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Old 05-20-2012, 05:37 AM
tonywintn tonywintn is offline
Drivin' the dream
Location: Tennessee
Join Date: Jun 2009
Posts: 187
Mein Auto: 2005 BMW X5 & 2007 Z4 si
If you have an Ohmmeter you can remove the rear seat, unplug the connectors on the tank lids on both sides. One side has 4 pins. Two are for the fuel pump, two for the level sender and the other side has two for the other level sender. If you see less than 1 ohm, that is the fuel pump pins. Fill your tank first so that you know the tank has fuel. Measure the resistance on each tank lid. If either one reads 980 ohms or higher, then something is wrong with that level sender. 980 ohms is the normal empty reading. If your gage on the dash reads empty all of the time, it could possibly be due to a faulty sender. The computer will drive the needle to empty if it detects a problem with either fuel sender. This ohm check will confirm which one or if even both are bad.
As for venting; fuel is like soda pop. It generates a vapor pressure. That pressure is vented through a carbon canister and then to atmosphere. The tank builds up a certain amount of pressure before it is vented. As the fuel heats up the pressure builds and gets vented through the canister. The charcoal captures the hydrocarbons and stores them until they can be burned off by the engine. Once the tanks cools the pressure drops inside the tank. Now venting must take place the other way. Air from the outside must enter into the tank so that the tank does not create a vacuum. It would be like canning tomatoes. You fill the jar and heat it, then seal the lid. After the jar cools it has a vacuum inside. The difference is your gas tank is made of a high density polyethylene, not rigid glass. So a vacuum can cause the tank to collapse and if severe enough, break the fuel pump/level sender components. That is why it is important that the venting system be clear of any obstructions. The vent pipe is mounted on the carbon canister in the rear of the car.
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