Check and/or clean your tank vents! - PeachParts Mercedes-Benz Forum

ROLLGUY · #1 10-24-2013, 01:19 AM

A guy got in touch with me the other day that had his car at a radiator shop to get his tank pulled and expanded. Evidently the vent lines were plugged, and the fuel tank shrunk so much, it would only hold about 12 gallons. The rad shop guy told him it would cost about $400 for a new tank plus labor. He called me and had his car towed to me. The car is a 1985 300D (former CA emissions) with Federal manifolds and air cleaner installed. The car is ALL ORIGINAL and near perfect, including a blue crack free dash!

I removed a tank from one of my parts cars and installed it. Here are a couple photos of the old tank:

I don't know how this was possible, but now I have seen it with my own eyes. Check/clean those tank vent tubes!

Edit: No he did not run over the tank with a car!

vstech · #2 10-24-2013, 01:35 AM

I've seen worse. I ran into a guy with a euro 240 that got an aux tank installed, because his tank would only hold... 6 gallons...!!! it was nigh flat. he thought it was because of the wrong fuel cap, and I explained to him about the bell behind the car... he'd drilled a hole in his filler cap to keep it from happening.
the amazing thing is it only takes about 8" vacuum to collapse the tank... and about 3 or 4 psi to refill it.
I've put the cap on, plugged the fuel line, and blew with shop air, and popped the tank back to nearly perfect shape. but the creases are always there, and I think they will rust. a tank swap is best.

Phil_F_NM · #3 10-24-2013, 01:51 AM

WOW!

Phil Forrest

Stretch · #4 10-24-2013, 03:22 AM

Indeed I have seen pictures of more sucked in tanks before - never seen it outside of this virtual reality though!

And people say the lift pump on an OM617 isn't butch enough...

jay_bob · #5 10-24-2013, 08:12 AM

Wow that looks like the science experiment we did back in 7th grade. Teacher took a 1-gallon rectangular container of duplicator machine fluid (remember that??, it was about 1977 or so), empty of course.

Put about a quart of water in it, and set it over the Bunsen burner. When the water started boiling he put the cap on it and shut off the gas.

The container looked exactly like that fuel tank. Amazing the power of 14.7 psi (or 1 bar for those of you across the pond) atmospheric pressure.

Smoker · #6 10-24-2013, 08:20 AM

Yup, I posted about this on the 300CD last week. I had the radio off an actually heard the sound of the metal on the tank popping in and immediately pulled over.

Was very hard to get the fuel cap off from all the vacuum, but got it off and drove home with the cap off and then replaced the little rubber line going to the vent that had collapsed inside.

I don't know that it has affected my capacity, I can see a depressed area (not big), so not going to worry about it. Last fillup I put 16 gallons in.

With the vacuum at the cap I don't doubt if I'd driven the rest of the 40 miles or so home I'd have had a tiny fuel tank.

Vacuum sucks....

PackerEdgerton · #7 10-24-2013, 10:24 AM

OK, all good information. Where are the vents, and how do you clear them?

Thanks,

Packman

charmalu · #8 10-24-2013, 10:25 AM

It`s amazing how a little vacuum can collapse a tank. Any time the Fuel Cap is released and there is a Whooosh sound, better check that little Bell Vent behind the L/R wheel.

Boneheaddoctor posted this a couple years ago when the tank in his W116 did this.

What happens when a tank vent gets clogged.

I posted this one before of a Rail Car collapsing.
Railroad tank car vacuum implosion - YouTube

Charlie

JB3 · #9 10-24-2013, 10:48 AM

amazing!

Smoker · #10 10-24-2013, 11:30 AM

On my CD it's on driver side, forward of the rear axle. You'll see 3 fuel lines, one terminates in a bell shaped rubber piece pointed down. that's the vent line.

The vent valve is attached to the bell and to one of those fuel lines with a rubber hose, mine hose was very old and had clogged. I cleaned the vent valve with carb cleaner, then tested it with a vac and pressure pump to make sure it opened and closed at spec (close, my guage isn't THAT accurate).

I have no idea how accurate my fuel gauge is since I haven't had the car that long. It was near red and I put 16 gallons in, it may have lost some capacity or it may be a dirty sending unit.

Quote:

Originally Posted by PackerEdgerton

OK, all good information. Where are the vents, and how do you clear them?

Thanks,

Packman

Stretch · #11 10-24-2013, 11:46 AM

Quote:

Originally Posted by charmalu

It`s amazing how a little vacuum can collapse a tank. ...

I'm sure you really mean it is amazing how much suck there is on that little lift pump that every one says is a poof!

ROLLGUY · #12 10-24-2013, 11:47 AM

Quote:

Originally Posted by PackerEdgerton

OK, all good information. Where are the vents, and how do you clear them?

Thanks,

Packman

A clog could be either in the steel vent line, or the vent valve itself. The vent valve is located under the car below the rear seat on the driver side. You can remove the vent assembly itself, and check if it is clogged by applying pressure and vacuum (I won't tell you specifically how to do this, you can figure it out

). If there is flow both ways (a little restriction should be felt), the vent is good. To clear the lines, remove the fuel cap and apply compressed air to the metal tube at the vent valve. If a restriction is present, a wire can be inserted in the tube (difficult but doable) to remove the clog. The line is in two parts, and both parts can be done separately. The rubber hose can be removed between the tank and the line going down to the vent valve and tested/unclogged separately. Chances are that a clog will be in the tube closest to the filler neck. The tube is actually higher than the level of the filler tube, so any restriction will be most likely between the filler and the high point (tight bend) of the vent tube (approx 18" of tube?). You can see the tube in the photo:

charmalu · #13 10-24-2013, 11:45 PM

Quote:

Originally Posted by Stretch

I'm sure you really mean it is amazing how much suck there is on that little lift pump that every one says is a poof!

Yeah it is a powerful little thing, I did read where someone said it could probably suck a Golf ball through the line.

I think there is something like 14.92" of air pressure per square inch at sea level at 55deg F. So that is quite a bit pushing on the tank with less pressure on the inside. so it will collapse. so if the pressure is equal on both sides the tank will retain it`s shape. Now if the pressure is greater on the inside, the effect will be the opposite, the tank will explode when it over comes the atmospheric pressure. Think I worded that correctly.

Charlie

Stretch · #14 10-25-2013, 09:54 AM

Quote:

Originally Posted by charmalu

Yeah it is a powerful little thing, I did read where someone said it could probably suck a Golf ball through the line.

I think there is something like 14.92" of air pressure per square inch at sea level at 55deg F. So that is quite a bit pushing on the tank with less pressure on the inside. so it will collapse. so if the pressure is equal on both sides the tank will retain it`s shape. Now if the pressure is greater on the inside, the effect will be the opposite, the tank will explode when it over comes the atmospheric pressure. Think I worded that correctly.

Charlie

Yep unlike unwanted wind it is better in than out in the case of a fuel tank...