Replacing Fuel Tank Breather Hoses
Every time you go to the gas station it happens: it smells like gas fumes. No, its not in your head, your car is leaking precious and expensive fuel.
The design of the BMW E21 fuel tanks (there are two of them, left and right) includes a breather hose system that lets air in the tanks as fuel is consumed and allows air to escape when you fill up. Unfortuantly, BMW did not use fuel grade hoses for these breather hoses and after a number of years of exposure to fuel, they fail and fuel leaks when you fill up.
The following article details the replacement procedure for these hoses. Good luck and breathe easy!
The first step to this project is to understand the layout of the breather system. Click on the thumbnails to see details.
Diagram 1: This is the most common layout. It appears that most cars with this setup also have a charcoal canister in the engine compartment and an expansion tank directly over the filler door inside the trunk/boot. This is the layout that appears in all US 320i's and most or all Australian E21's.
Diagram 2: This diagram is used in most European E21's. Cars with this setup generally do not have charcoal canisters.
Diagram 3: This diagram illustrates the system used by VERY early E21's, namely those produced before 8/77. Notice the single gas tank. I have never seen this setup and therefore do not know where the hoses actually run. Sorry. I also don't know how to replace the breather hoses on one of these cars. These instructions apply to cars that have the setups in diagrams 1 and 2.
There are three sizes of hose used. From the BMW HTK, they appear to be 6mm, 12mm and 14mm inner diameter. They can be substituted using 1/4, 3/8 and 1/2 inch hoses repsectivly. Keep in mind that you must use FUEL GRADE HOSE or you will just have to end up replacing these again soon. The 1/4 and 1/2 inch hoses seem to fit nicely. The 3/8 inch hose its a nice fit to the tanks (as will be discussed later) while its quite snug on the plastic t-connectors. My advice is to have some vegetable oil handy.
What you need (I am not responsible if this is not exact):
- 6 feet of 1/2 inch FUEL hose
- 2 feet of 3/8 inch FUEL hose
- 2 feet if 1/4 inch FUEL hose
- 10 - 1/2 inch hose clamps
- 4 - 1/4 inch hose clamps
Note, the designations "left" and "right" refer to the left and right sides of the car if you are sitting inside, unless otherwise noted. The bottom of the diagrams are the front of the car.
Its important to do these in this order (it will save you a lot of headaches):
- Remove the back seat. Its only necessary to remove the bottom of the seat.
- Remove the four screws holding the door over the right tank in place. If you have a 77-78 model with a door on the left side as well, remove this also.
- There is a short piece of hose connecting the in tank unit (it looks like brass, and should be obvious) to a hard line on the left side of the right access hole (looking at the trunk from inside). This is the fuel return line. Remove the clamp and disconnect the line at the car, not at the sender. Fuel will probably spill but its not under pressure.
- Get under the car. Each tank is attached with three bolts, two on the front that are obvious and easily acceable. Remove these. There is also a third at the back at the top of the tank. You don't have to remove this, just loosen it (the tank is notched here so it will slide off a loosened bolt).
- Pull down the left side tank until you have access to the top of it. Don't worry, you won't break anything, so you can use some force. You may have to remove a rear swaybar if the mounts are in the way. If you have an access hole over this tank, you can access the top of this tank from this hole instead.
- Disconnect the hose from the nipple at the top of the left side tank (center and right pink boxes on diagram 1).
- Push the tank back up into position.
- Now pull the right side tank down. This one, however, will be accessed from the access hole inside the car.
- Disconnect the breather hose from the nipple on the right tank (left pink box on diagram 1).
- Disconnect the two hoses on the left side of the "y"
shaped connector (looking at the trunk from inside, left side of green box in diagram 2). These should be two different sizes. Once these are disconnected, pull out the "y" shaped connector along with the hose that runs to the left side tank.
- If you have the setup in figure 1, disconnect the hose that runs between the t-connector in the trunk/boot (purple box in figure 1) and the expansion tank (above the filler neck) at the tank.
- If you have the setup in figure 2, there is small piece of hose connecting two small hard plastic hoses (yellow box of diagram 2). These reside above the right tank. Remove this as well.
- Now open the trunk/boot. Locate the expansion tank (it sits above the right wheel well. Disconnect the hose that runs to the filler (turquoise box in diagram 2). Pull out the expansion tank hoses and all.
- Remove the remaining hose from the "y" shaped connector. Using some oil, lubricate and attach the 2 feet of 3/8 inch to the end of the "y" connector (this is the hose one in the right purple box in diagram 2). It will be a snug fit and require some work.
- Now slide the hose back in the right side access hole, directed toward the left tank. There is an access hole in a piece of the car's body that seperates the two tanks. You will need to push the hose through this hole.
- Once again, pull down the left tank. Lubricate the end with some oil and attach the end to the nipple on the left tank. Pull out the slack from inside the car. Push the left tank back up into place and loosely secure the tank with one of the lower bolts.
- Determine how much slack must be removed to place the "y" shaped connector just to the right of the sender unit of the right tank (looking at the trunk from inside). You only want to do this once, so get it right.
- Once again, drop the left tank. Disconnect the hose from the left tank.
- Cut off the amount mesured in step 15. Attach the piece you just cut off to other end of the "y" shaped connector (the left purple box in diagram 2). Both hoses connected to the "y" shaped connector should have hose clamps now.
- Once again, run this hose over to the left tank. Drop the left tank, and once again attach the hose to the left tank, this time using a hose clamp.
- Go back to access hole. Pull the hose tight. Push the left tank into place and loosely attach one of the bottom bolts to hold it in place. Once again, pull out the slack. If you have any slack the hose will kink and only the right tank will fill.
- Now trim the short hose (the left purple box in diagram 2) so that you can attach it to the right tank (left pink box in diagram 1).
- Now replace the hoses that are attached to the section that you pulled out from the trunk/boot (in step 11) using hose clamps:
- If you have the setup in diagram 1, this means the two sections of 1/2 inch hose and the piece of hose that runs from the "t" shaped connector to the expansion tank above the filler. Cut the piece of 1/2 inch hose that runs to the filler the exact same size as the original piece. The slack should be left on the piece of hose that runs from the trunk to the gas tanks.
- If you have the setup in diagram 2, this means the two sections of 1/2 inch hose and the two sections of 1/4 inch hose. Cut the piece of 1/2 inch hose that runs between the expansion tank and the filler to the same size as the original piece. Cut the piece of 1/4 inch hose that connects to the expansion tank the same size as the original. Leave the slack on the ends the run from the trunk to the gas tank.
- Feed the hose(s) from the trunk through the metal tube that runs to the gas tanks.
- If you have the setup in diagram 1, trim the slack from 1/4 inch hose and reconnect it to the expansion tank in the trunk using a hose clamp.
- If you have the setup in diagram 2, trim the slack from the 1/4 inch hose and reconnect it to the section of 1/4" hose over the right tank (part #10 in diagram 2) with a hose clamp.
- Trim the slack from the 1/2 inch hose, and using lubrication, attach it to the remaining part of the "y" shaped connector using a hose clamp.
- Raise the right tank and install one of the bottom bolts loosely.
- Attach the short piece of hose (left purple box in diagram 2) to the right tank with a hose clamp.
- Attach the return hose that runs from the sender unit in the tank to the hard line on the body (this is the one you removed in step 3) with a hose clamp.
- Fully reinstall all 6 fuel tank bolts. Reinstall the cover plate(s) inside. Reinstall the seat.
- You are done. Treat yourself to a beer or likewise refreshing beverage.