FSI hesitation under light acceleration

Andrew

A2OC Donor
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
 

PlasticMac

Member
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.
 

steaman

Member
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
 

A2Steve

Member
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.
 

Andrew

A2OC Donor
image.png


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

image.png


How does outside air enter the tank?

Andy
 
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