I really do not know whether it is sucking air in or pushing warm air out but it seems to be happening in the SDLs and 300D. If the vent tube is OK then the tank should be at atmospheric pressure, there should be no hissing sound. Am I correct?

There are 2 ways I can try:

1) Connect compressed air into the tank from the fuel line and see it stays pressurized - It would not pressurize if the vent tube is OK.

2) Get spare cap, drill a hole, hook it up a MityVac and drive the car around. If it shows a vacuum then the tube is blocked.

Would any of these 2 methods work?

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.