All Things Mk5 > Mk5 General Area
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:
--- Quote from: f00glee on September 27, 2018, 09:56:56 am ---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.
--- End quote ---
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.
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