FSI hesitation under light acceleration

terrywindy1

Member
If anyone has an FSI with a slight hesitation when accelerating from lower RPM I may have found the problem by accident. I purchased my FSI about 5 years ago and as normal I have had a lot of frustration with it culminating in have a replacement engine at WOM,but even after that I still had this annoying hesitation. I thought it must the high pressure fuel pump as that was the same unit on both engines.Until about a month ago when I was filling with Tesco Momentum and the wind blew my fuel cap under the car because the rubber tether strap had broken about a month before. So I got straight on to Amazon and ordered a new fuel cap with tether for £7 It arrived next day. But when drove to work that evening the hesitation had gone completely and has not returned.Has anyone else found this cure?May work on other engine's?
 
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dj_efk

A2OC Donor
If anyone has an FSI with a slight hesitation when accelerating from lower RPM I may have found the problem by accident. I purchased my FSI about 5 years ago and as normal I have had a lot of frustration with it culminating in have a replacement engine at WOM,but even after that I still had this annoying hesitation. I thought it must the high pressure fuel pump as that was the same unit on both engines.Until about a month ago when I was filling with Tesco Momentum and the wind blew my fuel cap under the car because the rubber tether strap had broken about a month before. So I got straight on to Amazon and ordered a new fuel cap with tether for £7 It arrived next day. But when drove to work that evening the hesitation had gone completely and has not returned.Has anyone else found this cure?May work on other engine's?
Surely just a coincidence Terry? How could the fuel cap impact fuel delivery?
 

drewroberts

Member
If I remove the oil filler cap on an FSI when the engine is running it seems to want to stall itself and runs very lumpy can anyone explain that?
 

Clackers

Member
Surely just a coincidence Terry? How could the fuel cap impact fuel delivery?
I know I am going back many many years with this next comment, but I used to own Minis when I was younger and if the tank wasn't allowed to vent it would cause fuel starvation, you would drill a tiny hole through the fuel cap and this would allow the tank to vent. Maybe it is something along those lines? Especially if it is after a bit of a run in the car....but, my thought is the same as yours, possibly pure coincidence.
 

kp 115

Member
If I remove the oil filler cap on an FSI when the engine is running it seems to want to stall itself and runs very lumpy can anyone explain that?
I think there’s a carbon filter on the vent line to prevent fuel vapour release to the atmosphere, if the cap is off the pressures would be off I’d expect causing issues.
Hth
Keith
 

kp 115

Member
If anyone has an FSI with a slight hesitation when accelerating from lower RPM I may have found the problem by accident. I purchased my FSI about 5 years ago and as normal I have had a lot of frustration with it culminating in have a replacement engine at WOM,but even after that I still had this annoying hesitation. I thought it must the high pressure fuel pump as that was the same unit on both engines.Until about a month ago when I was filling with Tesco Momentum and the wind blew my fuel cap under the car because the rubber tether strap had broken about a month before. So I got straight on to Amazon and ordered a new fuel cap with tether for £7 It arrived next day. But when drove to work that evening the hesitation had gone completely and has not returned.Has anyone else found this cure?May work on other engine's?
Hi Terry,
Was the first cap the correct type ie petrol & not a diesel cap as it sounds like there may be a one way valve fitted in the cap to allow air in.
Maybe swap back to old cap & see if problem comes back.
Hth
Keith.
 

a2sumo

A2OC Donor
If I remove the oil filler cap on an FSI when the engine is running it seems to want to stall itself and runs very lumpy can anyone explain that?
It’s because the engine is designed as positive crank case pressure. Often the fsi can suffer bad running from the oil cap being loose (prongs on back bent) or the plastic housing being loose from someone ham fisted removing it when changing a coil pack on number 1 cylinder. Also runs bad if you loosen or remove the dip stick.
cheers Mike
 

PlasticMac

Member
Maybe the tank vent filter is blocked, and so not venting? New cap, with vent perhaps, is bypassing the filter?
Is the carbon filter easily accessible?
Mac.
 

Andrew

A2OC Donor
Maybe the tank vent filter is blocked, and so not venting? New cap, with vent perhaps, is bypassing the filter?
Is the carbon filter easily accessible?
Mac.
Yes. I have said before it is a pity it is translucent and looks an eyesore, looks like it is crying out to be changed but being charcoal it is natural black. A lifetime part but note the comments in the current lifetime gearbox oil thread.

image.png


image.jpeg


How the charcoal filter sits in the fuel deiverry system.

image.png


I have yet to tie in the charcoal filter with this diagram but looks like 4 is its feed.

image.png

Next need to formulate a mad theory as to why a leaking seal on a faulty fuel cap accounts for the hesitation and a new cap cures it.

Finally if you Google something like 'fuel cap and engine faults' lots of US hits about causing EML!

Andy
 
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PlasticMac

Member
As the charcoal filter is higher than the tank, it is quite possible for liquid (fuel or condensation) to lay at the lowest point, and so block the vent.
So, the vented cap theory has validity.
Maybe disconnect both ends and apply air pressure?
Don't advise "suck it and see though...
Mac.
 

terrywindy1

Member
Yes. I have said before it is a pity it is translucent and looks an eyesore, looks like it is crying out to be changed but being charcoal it is natural black. A lifetime part but note the comments in the current lifetime gearbox oil thread.

View attachment 77162

View attachment 77163

How the charcoal filter sits in the fuel deiverry system.

View attachment 77164

I have yet to tie in the charcoal filter return to the tank with this diagram.

View attachment 77165
Next need to formulate a mad theory as to why a leaking seal on a faulty fuel cap accounts for the hesitation and a new cap cures it.

Finally if you Google something like 'fuel cap and engine faults' lots of US hits about causing EML!

Andy
After about 400 yards of driving and released the hesitation had gone and shouting WTF with stupid smile.When got to work I did the same and googled same as you and came up with the American site's saying that a faulty "Gas"cap can cause an EML fault.
Unfortunately I didn't keep the old petrol cap it went straight into the recycling
 

PlasticMac

Member
Can you get at the tank end of the tube to the cannister? If so, give it a blast, even better if you can diss the canister end too, and then you're just clearing the tube. You can check the cannister too...
This is looking very promising...
Mac.
 

Andrew

A2OC Donor
Most interesting....


Andy
 

Andrew

A2OC Donor
Great sleuthing @Andrew Anyone know where this N180 solenoid is, and how it's controlled?
Mac.
I do not know the location of N80, (note N80 not N180 - the poster corrected themselves later in their post), but the fuel system schematic has it "near" the filter but means little but the diagram shows connected to and controlled by the ECU.
I will see if I can find anything later.

Andy
 

PlasticMac

Member
I do not know the location of N80, (note N80 not N180 - the poster corrected themselves later in their post), but the fuel system schematic has it "near" the filter but means little but the diagram shows connected to and controlled by the ECU.
I will see if I can find anything later.

Andy
More in this thread:
The OP feels that having "miss handled" it, while working, a little abuse seems to have sorted it out. In his case, it did come up as an error on VCDS, which cleared post abuse! I suppose a sticky solenoid after 15+ years, is nothing to complain about, once you know it's there!
Mac.
 
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