FSI hesitation under light acceleration

PlasticMac

Member
I have an idea of how the vapour/purge function works, but not how air is supposed to enter the tank, as fuel is used. The purge function takes vapour out, using manifold vacuum, controlled by the N80 solenoid. But, from @terrywindy1 has observed, it maybe that at low revs (low manifold vacuum), and a high j resistance path for air into the tank, and, maybe, an N80 that leaks (not closed fully), the fuel/air mixture is affected?
Mac.
 

PlasticMac

Member
Since emissions are so tightly controlled, (to the point where any vapour from the tank is recycled), the tank vent that allows air in, must also be controlled, probably by a one way valve (air in, no vapour out). So, that valve, probably only opens when the purge system creates a vacuum in the tank. It follows that the fuel tank cap is not (meant to be) vented.
Maybe @terrywindy1 could compare his new cap with my original?
I'm wondering if the "old" cap was leaking, - causing a problem) and the new one is not...
Mac.
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PlasticMac

Member
@terrywindy1 I suspect you're right. I was locked into thinking it was a vacuum lock thing, where the vent was blocked. Having realised how complex (isn't everything FSI?), the emissions system is, I understand that an unmonitored vent path (via the fuel cap) that shouldn't be there is going to cause problems.
Here's hoping the fix is permanent. Let us know in a few weeks. I think it is fixed.
Mac.
 

terrywindy1

Member
@terrywindy1 I suspect you're right. I was locked into thinking it was a vacuum lock thing, where the vent was blocked. Having realised how complex (isn't everything FSI?), the emissions system is, I understand that an unmonitored vent path (via the fuel cap) that shouldn't be there is going to cause problems.
Here's hoping the fix is permanent. Let us know in a few weeks. I think it is fixed.
Mac.
It has been about a month now, so probably about 300 miles now and no hesitation at all.
So good to get last small annoying problem sorted
 

Andrew

A2OC Donor
Good explanation of the vent purge circuit here :

Mac.
A helpful article, it pushes back the boundaries of ignorance. One point where our FSI differs is the statement

"The purge flow is monitored by a number of sensors. If the purge flow is less or more than is expected under the conditions, the computer illuminates the "Check Engine" light."

As far as I can see we have no flow sensors and assuming the N80 valve is fully functional any error in the flow rate will not result in an EML, (but may impact on engine performance). I can only think with the lack of flow rate sensor the ECU must assume some fixed quantity for flow, no doubt derived from a factory test rig, and use that as a minor contributer to the myriad of control calculations it uses to run the engine, oblivious that if the purge flow is different in case of a flow malfunction then the result of its calculations is slightly wrong.

Next need to think is it possible for a leaking petrol cap seal to impact on the purge flow.

Andy
 
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terrywindy1

Member
This part of massage from WOM I received when I asked if this was possible. "The fuel systems are sealed up until the engine where the evap valve controls the breathing. If extra air was getting into the system further back around the tank filler then it may cause issues".
 

PlasticMac

Member
Although the fuel system is "sealed" it must have path for vapour to be vented, and one for air to enter. In addition, there will be a safety pressure release valve. I'll ignore the filler for now, as it is normally sealed, except during filling.
The vapour vent path is via the canister and the air entry (I think) is via a non return valve, that only opens when the tank is at a negative pressure (vacuum).
The vapour vent is controlled with a solenoid valve, which, when opened by the ECU, purges the vapour (stored in the canister) back into the inlet path, utilising the vacuum in the inlet manifold. Since the volume of vapour that requires venting will be small (I think), the solenoid valve (N80) will not open very often, or for long. When it is open, and the purge process is active, a volume of fresh air, equivalent to the volume of vapour sucked into the inlet, will be drawn into the tank, via the non return valve. A vacuum will remain in the tank throughout, this is, (I think), important, because:

If the tank was vented to atmosphere (so not at a vacuum), the inlet would, effectively, have an air leak, (via the vent line/canister/N80
If there was a small leak to atmosphere via a worn filler cap, this would be most significant at low revs, where the volume of air via the leak would be high, compared with the volume of air flowing through the inlet, but not so at higher revs and higher inlet air flow.

This goes someway to an explanation of @terrywindy1 observations, but not all the way. So far, the effect of the fuel cap leak would only be felt when the N80 valve was open. So, (I think) the N80 valve is not closing completely.. It's not unusual for a solenoid valve not to seal when closed, after many years of operation, the seat will wear and deform.
My guess is that the hesitation at low revs, on an FSI could be caused by the combination of a leaky fuel cap (or, indeed anywhere is the vent line), and a leaky N80 valve, resulting in a small air leak into the inlet.
Replacing the cap solves the problem, because with the system now leak free, a vacuum is maintained in the tank, so no air leak.
Consider replacing the N80, your FSI is worth it.
PS: I've found similar problems symptoms on the Mk1 TT Forum, and replacing the fuel cap is a fix. As all modern cars have these vapour canister/purge functions, it's probably quite common, but not much recognised.

Mac.
 

Andrew

A2OC Donor
I assume 7 is the purge line.

image.png


Need to think to think about item 8, labelled as 'Gravity Valve'.

Andy
 

Andrew

A2OC Donor
Interesting...


Andy
 

PlasticMac

Member
I would say yes, in that they seem to be vent lines for the main tank and empty into the vertical expansion tank and then on into the purge system.

Andy
So, just the inlet that's still a mystery. Maybe gravity = one way or non return?
Edit: Or (anti) siphon perhaps?
Mac.
 
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Andrew

A2OC Donor
Still a bit of a mystery.

General googling of gravity valve in this context yields if a car rolls over in an accident gravity shuts the valve and prevents emptying of the tank.

Andy
 

PlasticMac

Member
Still a bit of a mystery.

General googling of gravity valve in this context yields if a car rolls over in an accident gravity shuts the valve and prevents emptying of the tank.

Andy
That, to me, is an anti siphon valve, only passes fuel when tank pressure is below (vacuum) ambient. So car upside down, (gravity) pressure of fuel in the tank is above ambient, so no flow.
Mac.
 

Robin_Cox

Member
Sorry for thread hijack, but can anyone comment on whether the AUA / 1.4i does the same thing as regards maintenance of the negative air pressure of the fuel system when running? We also get the sporadic hesitation in the mid-teens rpm, occasional spurious EPC error with sporadic open to earth fault of manifold barometric pressure sensor (all the warning lights including ABS coming on, resets when switched off and on again). Wondering if fuel cap replacement would resolve this final issue (I put a new one on when we first got the car as original had no strap but wondering if it is not doing what it is supposed to).
 

PlasticMac

Member
@Robin_Cox See Post 31, where @Andrew references similarities on a 1.4. Since the 1.4 is, like the 1.6, Euro 4, I'd think the emissions controls would be very much alike.
Check the filler cap, are the "O" rings nice and plump? Is it a snug fit in the filler neck? If you have any doubts, suggest you replace it.
Let us know how you get on.
Mac.
PS: Bit more on the 1.4 here :
Mac.
 
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steaman

Member
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.)


 

Andrew

A2OC Donor
Sorry for thread hijack, but can anyone comment on whether the AUA / 1.4i does the same thing as regards maintenance of the negative air pressure of the fuel system when running? We also get the sporadic hesitation in the mid-teens rpm, occasional spurious EPC error with sporadic open to earth fault of manifold barometric pressure sensor (all the warning lights including ABS coming on, resets when switched off and on again). Wondering if fuel cap replacement would resolve this final issue (I put a new one on when we first got the car as original had no strap but wondering if it is not doing what it is supposed to).
Hi,

I too would say the AUA tank purge system is most likely to be the same.

The tank is the same at the back and a quick glance at the workshop guide shows the charcoal filter and N80 solenoid valve to be present but different positions to the FSI. The filter is under the front O/S wheel arch and the N80 in the engine compartment on the left up near the top bend of the main air intake.

Andy
 
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