SevenSisters Posted July 17, 2004 Share Posted July 17, 2004 Fuel filters take out particulate debris from the gas so the injectors don't wear out too fast. Kind of like keeping the oil filter clean in an oil burning boiler or furnace so the nozzel doesn't get a goofy wear pattern and spray the oil unevenly. FWIW, Steel fabricators use high pressure water with a little abrasive in it to cut armor steel like butter. Fuel injectors get "dirty" because they live in a hot environment and soluble gum components of the gasoline build up inside the precise metering orifice. Other components of the fuel, like anti-knock additives, build up on the outside tip of the injector like that brown fluffy stuff that builds up on your spark plugs. You'll never filter that stuff out of the gas. Quote Link to comment Share on other sites More sharing options...
frag Posted July 17, 2004 Share Posted July 17, 2004 Legacy 2.5, the first time i read here about relieving fuel system pressure by opening the gas cap, I made a test. I have an inline fuel pressure gauge right under the hood between the filter and the injectors. I stopped the engine and as always the pressure stayed at 30 psi (It take a couple of hours for the residual presure to go to zero). Then I opened the gas cap and looked again at the pressure gauge : the needle was still at 30 psi. What should I conclude from that experience in your opinion? Cheers! Gilles Quote Link to comment Share on other sites More sharing options...
SevenSisters Posted July 17, 2004 Share Posted July 17, 2004 You can conclude that opening the gas cap doesn't relieve the pressure in the fuel supply line, filter, fuel rail, etc. Disconnect the power to the pump and run it dry before disconnecting anything like the directions say to do. Quote Link to comment Share on other sites More sharing options...
legacy2.5 Posted July 17, 2004 Share Posted July 17, 2004 reply to frag... think about this, first i assume that your in line fuel pressure gauge is between the fuel rail and the fuel filter. i dont doubt that the pressure in the rail will not go down for an hour. by relieving the gas cap your remove the pressure from the gas tank. you can actually hear it relieve when you open the cap. when gas is taken from the tank it has to be replaced by something (which is air) which builds pressure in the gas, and when you remove the cap, like when you put gas in the pressure relieves. if you tried the test i talked about earlier in this post you would see what i am talking about. i dont know your automotive backround, but i have been a tech for 10 years, went to school for automotive, and i am constantly going back to school. i have worked on import cars for at least 7 years, and work on them everyday. this is a real world situation i deal with every day, and have practiced the "test" i was talking about years ago with the same results, there is hardly any spilled fuel when you remove the gas cap before changing the fuel filter, compared to not removing the gas cap. try it yourself. i have a shop full of 8 techs that all agree, and have been schooled, and even the factory service manuals instruct the technician to remove the gas cap to reileve system pressure. so now you tell me if it does not relieve pressure. Quote Link to comment Share on other sites More sharing options...
blitz Posted July 17, 2004 Share Posted July 17, 2004 legacy2.5, Clarification please. Are you suggesting that the entire fuel tank is being pressurized by the fuel pump as a part of normal operation? Quote Link to comment Share on other sites More sharing options...
legacy2.5 Posted July 17, 2004 Share Posted July 17, 2004 i am saying that as fuel is being used from the tank, something needs to occupy the area where the fuel "was", air replaces the missing fuel. this air, ambient temp., and vapors from the fuel create a pressure in the tank. when you remove the fuel cap you relieve the pressure in the tank. you hear this pressure relieve when you open the fuel cap. this pressure is the same pressure that would make gas spray all over when you remove a fuel hose off the filter. removing the gas cap relieves this pressure, which in turn keeps gas from spraying all over when you change your fuel filter. just try it on your own vehicle, most subaru filters are easily accessible. Quote Link to comment Share on other sites More sharing options...
blitz Posted July 17, 2004 Share Posted July 17, 2004 Any tank pressure is a result of thermal expansion such as might be caused by bringing a vehicle into a heated service bay from a frozen parking lot, or by mid-day heat following a cool night. Understandably, removing the cap would allow this pressure to escape. My question is; how does removing the cap relieve the 40 PSI (or thereabouts) of system pressure forward of the pump (i.e. at the fuel filter and associated lines)? Quote Link to comment Share on other sites More sharing options...
toddkageals Posted July 17, 2004 Share Posted July 17, 2004 Any tank pressure is a result of thermal expansion such as might be caused by bringing a vehicle into a heated service bay from a frozen parking lot, or by mid-day heat following a cool night. Understandably, removing the cap would allow this pressure to escape. My question is; how does removing the cap relieve the 40 PSI (or thereabouts) of system pressure forward of the pump (i.e. at the fuel filter and associated lines)? It doesn't. There is a check valve in the pump that keeps pressure/fuel from flowing back into the tank. However, if you don't relieve tank pressure too, it can spray out a lot of gas when you open the line to the filter. So....you are both right! Todd Quote Link to comment Share on other sites More sharing options...
legacy2.5 Posted July 19, 2004 Share Posted July 19, 2004 thank you, blitz. sorry i could not explain it better. i know from experience about removing the fuel cap relieving pressure from the system. almost want to say that instead of some people doubting what others say on this forum, listen more. i know i try not to steer anyone in the wrong direction on this forum. i try to use my experience to help some others, and in the process hopefully learn more. i have been saying since the first post's on this topic to try removing the hoses with and without removing the fuel cap. certian people on here like to go by "theory" not how it really works. theory always works out on paper, but not in the real world. i never said i knew exaclty how come removing the cap relieves pressure, i said i knew this from years of working in the automotive field. Quote Link to comment Share on other sites More sharing options...
blitz Posted July 19, 2004 Share Posted July 19, 2004 No prob legacy2.5, I do understand your point about the tank pressure. I just wanted to clarify about high system (pump) pressure needing to be bled down before pulling any filter hoses. It was the following quote that concerned me on account of not being entirely clear on this. now try the same vehicle after you reconnected the hose, start the vehicle up, let it run, turn it off and remove the gas cap and then try to remove the hoses and i bet hardly any fuel will spray out I just was worried someone might misunderstand and end up with a faceful of fuel. Quote Link to comment Share on other sites More sharing options...
legacy2.5 Posted July 19, 2004 Share Posted July 19, 2004 blitz, i was not referring to your comments. you seemed to understand what i was getting at. i was glad you were able to help get across what i was trying to say. if you read through the whole thread on this topic, you will see what i mean. Quote Link to comment Share on other sites More sharing options...
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