We are discussion secondary evap systems here, a series of device(s) which capture petrol fuel vapours of which when released to atmosphere created ozone brown clouding in urban conurbations.
Upon start up the engine manifold vacuum system is connected by a purge valve to the tank to burn off these vapours and create a partial vacuum in the tank up to about 2kp of negative pressure. This pressure is monitored by the system and if there is a leak to atmosphere it will throw a code. Consequently keeping the fuel cap tight through the cap ratchet system makes sure you don't induce a code and CEL on the dashboard.
Yes as Grover as taken the time to point out, there is a carbon filter canister located on the car, (and on any modern petrol driven car) that absorbs these vapours since it is filled with a carbon stack filter.
Should the carbon filter become saturated with raw fuel it will affect how the car runs and be pollutive. ( Such as high HC tail pipe outputs which will affect air fuel ratios and 02 sensors going haywire & emission compliance failures )
The system is monitored by the PCM through the I/M 240 readiness monitors and it will show closed , open or inconclusive, should a fault or potential fault (s) be present. (like leaving the cap loose! )
As a comment these systems are designed NOT to work above 6000 ft since they can affect driveability. Consequently it is not unusual to have a car throw a CEL if you have gone to higher altitudes above 6000 and the PCM is doing a re-learn.
For that reason most places where an e test is performed and the I/M 240 monitors are part of the E test you are allowed to have one monitor open which is usually "surprise surprise" the Sec/Evaps system! There are some places that require a closed monitor , California and parts of Arizona off the top of my head require all I/M 240 monitors to be closed at test time.
The biggest cause of saturated carbon filters by the way are are drivers/users who top off the tank by brimming it. If you are a Brimmer with a "Bimmer" (BMW aka Bavarian Money Waster )
refrain from doing that ! You can end up with an expensive emissions related repair .
With a
slight negative pressure normally existing in the tank especially after a long run & you are about to fuel up, a
slight hiss of air entering the tank as you open the cap is normal especially on hot days.
Gasping or gulping expulsion of fuel especially during filling is a result of the check valve sticking in the system, you have pipework valve issues as mentioned.
The MB system is fairly standard across the models, like Toyota & other Asian makes . For me Ford is super complicated where it doesn't need to be and Chrysler is-----Well something else! With three systems and you don't know what you are getting until you get under the car! !!!!
By the way if any of you have a Prius in the family, the tank is equipped with a bladder that closes the space inside the tank to prevent a large amount of empty tank space building up as you use the fuel approaching to "empty" .Of course the space is where the larger volume of negative pressure exists on more normal cars and the potential of pollutant expulsion is greater. Hence no "hiss" on a "Prissss"--Ooowh poet and didn't know it!
If you have problems with not getting enough fuel in the tank on a Prius then you most likely have a bladder problem and that can be a real expensive pi$$$er!
Take care
Tuercas Viejas
