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Old 11-07-2013, 10:00 AM
VA-Merc VA-Merc is offline
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Join Date: Jun 2012
Location: Monroe, VA
Posts: 135
I wouldn't necessarily let this go, on my old 300SD it actually started to collapse the tank and the engine was starving for fuel on a road trip up to NH. The vent allows the tank to equalize with atmospheric pressure. On my 300sd, I had a heck of a time getting it to work right. I read some posts where it was recommended to take an air compressor and shoot air up it to blow it out, but mine was so clogged that didn't work for me. I tried running weedwacker line up out after I pulled off the little bell shaped thing near the fuel tank, but that didn't work for me either. Finally, I ended up pulling the trunk back panel off, found the vent line, cut it with a brake line cutter, and made a new vent line out of brake line (just connecting the old line to the new line with a short rubber hose).

Hopefully yours isn't so bad...my SD was considerably older. Connerm is right that the end of the line is underneath the car. If you back the car up on ramps or jack the car up considerably in the back, you should be able to find a little bell shaped piece. This is the end of the vent line, but the line runs all the way up to the top of the tank...this is how it equalizes pressure. If you follow the line from the bell shaped piece, you will find a valve in-line. This is a two way valve that allows the tank to either pull air in or push it out. Try blowing on it in both directions to see if you can easily move air both ways. You shouldn't have to blow very hard to make it do this...if it is hard to get air through try cleaning it with some brake cleaner or solvent and then test it again. If you have access to an air compressor, you will want to blow air through the line that runs up to the tank (with the gas cap off) as well. This should clear out any obstructions. If you don't have an air compressor, at least connect a short piece of rubber hose to see if you can blow air with your mouth up into the line relatively easy (again, this will only work if the gas cap is off).

If you don't feel like messing with all of this before your trip, you can drill a small hole in your gas cap for a temporary fix. I had to do this to get my SD back from that one roadtrip where the fuel kept starving due to the plugged air vent. When the vent line is free of obstructions, there shouldn't be much vacuum or pressure when you pull the gas cap off after driving.

Here is another thread that has a diagram...it may help you a bit: fuel tank vent?


Hope that helps!
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97 S500 (90,000 miles) - wife's car
87 300D (298,000 miles and a replacement #14 head)
94 Suburban K2500 - need something to haul firewood
83 300SD (343,000 miles) (sold)
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