Fuel overflowing tank! Will not drain
 

Hi all. It has been sometime since I have posted. Been lucky I guess. I own a 1984 300D. She runs great except...when I got home from work I noticed a big puddle beneath the benz. It is diesel fuel which had got into the trunk.

So here is the problem:
Fuel is backing up out of the tank, it is over flowing. The car still runs, so fuel is getting to the engine, but why is the tank backing up past the filler neck through the fuel cap? Fuel gage reads 3/4 full so there is not too much fuel in the system. In fact, I filled up (a full tank) 2 days ago and have put nearly 100 miles on the car so I know there is no way the tank is too full. Unless of course the miracle finally happened and my car is now running on air...ha ha.:P

If anyone can help with this I would really appreciate it. We are a one car family so this would really put us in a bind if I cannot solve this.

Thanks in advance

Thomas

I'm not familiar with the 123, but do you possibly have a leaking fuel return hose in the trunk?

No, no leaking return hose. The fuel in the trunk is not the issue it was just the sign that something was amiss. I just cleaned out the little bell shaped vent thing that runs back to the fuel tank. It was a bit clogged. I ran her around the block a few times, but still the fuel remains at the top of the filler neck. It still will not retreat. WTH?

I was wrong though about the miles I have traveled since I filled up. I just checked the trip at it reads 40 miles. But still, at about 22 mpg a too full fuel tank should not be the issue. Besides, if that were the problem I would have noticed this overflow 2 days ago.

Perhaps you need a new seal on the fuel cap.

Your car is creating fuel. Put a tank in the trunk to collect it and sell it to me. :D

Seriously, maybe you have a hole in the tank. That is the only way I can think of that fuel would end up in the trunk.

There is a possibility your vent line is blocked and the tank has collapsed a little with the created vacuum. I can not figure any other way the fuel could still be at the top of the neck. Forty miles driving should have dropped the level about a gallon and a half.

check the condition of both the seal on your fuel cap, and the seal that surrounds the fill neck mine were very cracked, as you drive, the fuel sloshes around and splashes against the cap, then the fuel dribbles down the neck and leaks into the trunk. I bet. even if you have the amazing fuel producing car, a new seal (about $2) will hold in in.

YOu may have just left the cap loose or crooked. Also there is a drain from the filler area that runs through a tube through the trunk on some models. check the drain from the little filler door area.

The tank must have been filled from a cold tank and the heat is expanding the fuel now. Drive it and it will have to go down.

Tom W

Thanks all for responding. The seals are good, but I will replace them anyway.

The problem went away for a while. So I figured that is was simply diesel fuel expanding with heat. But today another huge puddle of fuel underneath the car. I am at less than a quarter tank. What the heck is going on?

In any case, Friday I am taking the car in for a valve adjustment. I'll ask my mechanic to see if he can locate a hole in the tank or whatever. I hope it is not too serious. This is car is a 97 percenter, and I want it to stay that way. Thanks all for responding. If I figure it out I'll post the solution.

Thomas

What did the answer to this turn out to be? I've got the exact same problem which just started on my 240D this last fill up.

It could be the Tank Vent is plugged up higher than the Bell

The OP did not get the best of responses. So He might not have come back.
His last post was Sep 2008.

Is your problem exactly the same as the Original Posters?

I find it hard to believe that He had 1/4 Tank of Fuel and that Fuel was getting pushed out of the Fuel Fill neck unless his Fuel Tank was collapsed.

You might want to remove the Sheet Metal Barrier in the Trunk and get a good look at the Fuel Tank.

In the pic is the Vent Line in Red and the Vent Line Hose circled.
If the Circled Hose collapsed that would render the Fuel Tank Vent useless.

what part of the country are you in?
is rust a problem with your car? have you looked at your filler cap gasket?
look in the trunk, there is a rubber hose that connects to your filler neck that can leak fuel into the trunk, and it'll leak onto the pavement on the passenger's side back tire area.

The problem started today (well, when I got home today I noticed that there was a major diesel spill where I had been parked, so I guess it started after my last fill, which was Monday.) I noticed some diesel pooled in the pass fender well in the trunk (there are no plugs in the fender wells or carpet in the trunk, I assume they were removed by the previous owner because the trunk seal and or the rear window seal leak so I am still guessing that the fuel blew up through these holes as there was no fuel streak indicating from where else it may have come.) I pondered on this a bit and then I also noticed the two small spots of diesel under the pass side aft of the rear axle. One of these spots coincides with the drain from the filler door (I think) and the other, I assume, is from where ever the trunk drains. When I drove away, I thought I could see a stream of fuel after I turned right, but when I pulled over I could not confirm this. After I got home (I parked a little up hill with the street crown lifting the driver's side) I removed the filler cap again and fuel started to pour out. I put the cap back on and parked in the opposite direction so that there was not so much fuel resting against the fuel cap.
When I filled, I let the pump switch off automatically and did not top off, which is my normal procedure. When I calculated the mileage I found it to be higher than it should have been (26 something vs my usual 22-23), but since I only fill about once a month (too many vehicles) I couldn't remember whether there had been many freeway miles this last month (which would have easily explained it.) Since the fill, I have driven 22 miles which is what I typically get from a gallon in the city, and that doesn't count what I have left on the streets of Seattle, which I would estimate to be at least a pint to a quart, so far. With the car parked as it is, there is no fuel showing in the neck. I should also point out that the weather has been warm lately, though only in the low 70's today (high 80's on Monday when I filled.) Right now it is about 60F so it's cooled quite a bit.
It seems from the MB manual that there is positive and negative ventilation of this tank and it would make sense that if the tank couldn't exhale it might burp up some fuel, but if I am reading things right, the tank filler should not be able to trap fuel above an air bubble which could then expand in the heat. I'll post more info if I can think of it.