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Starting Problem and Vacuum in Gas Tank

Yesterday my 1996 Mercedes C220 would not start. It cranked like crazy but would not fire. Some things I suspect contributed: The gas tank was below 1/4 full. It was cold ... about 25F. The night before I had put 140 lb. of sand in the trunk just before driving it about 15 miles home and parking it. My theory was that because of the weight of the sand and the low gas level, gas was not able to get to the fuel pump. So this morning I was going to put in a couple gallons of gas from my garage gas can. When I opened the gas cap there was a serious whoosh of air into the tank; there had been a substantial vacuum inside the tank. I put the cap back on, without adding any gas (the sand was still in the trunk), and the car started right up. Here's my question(s): 1) Did the sand contribute in any way to the problem?; 2) Did the low gas level contribute?; 3) Did the cold weather contribute?; 4) Do I have a mechanical problem I need to take care of? Thanks.

Comments

  • To me, the fuel pump should be able to override a vacuum in your tank.

    But in theory, a vacuum in your tank would fight the flow of fuel out of the tank. The sand is a red herring. The low gas level creates a larger vapor space in the tank. The cold weather also creates a vacuum as the volume of the vapor space does not contract much to compensate for the decreased temperature and you can create a lower pressure system this way, because if the volume remains constant, then a decrease in temperature directly causes a decrease in pressure. These two things combined created a vacuum space in your tank (as compared to the outside air).

    I still question though whether this should be able to fight against a fuel pump. They are designed to start the car under these conditions.

    Bad fuel pump? Partially clogged fuel system could contribute to this? Anyone else have thoughts?
  • A bad gas cap will create a vacuum in your gas tank. The fuel pump will pump fuel until the vacuum overrides the pumping action of the fuel pump. Replace the gas cap.
  • This is a clear sign of a problem in the EVAP system, specifically the charcoal canister.

    Your gas tank needs to breath in, to allow air to displace the gas pumped out and when the gas contracts during cooling, and out, because the volume in the tank inceases (1) whenn you fill it, (2) when the gas warms up in the sun, and (3) when the fuel is agitated as you drive (agitated fluid takes up more volume). In order to allow it to breath while still preventing hydrocarbon molecules from getting into the atmosphere, it breaths ina and out through an activated charcoal bed. The component that contains this is called the "carcoal canister". The charcoal catches hydrocarbon molecules in air that's escaping the tank, and when you start the car a solenoid-operated "purge valve" and allows the HC to be drawn into the engine's intake system and burned.

    If the vent line to the canister becomes blocked or kinked, or if the charcoal bed becomes saturated with gas, the tank will be unable to breath in. A vaciuum will form in the airspace and the vacuume will prevent the pump from being able to pump gas out. It also places excess stress on the pump and can cause premature pump failure.

    I've attached an "exploded view" drawing of my owne system just to illustrate the typical system. I don;t heva one specifically for your system, but I'm coonfident that it'll be similarly constructed.

    http://tijil.org/Scion_Docs/05_tC_Shop_Manuals/Repair Info/Repair Manual/Fuel/Fuel Tank Assy/conponen.pdf

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