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  #1  
Old 09-09-2019, 07:35 PM
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Why does fuel tank over flow

I filled the 84SD with fuel yesterday and drove about 1 mile home. Today, there was fuel on the ground in the area of the tank filler. I expected that one of the bottles of fluid that I keep in the trunk had leaked.



It turned out that there was diesel fuel on the ground. I carefully loosened the filler cap and fuel gushed out. Today's temps were in the 90s but this has never happened on this car. What gives? It seems like a stopped up vent would result mostly in vacuum from driving.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.
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Old 09-09-2019, 07:47 PM
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The fuel wasn't in the 90s when pumped into the tank, it was whatever the earth temperature was in the tank it was pumped out of. Let it warm up to whatever the ambient temperature is and it expands. If your tank vent is clogged, guess what.....you now have pressure in the tank. Full tank with fuel sitting in the neck = fuel shower when opened. Fix your tank vent.
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Old 09-09-2019, 09:57 PM
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You did not drive far enough for this but also fuel that passes through the fuel injection pump cools it and the return fuel carries heat back to the fuel tank.

Note a few people that have had clogged fuel tank vents have had their fuel Tanks implode/collapse.
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Old 09-09-2019, 11:11 PM
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I never stuff my tank. I always put it on automatic fill on the fastest notch and when it kicks I pull out the nozzle and move on.
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Old 09-10-2019, 10:47 AM
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Quote:
Originally Posted by t walgamuth View Post
I never stuff my tank. I always put it on automatic fill on the fastest notch and when it kicks I pull out the nozzle and move on.

I go to the next nickle after the pump shuts off and add 1 oz 2 stroke oil/gallon of added fuel. Never had a problem doing things that way. I'll clean &/or replace the vent valve.


FSM says that it is above left axle on a 126.
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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.
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Old 09-10-2019, 12:35 PM
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2 stroke oil goes in first....
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Old 09-10-2019, 02:40 PM
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Quote:
Originally Posted by resto108 View Post
2 stroke oil goes in first....

It can but is not necessary because 1) oil will naturally diffuse from areas of higher concentration to areas of lower concentration



and


2) return line will mix the oil and fuel.


Putting it in 1st requires that you know how much fuel will be bought. I frequently buy about 10 gallons of fuel give or take and there has always been sufficient capacity to add 10 oz (1/3 of a quart) to the tank.
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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.
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  #8  
Old 09-11-2019, 12:38 AM
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I had a similar problem

I had a similar problem on my w124. I would fill up, drive home a few miles (ie, there should now be a bit less fuel in the tank, right?), and maybe 2 days later there would a fuel leak under the car that was like 16 inches x 16 inches, ie, pretty big. Couldn't figure out what was going on.

Had my mechanic look at it, and there's a bell shaped thing under the fuel tank. you might be able to see it without having to jack the car up if you get your head in there and look around. I forget its function, handles some fuel overflow or something.

End of story, unless I wanted to do some work under the car (I didn't), the solution was to not fill up to the very brim. Somehow what was happening is that if I fill up completely, some of the excess fuel goes into that overflow hose and down to the bell shaped thing. Now that the car is 28 years old, something is not working like it's supposed to, pressure builds up, and that fuel overage ends up on the ground.

Solution for me: As soon as I hear the fuel pump getting to where it'll shut off soon, I stop pumping right then. Problem solved in my situation.
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Old 09-11-2019, 09:52 AM
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Quote:
Originally Posted by jbach36 View Post
I had a similar problem on my w124. I would fill up, drive home a few miles (ie, there should now be a bit less fuel in the tank, right?), and maybe 2 days later there would a fuel leak under the car that was like 16 inches x 16 inches, ie, pretty big. Couldn't figure out what was going on.

Had my mechanic look at it, and there's a bell shaped thing under the fuel tank. you might be able to see it without having to jack the car up if you get your head in there and look around. I forget its function, handles some fuel overflow or something.

End of story, unless I wanted to do some work under the car (I didn't), the solution was to not fill up to the very brim. Somehow what was happening is that if I fill up completely, some of the excess fuel goes into that overflow hose and down to the bell shaped thing. Now that the car is 28 years old, something is not working like it's supposed to, pressure builds up, and that fuel overage ends up on the ground.

Solution for me: As soon as I hear the fuel pump getting to where it'll shut off soon, I stop pumping right then. Problem solved in my situation.
Sounds like you are describing the Tank Vent pipe which on mine is in front of the drivers side rear Wheel. See pic.
Note this is someone else's pic. I think I have read they all don't have the same bell shaped end to them.

Also in the diagram 77 is a hose. I suppose it is possible for that hose to get old and kink and or collapse with age.
Attached Thumbnails
Why does fuel tank over flow-tank-vent-picture-large.jpg   Why does fuel tank over flow-fuel-lines-hoses-fuel-tank-x.jpg  
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  #10  
Old 09-11-2019, 04:38 PM
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The valve is supposed to open And vent at 30-50 mbar. This equates to ~.5 to .75 psi.

The valve is above the driver side rear axle on a 126. The line comes down from the tank area, connects to the valve and goes back up through the sheet metal. I'll track the end of the line down in the next day or so.

The cone shaped thing is simply one method of terminating the vent line. It is not the valve. There are apparently several systems all slightly different.

My plan is to pull the valve itself to replace with the one off of the parts car. I'll track down the end of the line in the process. It "looks" like the end may be behind removable wall in the trunk. I have the trunk liner out to track down a water leak and will combine the vent project with the leak project.

I'll leave the filler cap loose so that using fuel doesn't create a vacuum and collapse the tank or starve the engine of fuel. This clogged valve issue is covered in section 47-720 of the 126 FSM.
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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.
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  #11  
Old 09-13-2019, 11:42 PM
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Quote:
Originally Posted by Junkman View Post
The valve is supposed to open And vent at 30-50 mbar. This equates to ~.5 to .75 psi.

The valve is above the driver side rear axle on a 126. The line comes down from the tank area, connects to the valve and goes back up through the sheet metal. I'll track the end of the line down in the next day or so.

The cone shaped thing is simply one method of terminating the vent line. It is not the valve. There are apparently several systems all slightly different.

My plan is to pull the valve itself to replace with the one off of the parts car. I'll track down the end of the line in the process. It "looks" like the end may be behind removable wall in the trunk. I have the trunk liner out to track down a water leak and will combine the vent project with the leak project.

I'll leave the filler cap loose so that using fuel doesn't create a vacuum and collapse the tank or starve the engine of fuel. This clogged valve issue is covered in section 47-720 of the 126 FSM.
It is in the same section number in the engine part of the W123 Manual.

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