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  #1  
Old 08-18-2006, 11:06 PM
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W123 belching fuel from the filler cap

Anybody experienced this; my 82 w123 diesel is purging fuel from the filler cap at a rate of about 5 gallons per 100 miles. I come home and there is a trail of fuel following me up the driveway and running down the rear fender of the car. I'm baffled. An Ideas??? thanks

82 w123 186000 mi,
86 Golf diesel NA 136000 mi,
05 F350 6.0 powerstroke 30000 mi,
77 Bronco soon to be powered by an MB 300TD
and a 1000L biodiesel batch reactor in the garage

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  #2  
Old 08-18-2006, 11:11 PM
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interesting... hows the vent line? id clean the vent check valve and see how that does... the only way i could possibly see the return fuel making the tank overflow is if the tank is collapsing slightly while driving and thus decreasing the capacity and the fuel is spilling out..unless you have a very special fuel tank that makes its own fuel
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  #3  
Old 08-18-2006, 11:38 PM
guage's Avatar
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Did you have a full tank?
If so it may be time for a new fuel cap or
the rubber gasket on the inside of the cap if that is offered.
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  #4  
Old 08-18-2006, 11:42 PM
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Is it possible that one of the two tabs on the fuel cap failed to engage?
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  #5  
Old 08-19-2006, 01:53 AM
RAYMOND485
 
Join Date: May 2005
Location: CALIF
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Fuel Cap Leaking

1984 300d Turbo 140k
Replace The Cap With Large Cap Gasket I Use A Small Gasket And It
Leak When Fuel Full, New Fuel Cap No More Leak
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  #6  
Old 08-19-2006, 09:40 AM
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thanks for the responses-
Drove the car last night with a full tank. no purging. this happens irregularly.

I checked the gasket in the cap and the lugs previously. everything looks right and the cap closes securely. I know it seals because there was a whoosh sound when I opened it the first time yesterday. I think the tank had a slight vacuum. I've cleaned the overflow tube before but don't see how it could effect the tank volume, pressure, or vaccuum.

the tank collapsing is an interesting thought but I don't know where the vaccum would come from unless the return line is blocked and the tank is collapsing as fuel is drawn out. I'm going to apply air pressure to the return line today and listen for air / bubbling at the filler opening.

The day it lost 3-4 gallons it started on 3/4 full and by the time I got home, it was at 1/4. I drove a distance of 115 miles smelling fuel the whole way literally watching the guage drop.

Any other thoughts are appreciated.

Rgds

Last edited by standleys; 08-19-2006 at 09:45 AM.
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  #7  
Old 08-19-2006, 09:47 AM
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yea that woosh sound is something you dont want... remove and clean the vent line check valve.. its located on the drivers side right in front of the trailing arm.. looks like a little bell (which is part of the assembly of the valve) remove it and clean it out, i just blew somewater through it and air...no more woosh
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  #8  
Old 08-19-2006, 09:52 AM
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Don-
didn't know there was a vent. thougt the feed/ return lines meant a closed system. I'll find the sucker and clean it. thanks
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  #9  
Old 08-19-2006, 10:30 AM
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Quote:
Originally Posted by TheDon View Post
yea that woosh sound is something you dont want...
Always had the woosh sound and never had problems. Are you talking of excess pressure or vacuum? They might sound the same...
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  #10  
Old 08-19-2006, 10:33 AM
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Quote:
Originally Posted by MBeige View Post
Always had the woosh sound and never had problems. Are you talking of excess pressure or vacuum? They might sound the same...
either or you dont want a woosh sound... i collapsed my tank i can only fit maybe 18 gallons in and thats from empty.ill have to remove my tank and fix it or put some slight pressure to the tank and listen for a pop ...
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  #11  
Old 08-19-2006, 11:07 AM
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Quote:
Originally Posted by standleys View Post
the tank collapsing is an interesting thought but I don't know where the vaccum would come from unless the return line is blocked and the tank is collapsing as fuel is drawn out.
The problem with that theory is that fuel burned by the five cylinder device in the front of the car is not returned to the fuel tank. (Perhaps you have noticed that the tank must be refilled periodically.) So it is critical that fuel not returned to the tank be replaced by air. Hence the need for a mechanism (vent) to relieve both positive and negative pressure in the fuel tank. The valve which controls this function will allow slight pressure differentials to occur (both positive and negative) so a slight rush of air when removing the fuel cap is not necessarily indicative of a problem.
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  #12  
Old 08-19-2006, 11:09 AM
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Quote:
Originally Posted by tangofox007 View Post
so a slight rush of air when removing the fuel cap is not necessarily indicative of a problem.
in a gazzer
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  #13  
Old 08-19-2006, 11:23 AM
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Quote:
Originally Posted by TheDon View Post
in a gazzer
According to the service manual for the 617.95 diesel engine, the fuel vent valve will permit positive pressures of 30-50 mbar and vacuum levels of 1-16 mbar. Additionally, a valve in the fuel cap will relieve positive pressures of 100-300 mbar.

Clearly, the system was not designed to provide a "zero" pressure differential at all times.
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  #14  
Old 08-19-2006, 11:25 AM
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i dont get a woosh at all anymore.. and im getting 24-25 mpg consistantly

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