Seeking data for stumbling at start-up

[ROH ECHT]

OK, I know one cause for stumbling when starting the engine can be when fuel has leaked from the injectors and sits in the combustion chambers and this excess of fuel is what will cause stumbling at start-up. This is only happening when the car has sat for some time.

So, after giving it some thought, I decided to hook up the vcds and look at the rail pressure before starting it after it sat for 21 hours. The rail pressure was at 7.0 bar...and I am thinking that perhaps it should be as it was upon shut-down. But I am unaware of what the system does when you turn the ignition on...as you do to enable vcds...does the injectors open to prime the cylinders with fuel, or not?

I did start it up and it again stumbled...and then I shut it down after a bit and watched the rail pressure for 20 minutes and it remained at 51.2 bar. So, it is not leaking fuel out of the injectors and is opening when the key is turned to on(not start) or it is leaking...very slowly and causing the stumbled start-up. If neither is happening...what else may be causing it? (←semi-rhetorical)

So, my request is if anyone with knowingly good or near new injectors will look at their rail pressure "actual" after your car has sat for near 20 hours. Hook up vcds and pull up MB 106, 140, 141, or 230 and turn the key on(but not to start) and report what your rail pressure is before starting the engine when good and cold. Can anyone do this please?

If you know your injectors are old and you have the same symptoms...go ahead and post your findings as well. Thank you.

[f00glee]

See the following link for my chasing of a spluttery start issue, finally solved with inlet/injector clean at R-Tech.

http://www.mk5golfgti.co.uk/forum/index.php/topic,114226.0.html

May contain some useful information, some fuel pressure tests etc.

[ROH ECHT]

See the following link for my chasing of a spluttery start issue, finally solved with inlet/injector clean at R-Tech.

http://www.mk5golfgti.co.uk/forum/index.php/topic,114226.0.html

May contain some useful information, some fuel pressure tests etc.

Thank you. I know my LPFP needs changing...the low pressure only goes to 5.5 bar when it primes and I have seen where others see 6.5 bar when primed. Also, when I do a full gear pull the pressure drops to 3.7 bar. The stumbling start-up is new and my hopes are only to determine what the rail pressure looks like on other MK5 GTI's so I can help others in the future while being confident this is one test to suggest. If the rail pressure is low after sitting...it has to be either the injectors, the hpfp's outlet or inlet valve, or FPRV are leaking. Mine has a new fprv and hpfp.

[Dan_FR]

The low Pressure has a controller which varies the pressure depending on load/demand. Its not a case of 'it should always be at 5 bar' etc.

To answer your question, after the car has been switched off, the fuel pressure rises quite quickly initially due to heat soak of fuel etc... but after being stood for like 20 hours it dissipates to nothing more than the LPFP priming pressure which is around 5-6 bar. This still has to build to 50 bar before the car will start cleanly.

[pudding]

3.7 bar under load is pretty low.  Any fault codes or unusually high positive lambda adjustments?  I don't think I ever saw mine dip below 4.5 bar under hard load at Stage 2+ levels of tune.

Remember to pull the LPFP fuse the night before if you want to do a drain back test.  From cold, the engine fires from LPFP volume + wide open injectors.  The HPFP can't generate 50+ bar from a few seconds of cranking at 300ish rpm.  When hot, there's usually enough residual pressure in the HPFP rail for quick starting.

What are your plugs and coils like?  If you had a dribbling injector, you would know!  If you took the plugs out after leaving it for 20 hours, you would smell neat fuel in the offending cylinder.

[ROH ECHT]

The low Pressure has a controller which varies the pressure depending on load/demand. Its not a case of 'it should always be at 5 bar' etc.

To answer your question, after the car has been switched off, the fuel pressure rises quite quickly initially due to heat soak of fuel etc... but after being stood for like 20 hours it dissipates to nothing more than the LPFP priming pressure which is around 5-6 bar. This still has to build to 50 bar before the car will start cleanly.

Hmmm? Then why would another guy's MK5 be at 30 bar when his car has sat for three or four days? I learned before posting this topic that pressurized fuel is trapped in the rail. So, what is your rail pressure when it has sat over night?  Is it 5 to 6 bar as you stated? Have you checked or are you just saying?

[ROH ECHT]

3.7 bar under load is pretty low.  Any fault codes or unusually high positive lambda adjustments?  I don't think I ever saw mine dip below 4.5 bar under hard load at Stage 2+ levels of tune.

Remember to pull the LPFP fuse the night before if you want to do a drain back test.  From cold, the engine fires from LPFP volume + wide open injectors.  The HPFP can't generate 50+ bar from a few seconds of cranking at 300ish rpm.  When hot, there's usually enough residual pressure in the HPFP rail for quick starting.

What are your plugs and coils like?  If you had a dribbling injector, you would know!  If you took the plugs out after leaving it for 20 hours, you would smell neat fuel in the offending cylinder.

My lpfp is old and tired. When I did data pulls last year from 2k rpm to redline it was dropping to 3.5 bar just near redline. When I installed the PM3 last summer it went back to staying above 4 bar. Now it is dropping below 4 bar again. I've known the lpfp is gone and I have a TTRS ready to go in.

I haven't pulled my plugs but I may do so in the next few days. I run NGK PFR7B and coils are good. Car runs great and I have no codes at all. When I pull the plugs...I may dip a long swab down into each cylinder to check for fuel or I can bore-scope them, but after I look at the rail pressure again. Yeah, it is only stumbling when I start it after sitting overnight.

I will pull the fuse and check the low side pressure when cold...I thought the test I did today would be good enough but I will do as you suggested as I have doubt of leak back when it is checked when hot.

[ROH ECHT]

OK, so now...Today I hooked up VCDS after a drive. The test is supposed to check for leaks when the engine is at 80°C or above. Anyhow, after parking at home, I let it sit for exactly ten minutes and then I hooked up VCDS and turned the key to run. The temp was at 88°C, the high pressure was at 119.x bar, and the low pressure was at 6.5 bar. Then I started it and pressures dropped to 50 bar and 5.8 bar at idle.

This suggests the pressure is holding unless the heat has a greater effect on the pressure than the leak in an injector can affect pressure or release. Perhaps the pressure in the rail would've been higher...just not sure.

[pudding]

Both low and high rails seem to be holding pressure OK.   How long does it stumble for?  Mine occasionally has a momentary cough after catching, then smooths out immediately.  This usually happens when ambient temps fall.  During the summer it's always a super quick and smooth starter.

The starting routines are fixed.  X amount of injector duty vs water and air temp.  Lambda doesn't kick in until around 40 deg C water temp from what I remember.  Therefore the engine has no closed loop control for the first few minutes of driving.   When things are fixed like that, there can be occasional rough running if the moon and mars aren't in alignment, you know, kind of like the situation when your gf is ready for sex.

I think by a process of elimination, your LPFP could be suspect.  It's getting tired.  It may have pressure, but perhaps it's volumetric flow is reducing.  Everything deteriorates with age.

[ROH ECHT]

Both low and high rails seem to be holding pressure OK.   How long does it stumble for?  Mine occasionally has a momentary cough after catching, then smooths out immediately.  This usually happens when ambient temps fall.  During the summer it's always a super quick and smooth starter.

The starting routines are fixed.  X amount of injector duty vs water and air temp.  Lambda doesn't kick in until around 40 deg C water temp from what I remember.  Therefore the engine has no closed loop control for the first few minutes of driving.   When things are fixed like that, there can be occasional rough running if the moon and mars aren't in alignment, you know, kind of like the situation when your gf is ready for sex.

I think by a process of elimination, your LPFP could be suspect.  It's getting tired.  It may have pressure, but perhaps it's volumetric flow is reducing.  Everything deteriorates with age.

I agree with my lpfp being tired and suspect. The stumble has occurred in the past 12 years but when so...was just the once and totaling a handful of times over the years. Now it is every time I start it when it has sat overnight.

That other guy posted screen shots of his pressures...and when only primed, his low side pressure was 6.5 bar and mine is 5.5 bar. My plan is to install my lpfp upgrade first...see what happens then.

It only stumbles briefly when initially cranking...sounds like just a few revolutions for no more than two maybe three seconds tops. It doesn't just start up quickly as it normally did...and so, it just takes a bit longer to get running due to the brief stumble. Once it has caught on and is running...it is all normal. This topic is really just to find out what others see for rail pressure when it is checked...so to show if the pressure trapped in the rail at the previous shut-down remains and at what level. I now the other guy on the other forum, to be knowledgeable in diagnosis and mk5 building, is saying 30 bar after sitting 3 or 4 days. And that perhaps the difference between my 7 bar and his 30 bar is due to mine sits in my garage in Oregon and his may be parked outside somewhere warmer. I just do not know. But was hoping others would participate in posting what their rail pressure is after cooled and when the engine is at ambient temps with the knowledge of the condition of their injectors. Can they have a slow leak and bleed off pressure even though they show what mine did in the heat soak test? We'll hopefully see.