FSI hesitation under light acceleration

Found this:

All vehicles sold in the United States (since at least the 1980s, probably the 1970s or earlier) are required to have a fuel evaporative control system (called an EVAP system in automotive jargon) which collects expanding fuel vapor from the fuel tank in a charcoal-lined canister while the engine is stopped and then releases the collected vapors (through a "purge valve") into the engine intake for burning when the engine is running (usually only after it has reached normal operating temperature.) The fuel evaporative control system is also required to include a gasketed filling cap which seals the fueling inlet to prevent vapors from escaping directly from the tank through it. Modern vehicles with OBD-II emissions control systems will turn on the MIL (Malfunction Indicator Light, a.k.a. "check engine" light) if it is detected that the gas cap is missing or loose and so not sealing. (The general purpose of this light is to indicate when any of the emissions controls are not working properly.)
Very much the same as I have read in my general reading but good for the record you have posted it, thanks.

I am still at odds with the last but one sentence, "will turn on the MIL (Malfunction Indicator Light, a.k.a. "check engine" light) if it is detected that the gas cap is missing or loose and so not sealing." Surely that implies the existence of a sensor to detect low purge pressure/flow rate which we do not appear to have and as such our ECUs will be unaware something wrong.

Andy
 
I agree with @Andrew.
The only way a filler cap leak would, possibly, be detected is if an inlet air leak was detected at, and only at, the same time as the N80 valve was open. Such a combination of inlet air leak and N80 open could only mean a leak in the purge circuit, most likely to be the filler cap.
Mac.
 
Just tried little experiment,I loosened petrol cap just over half a turn.when I started engine revs jumped between 700rpm and 900rpm for maybe a second. I drove up the lane for a mile and the hesitation is back,stopped and tightened fuel cap hesitation gone
 
Very much the same as I have read in my general reading but good for the record you have posted it, thanks.

I am still at odds with the last but one sentence, "will turn on the MIL (Malfunction Indicator Light, a.k.a. "check engine" light) if it is detected that the gas cap is missing or loose and so not sealing." Surely that implies the existence of a sensor to detect low purge pressure/flow rate which we do not appear to have and as such our ECUs will be unaware something wrong.

Andy

On our Mercedes the engine malfunction symbol was lit and the workshop asked me to check that the fuel cap was properly tightened
 
Just tried little experiment,I loosened petrol cap just over half a turn.when I started engine revs jumped between 700rpm and 900rpm for maybe a second. I drove up the lane for a mile and the hesitation is back,stopped and tightened fuel cap hesitation gone

Well that's a new one for FSI owners to check and porbably one of the cheapest.
 
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Still thinking about the two vent pipes 11 and 13. It struck me why two? The answer I suspect is connected with that strange looking bundle of three "pipes" that comprise the vertical rise to the filler. Pipe 13 I think is part of the purge circuit and maybe pipe 11 is to do with the introduction of outside air into the tank as petrol is used. This would mean outside air enters the right side pipe at the filler neck but how I am not clear. It all depends on the arrangement at the filler neck and the diagram lacks detail here.

The best tank picture I could find but poorly lit and not sharp...

image.jpeg


A bit more detail from the workshop guide

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How does outside air enter the tank?

Andy
 
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Well that's a new one for FSI owners to check and porbably one of the cheapest.
Just tried little experiment,I loosened petrol cap just over half a turn.when I started engine revs jumped between 700rpm and 900rpm for maybe a second. I drove up the lane for a mile and the hesitation is back,stopped and tightened fuel cap hesitation gone
Hi all, very interested in reading this thread as will give this a go next if replacing the incorrectly fitted timing belt doesn’t resolve the rough idle and hesitation. Should replacing the fuel cap be combined with replacing the N80 purge valve? And if so, can anybody confirm if this is a competent DIYer job with easy access to the valve or do other engine parts have to be removed to get there? The car is in the garage tomorrow for the timing belt and so just wondering whether it would make sense to swap the N80 whilst that job is underway if it’s not a DIYer job. Thanks
 
no thats on the high pressure side, just a little dribble when you change the pipes over to the new n80
 
no thats on the high pressure side, just a little dribble when you change the pipes over to the new n80
Excellent - will try that once I see how the hesitation is after the timing belt is sorted today. Did you notice a difference after swapping out the N80?
 
not really mine was a combination of things, at the price i did it to eliminate it. its a 20 year old solenoid at the end of the day
 
Sorry, only just spotted this one.
I think most failure modes of the N80 would trigger an error, most likely to be P0444 or P0458. But, if it was leaking, due to worn seat, probably not.
I think I'd go with @steve54 and replace it anyway.
Mac.
 
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