My 86 300SDL often has a noteable pressure release when the fuel cap is removed. Seems like everone else notices a vacuum.

I know something is plugged up but what causes the pressure?

I doesn't seem to affect the operation of the car and doesn't cause any fuel leaks.


P E H

either pressure or vacuum can be normal! According to the 124 CD manual vacuum of 1 to 16mb or pressure of up to 50mb may occur during normal operation.The fitting on the end of the vent pipe controls vacuum (bell shaped rubber cover) and a valve in the filler cap controls the pressure. Don

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Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles

oldiesel.

So U are saying the tank on a 126 300SDL in not vented except thru the filler cap?

A vacuum in the tank I can understand because fuel is being removed, but where is the pressure coming from? Is it just from expansion of the fuel due to temperature change?

P E H

The air in the tank will expand (or attempt to expand) far more than the fuel when the temp increases.

Could the pressure be coming from the return line pumping fuel into the tank?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

i always have pressure in my 16 Valve. i wouldn't worry about it. but the only thing i can think is that gas vaporizes so i may just be getting a lot of vapor pressure.

diesel doesn't vaporize as readily. what conditions do you drive in.

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Current
2006 E320 CDI, 57K bought at 67K "Liesl"
1986 190E 2.3-16, 198K bought at 56K "Brigitta"
1987 300TD (Chuggin Along), 292K "Friedrich"
1995 E320 Wagon, 200K "Louisa"

Past
1987 300SDL, (sold) 125K, bought for 1$ "Kurt"

The older cars were equipped with a vent and a fuel/air separator in the trunk with a bleed line that vented pressure to the right rear corner of the car, under the rubber trim around the bumper where it joins the body. When it would clog, usually due to overfilling and the rubber parts getting gooey, it would stop venting positive pressure and generate a vacuum. When it got bad enough, the vacuum would stop fuel flow and the engine would stop running. So, significant vacuum is not desirable.

The pressure in the tank comes from temperature changes that affect Diesel fuel density, as well as the air temperature of the trapped air. The return line to the tank gets warmed fuel, that over a long drive, warms up the bulk fuel temperature and increases the vapor pressure. Newer cars have fuel heaters, and this effect is more pronounced. I doubt you are getting much more than a "whooshh" of air - it is not like the cap is going to fly out of your hand. At least that is how my cars appear to work.

Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Greg,

Lately just to the markets and Walmart ETC, ,about a 40 mile round trip.

However I will be driving to FL from PA in JAN.

P E H

Maybe if the fuel system is sucking in air someplace. It would have no place to go other than down the return line into the tank and out the vent back there.

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Broadband: more lies faster.