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#1
11-22-2022, 08:03 AM
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W126 350SD pressure in fuel tank
Each time I’ve tried to open the cap on my 200k mile w126 350sd, there is pressure from the cap. Kind of like some gas vehicles get.
I don’t see signs of air bubbles in the fuel, and the car is clean, no vents that I see have issues. Maybe elsewhere inside… What should I check as the culprit? Car runs fine. It’s just the only one of my diesels that builds pressure held in by the cap. Don’t want to induce leaks of my valuable $5.75/gal diesel. Thanks!
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Current Diesels: 1981 240D (73K) 1982 300CD (169k) 1985 190D (169k) 1991 350SD (113k) 1991 350SD (206k) 1991 300D (228k) 1993 300SD (291k) 1993 300D 2.5T (338k) 1996 Dodge Ram CTD (442k) 1996 Dodge Ram CTD (265k) Past Diesels: 1983 300D (228K) 1985 300D (233K) 
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#2
11-26-2022, 05:53 PM
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Mine (1987 300SDL) has always built up some pressure in the tank. When you take off the cap it hisses out. I've never really thought about it.
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#3
11-26-2022, 07:02 PM
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Kink or internal blockage in the fuel line to fuel tank valve.
__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014 79 W116 300SD 'Stormcloud' RIP 04/11/2022
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#4
11-26-2022, 07:14 PM
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Alec300SD, What is/where is the fuel tank valve?
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#5
11-26-2022, 09:43 PM
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Quote:
Just don’t want to blow something.
__________________
Current Diesels: 1981 240D (73K) 1982 300CD (169k) 1985 190D (169k) 1991 350SD (113k) 1991 350SD (206k) 1991 300D (228k) 1993 300SD (291k) 1993 300D 2.5T (338k) 1996 Dodge Ram CTD (442k) 1996 Dodge Ram CTD (265k) Past Diesels: 1983 300D (228K) 1985 300D (233K)
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#6
11-27-2022, 12:35 PM
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Pressureized Fuel Tank
Your W126 is far newer than my oldies but mine all have a vent hose that goes out the very highest part of the tank then exits the trunk and has a rubber bulb thingie (technical term there) with lips forward of the rear end on the left side .
The deal is : excess fuel droplets are supposed to collect there and the Mechanic is supposed to give it a gentle squeeze whenever the car is up on the hoist . Often the 'lips' are glued shut by ancient fuel residue . Might give that a look, I agree about not wanting any moisture in there .
__________________
-Nate 1982 240D 408,XXX miles Ignorance is the mother of suspicion and fear is the father I did then what I knew how to do ~ now that I know better I do better
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#7
11-27-2022, 01:27 PM
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Quote:
__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014 79 W116 300SD 'Stormcloud' RIP 04/11/2022
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#8
11-28-2022, 03:40 PM
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Alec300SD, Thanks for the info. I found and removed the valve and observed this: When you blow through the valve it takes a small amount of pressure. If you suck on it there is no resistance at all. This would prevent any vacuum from building up in the tank, but would allow a slight pressure to build up. I never knew it was there. (Old dog can still learn a little.)
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