Seeking help for stumbling at start-up

[ROH ECHT]

OK, I know one cause for stumbling when starting the engine can be fuel 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...and do the injectors open to prime the cylinders with fuel, or not? Although I think not.

I did start it up and it again stumbled...and then I shut it down 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 at a rapid rate and it must be leaking very slowly and causing the stumbling start-up.

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 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.

 

[GTI's]

Going in car through pass. side

Opening the DS door and closing DS door

This car has sat for days

 

[ROH ECHT]

Thank you...my rail pressure at 7 bar seemed to suggest I am on to why it is stumbling upon start-up. I believe it is time for new injectors. I need to add in my first post; I seek data from those with knowingly good injectors...preferably near new, LOL. And, if anyone knows their injectors are old and they have the same symptoms...go ahead and post their findings as well.

 

[GTI's]

I see you did not check the pressure at +80°c coolant temp then shutdown engine and observe rail pressure. https://www.youtube.com/watch?v=k50Z5JLfuJs

 

[ROH ECHT]

No...I did not. However, I did run it for a few minutes, where I wrote "then I shut it down", after checking the rail pressure cold but I did not note the temp. I wish they explained more about what they were looking for other than the fuel rail pressure goes up when shut off. What does that mean if it does and doesn't? It leaves one to assume that if the pressure does increase...I suppose normal. And so, if it does not...the injectors or FPRV leaks.

 

[ROH ECHT]

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. The increase in pressure is due to heat on the fuel trapped in both the low and high sides. 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 or release. Perhaps the pressure in the rail would've been higher...just not sure.

 

[GTI's]

Appears normal this is my 10 min log today http://log.malonetuning.com/chart/2...DsEbDv0wWNU3DlFRRU0dNGUzDtwFZwqPDn8ONZwEAAA== started it up after the ten minutes and at idle back down to 50+/-.

 

[ROH ECHT]

Cool, thank you. I just may be surprised and find that the lpfp swap may be all that is wrong. Don't know how exactly it would make it stumble at initial cranking...but going to do that first and see. Still wonder why my rail pressure was just 7 bar and yours was 30 bar. Is it warm there when you checked it? My garage that morning was probably 60°F.

 

[GTI's]

The temperature was in the low to mid 70°F both times.


24hrs later at mid 70°F ambient temp

 

[ROH ECHT]

OK, cool. Someone in the UK replied...again, I am only asking for folks to post their rail pressure when the car has sat for near 24 hours...he replied the the rail pressure drops upon shut-off to what ever is in the low-side.

This is his quote on 29 SEP:

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.

My response was;

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?

.......no response as of today

 

[GTI's]

Been looking at the SSP for FSI fueling and I am not see how the rail pressure will drop down to low side pressure after shutdown and waiting tell it cools down. The Hpfp has an outlet valve that closes rail pressure from the pump chamber.

The rail pressure this evening at 75°F was at 27bar and low at 6.18bar.

 

[707Stang]

Been looking at the SSP for FSI fueling and I am not see how the rail pressure will drop down to low side pressure after shutdown and waiting tell it cools down. The Hpfp has an outlet valve that closes rail pressure from the pump chamber.



The rail pressure this evening at 75°F was at 27bar and low at 6.18bar.

If you have a seeping injector you will lose pressure OR if that check valve is leaking you will lose pressure. The system is similar in theory to a Common Rail diesel.
I wish I had my ATG Direct injection book handy


OP: do you have a bunch of smoke comin out of the back of the car when you start it, or more that usual?


Sent from my iPhone using Tapatalk

 

[ROH ECHT]

Been looking at the SSP for FSI fueling and I am not see how the rail pressure will drop down to low side pressure after shutdown and waiting tell it cools down. The Hpfp has an outlet valve that closes rail pressure from the pump chamber.

The rail pressure this evening at 75°F was at 27bar and low at 6.18bar.

Exactly...my understanding as well is it remains in the rail unless leaking from an injector or FPRV...and at 7 bar in less than 24 hours points to mine leaking. With seeing a puff of smoke at start up and the very brief stumbling, I feel I can rule out the FPRV.

If you have a seeping injector you will lose pressure OR if that check valve is leaking you will lose pressure.

OP: do you have a bunch of smoke comin out of the back of the car when you start it, or more that usual?

Not a bunch of smoke...and yes I do see a small puff, that doesn't last, immediately after starting it.

 

[vwengineer]

OK, cool. Someone in the UK replied...again, I am only asking for folks to post their rail pressure when the car has sat for near 24 hours...he replied the the rail pressure drops upon shut-off to what ever is in the low-side.

Hey!

Had a look today, car sat for about 36 hours (temperature is 10°C) so not sure if it's relevant to you:

230: spec: 49.9bar / actual: 5.8
231: 5 bar / 5.4 bar / load: 5.9%

It starts normally although not as quickly as when hot.

 

[GIACUser]

Couple other possibilities

1. Air in fuel line. VCDS is measuring pressure but it sees air or fuel pressure as one. (pin hole leak intank tubing)
2. Could you have some moisture/water in fuel.

pull each plug before starting next day to see if any of the cylinders have a little fuel in them.

Sorry just grasping for ideas. Probably won't help.