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  #1  
Old 12-13-2009, 05:34 PM
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Fuel Cap Pressure instead of Vacuum

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

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  #2  
Old 12-13-2009, 08:04 PM
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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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Old 12-14-2009, 12:09 AM
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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
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Old 12-14-2009, 01:18 AM
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Quote:
Originally Posted by P.E.Haiges View Post
...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.
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Old 12-14-2009, 01:21 AM
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Could the pressure be coming from the return line pumping fuel into the tank?
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  #6  
Old 12-14-2009, 01:46 AM
Greg
 
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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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  #7  
Old 12-14-2009, 10:20 AM
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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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Old 12-15-2009, 05:57 AM
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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
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  #9  
Old 12-15-2009, 03:01 PM
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Quote:
Originally Posted by kerry View Post
Could the pressure be coming from the return line pumping fuel into the tank?
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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