I installed the repaired dented tank and rebuilt squashed sender this past weekend in the 300TD and thought I was done. The ride into work yesterday was fine but when I went out at noon the motor would not start. I gave the primer about 20 pumps and nothing was flowing. I removed the filler cap and swoosh!! air rushed in and I heard a bonk from the tank.

@##@%*^% I can't believe it!! One of the disadvantages of filling up in NJ is that there are no self-serve pumps, so I never get the opportunity to open the filler myself. Fortunately the tank seemed to pop back into shape. Unfortunately I could see a slight impression of the sender on the tank bottom. Sure enough when I got the engine fired and drove the fuel gauge was going nuts with the needle cycling between full and empty.

Last night I pulled the sender and found the bottom of it was squashed again but not as severely as the first time. At least the center rod was not broken as before. However the rod must have been pressed further into the cap as the sender wires were all floppy. They must have been shorting on the can making the needle swing. I left a little tail of wire at each solder joint from my previous repair so I was able to heat and retension the wires. I had to hammer the can back into shape and remove about an 1/8' from the length to match the shortened rod. The sender now looks really ugly but it works ok. Fortunately the loss in length was not enough to throw off the readings.

I checked the 2 small vent lines from the tank and one of them had some fuel inside but all the lines seemed clear both to the tank and canister. I filled the tank Sunday night so maybe this problem occurs only with a completely full tank.

Anyway rather than trying to sort out the venting system I popped the black cover off the fuel filler cap. I drilled 2 .050" diameter holes that fell inside the center boss through both the steel cap and spring washer. This puts the holes in direct communication with the filler tube but hides them from splash under the center boss. I also drilled the same size holes in the plastic cap and oriented them 90 degrees from the holes in the metal cap.

This morning I checked the cap a couple of times coming to work and there was no vacuum at all so hopefully I have it knocked this time. I guess the next full tank will tell the tale...

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

Sybill the SDL does a similar whoosh at cap removal but I drive that car so infrequently that I've been unable to determine if it is vaccum or pressure...I was thinking it was pressure but, on further reflection, that doesn't make any sense. It would almost HAVE to be a vacuum.

There's SO much wrong with the SDL...

Lilly the 300D apparently had a plugged breather but I never knew it until I pulled the tank recently for anti-fungus scrubbing and treatment. Lord only knows where it was equalizing the pressure...the cap probably.

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Never a dull moment at Berry Hill Farm.

Leo,

Just don't let the vac build up to tank crushing proportions!! You might want to leave the cap a little loose or remove the rubber seal.

I think we all feel your pain about problems. I'm in the process of restoring one of my W109's but it seems I keep ending up under the 300TD!! The same thing happened when I was using the 250SE as a daily driver....

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

My 123 does that...its vacume...I have fished a cable as far as it would go into the vent line from under the car and found no restrictions.....

Don't remember if I noticed the same thing on the W123...

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

There are several factors that contribute to positive pressure. The vent valve requires more positive pressure to open than it requires negative pressure. (30-50 mbar positive versus 1-16 mbar vacuum.) The fuel cap also vents positive pressure when it exceeds 100-300 mbar. So evidently the designers believed that some positiive pressure was acceptable.

One way to check the vent line is to attach a rubber hose. Blow into the rubber hose to pressurize the tank. Then place the end of the hose in a container of water. If bubbles appear, you know the line is clear. The vent valve itself should offer very little resistance to air flow in the direction of the fuel tank; a bit more resistance in the opposite direction.

hmmmm never though of that. I need to try that.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Should be pretty straight forward to determine whether you experience vacuum or pressure when you remove the cap.

If the vent is blocked though, it doesn't take much of a pump to suck a fuel tank flat! Think about how many square inches of surface there is on the inside of a fuel tank. Then imagine a pump sucking fuel/fumes out of it. I saw a fuel pump collapse a fuel tank on a ground power unit in the USAF because the vent hole was blocked.

I check the vent opening under the 300D every time I'm under there doing other checks/maintenance.

Wes

Sometimes I hear a quick "woosh" when I open the fuel cap, but I can never tell if the air is rushing in or rushing out. But given the vent valve parameters, a little vacuum or pressure is acceptable.

I hooked my Mityvac to both the lines going to the tank and to the ones going to the canister and I could not generate any vacuum in either direction. I'm not sure how the system built up that much pressure. I'm not a fan of quick fixes but I'm glad I drilled the vent holes in the cap. I filled the tank up coming home from work and when I got home there was no vacuum present at all. That little mechanical pump must be working much easier now.

I'm sure the two little .050" vent holes I drilled won't add to global warming. Gosh with all the talk about R12 on this list I'm surprised we even still have an ozone layer...

__________________
Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

Cow farts do far worse to the ozone than the R-12. IMHO

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I used to think much the same thing...until I had an opportunity to sit and talk with a retired DuPont chemist for an hour one afternoon. No big deal here but in Kiwiland, southern Oz and Taz the hole is a very real issue.

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Never a dull moment at Berry Hill Farm.