Lets not get catch tanks and carbon canisters mixed up here guys.
Anyway - I found the Bosch reference material on the subject - its considerably more interesting than you might first think !
Firstly - a diagram shows that the carbon canister has three connections. One at top where fuel vapours arrive from the tank breather. Secondly for a connection to the 'EVAP' or 'purge' valve that allows the engine to ingest metered doses of fumes. Lastly - and importantly - a vent to atmosphere from the bottom of the carbon can... This to allow air to be sucked up with the fuel vapours such that they are purged into the engine - this process also regenerates the activated charcoal filter within the 'carbon canister'.
Additionally it describes the primary purpose to prevent unwanted HC emissions from the fuel tank and the secondary one as preventing build up of pressure in the fuel tank.
There is a 1-way valve shown on the EVAP valve which ensures fumes can only flow from tank to manifold - and not the other way - a bad day if you have nasty misfire !
All this EVAP nonsense gives the closed loop lambda adaptation a bit of a headache as it gets affected in varying degrees by this essentially unmetered flow (by the MAF) of air and fuel vapour. It explains why the duty cycle on the EVAP is low at low engine speeds and higher at high engine speeds - intake air mass to be more accurate.
It also means my theory of the carbon can being blocked as the root cause of excessive vacuum on the fuel tank has some merit. Cos if that vent on the carbon can is blocked, then the EVAP system can only ever suck fumes from the tank and never gives it a chance to breath properly.
There is also the possibility that some of these scenarios of excessive vacuum in the fuel tank are caused by a failure in the purge valve itself and that the ECU has no control of the amount of regenerative flow - and therefore vacuum on the tank. This kind of mechanical failure on the purge valve will be undetected as long as the ECU 'sees' a good electrical connection to the purge solenoid. Or it could be a mistake in an EVAP map on an upgraded ECU that poorly controls the purge valve.
Notably - when active lambda is turned off (WOT for example) only small amounts of EVAP purge are permissible as there would be no way to compensate for mixture deviations... Thats a generic thing, but at WOT on our cars we have boost in the IM and so there is nothing being purged by the EVAP anyway.
I for one am more of the mind now to redirect my own tank breather to atmosphere and dial out all the EVAP nonsense. I dont think the extra HC emissions from my car will harm mother earth - and by giving my ECU all that extra work to do by continually adjusting mixture for it then I reckon I'll easily be carbon neutral on this one pretty quickly.
Anyway - I found the Bosch reference material on the subject - its considerably more interesting than you might first think !
Firstly - a diagram shows that the carbon canister has three connections. One at top where fuel vapours arrive from the tank breather. Secondly for a connection to the 'EVAP' or 'purge' valve that allows the engine to ingest metered doses of fumes. Lastly - and importantly - a vent to atmosphere from the bottom of the carbon can... This to allow air to be sucked up with the fuel vapours such that they are purged into the engine - this process also regenerates the activated charcoal filter within the 'carbon canister'.
Additionally it describes the primary purpose to prevent unwanted HC emissions from the fuel tank and the secondary one as preventing build up of pressure in the fuel tank.
There is a 1-way valve shown on the EVAP valve which ensures fumes can only flow from tank to manifold - and not the other way - a bad day if you have nasty misfire !
All this EVAP nonsense gives the closed loop lambda adaptation a bit of a headache as it gets affected in varying degrees by this essentially unmetered flow (by the MAF) of air and fuel vapour. It explains why the duty cycle on the EVAP is low at low engine speeds and higher at high engine speeds - intake air mass to be more accurate.
It also means my theory of the carbon can being blocked as the root cause of excessive vacuum on the fuel tank has some merit. Cos if that vent on the carbon can is blocked, then the EVAP system can only ever suck fumes from the tank and never gives it a chance to breath properly.
There is also the possibility that some of these scenarios of excessive vacuum in the fuel tank are caused by a failure in the purge valve itself and that the ECU has no control of the amount of regenerative flow - and therefore vacuum on the tank. This kind of mechanical failure on the purge valve will be undetected as long as the ECU 'sees' a good electrical connection to the purge solenoid. Or it could be a mistake in an EVAP map on an upgraded ECU that poorly controls the purge valve.
Notably - when active lambda is turned off (WOT for example) only small amounts of EVAP purge are permissible as there would be no way to compensate for mixture deviations... Thats a generic thing, but at WOT on our cars we have boost in the IM and so there is nothing being purged by the EVAP anyway.
I for one am more of the mind now to redirect my own tank breather to atmosphere and dial out all the EVAP nonsense. I dont think the extra HC emissions from my car will harm mother earth - and by giving my ECU all that extra work to do by continually adjusting mixture for it then I reckon I'll easily be carbon neutral on this one pretty quickly.
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