it rose proportionally to about 6 bar as the low pressure was completely bled. So that told me that the internal regulator in the HPFP was being piloted to isolate any pressure allowed to the rail. 

I unplugged the regulator on the HPFP and the car ran fine, albeit at only 7 bar in the fuel rail. So it didn't have any real power but ran just fine, with the ECM simply not allowing any boost. Drove for an hour with no blips at all.

I replaced the HPFP with yet another one, and voila the stalling disappeared. So was it possible that I had a bad regulator in the original pump and in the first replacement? Looks like it. Or the first pump was exhibiting another failure but I replaced it without really diagnosing the nature of that failure. 

Looking at the internals of the HPFP the piloting action and the nature of the regulator solenoid armature can possibly create a pressure lock that keeps even the low pressure pump from getting pressure to the rail. The only solution is to relieve the pressure acting on the pilot to allow opening of the internals.

I've been driving it today and the modules seem to have learned each other well again as the operation has smoothed out (mostly). The interesting thing is that the boost stops at 10 psi with the 110 bar '06 PLV, and with the 120 bar PLV I was sure I was able to get 11-12 psi. I'll have to re-install the original PLV.

Also, when the engine is cold (temps in the teens here) I still get a 50 rpm hunt when the throttle is around a constant 10%. I watch the throttle position sensor hunt between 9.5 and 11.5 %. Did the throttle adaptation a couple of times, but didn't help. Weird thing is my 05 Passat V6 4motion did the exact same thing. Thoughts?


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## ROH ECHT (Dec 9, 2010)

I have been thinking about this and am now leaning toward the pressure regulator of the hpfp. Not sure if can be tested but I remember someone posted what the resistance should be...only, unsure as to which posts are to be tested. This means of course replacing the hpfp if the regulator is failed. 
.....$#[email protected] just typing all of that ↑ and looked again at your post for more reference, when I saw you just replaced the hpfp.:facepalm:

I think you have more than one issue...always consider there are more than one, so you do not get stumped. Yes, try the other PLV and then we address the other symptoms.


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## DarinDux (Jan 30, 2018)

"Also, when the engine is cold (temps in the teens here) I still get a 50 rpm hunt when the throttle is around a constant 10%. I watch the throttle position sensor hunt between 9.5 and 11.5 %. Did the throttle adaptation a couple of times, but didn't help. Weird thing is my 05 Passat V6 4motion did the exact same thing. Thoughts?"

It's the Torque Converter. I monitored the Trans module MB 7, and when the TC is 'closed' (lockout) - line 3 - the slippage when cold alternates between 0 and 160 RPM - item 4 - in time with the idle surging. As the trans warms up the surging lowers to 120 rpm or less (within VAG spec) but there is still noticeable varying in engine speed.

So I'm going to do a trans fluid and filter change this weekend to see if I can make it go away. Most likely this is the original fluid at 126K. I do not want to do a torque converter!


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