Project: Fuel tank vent mod


About one month after I completed the extraction and re-installation of the PSD from my truck, necessitated to replace a rusted out oil pan, I noticed diesel fuel on the floor of my garage under the rear of my truck.  Sure enough the steel rear fuel tank had also rusted through.  Why in the world Ford ever put a poly midship tank in, and a STEEL rear tank, I'll never figure out.  So, I contacted Mike at MTS Company, www.mtscompany.com, and ordered a replacement poly rear fuel tank.  I figured that since I would have the tank off of the truck, I might as well do the Fuel Tank Vent Mod that has been talked about in the '94-'97 TDS.com forums.  I grabbed the digital camera and went to work.


Overview of Project


Ford installed a roll-over valve on the top of the fuel tanks that has a small 1/4" rubber hose that has a vented cap and is clipped to the frame.  The roll-over valve's purpose is to prevent fuel from escaping out of the vent in case the vehicle rolls over.  There is also a rubber hose inside of the filler neck that the fuel actually goes through to get into the fuel tank.  The steel filler neck and rubber hose you see by crawling under the truck are merely channeling air between them and the internal rubber hose to let air escape from the tank as it is being filled with fuel.  This system can definitely be improved upon.

The Fuel Tank Vent Mod uses the roll-over valve attached to a larger diameter fuel line that runs from the fuel tank to a fitting installed on the steel filler neck.  This assists in evacuating air from the tank during re-fueling.  The fuel cap is designed to vent naturally, so there should not be a pressure problem within the tank.  The fuel line connecting the tank to the filler neck will contain any fuel should there be a roll-over situation.

The installation of the new Ford roll-over valves and the modification of the steel filler necks are exactly the same on the front and rear, so what I have described below applies to both.  Just getting access to the top of the tanks to do the work is different. 


Materials Used


1)    Approximately 7 1/2' of 5/8" fuel line.  I used 3' for the front vent, and 4 1/2' for the rear vent. I obtained my fuel line from NAPA, #H-183, for $17 including 4 hose clamps, #705-1313.

2)    2 - Brass hose barbs, Watts AB #A-493, $1.77 each, with 5/8" barb and 1/2" thread.

3)    2 - Ford roll-over valves, #F4TZ-9B593-H, $12.99 each, plus shipping.

4)    2 - Ford valve grommets, #F6TZ-9B076-AA, $3.99 each, plus shipping. You can probably use the existing grommets as I saw no difference when compared to the new ones.

5)    Fresh pack of JB Weld, $3.97.

Total cost of project is about $65.


Modifying the steel filler neck


Unscrew the three  7 mm hex head screws holding the filler neck to the bed, located around the fuel filler cap.  Unscrew the two hose clamps where the steel filler neck meets the rubber neck, and gently pull the steel neck out away from the rubber neck.  In doing so, you are also removing a long rubber hose that is attached to the inside of the steel filler neck, that goes through the rubber filler tube into the tank.  Keep pulling as it's longer than you'd expect.  You will be removing and discarding the rubber hose and internal steel fitting in the following procedure:


  With the filler neck out of the truck, clamp it in a vise, carefully, as not to crush it.  Use a long screwdriver in the fuel cap end and begin tapping on the ring inside the filler neck that has small crescent shaped holes visible as you look down into the filler neck.  Tap with a hammer and pull on the hose sticking out the other end.  The object is to get the steel insert part with the crescent shaped holes all the way to the other end of the steel tube.  
  With the steel insert and rubber hose at the bottom end of the filler neck, grab the rubber hose firmly and pull it off of the steel insert.  Next, grab the steel insert with vise grips to hold it and use a reciprocating saw to cut a slot in one side of the insert.  
  Using the vise grips and a screwdriver, pry on the insert to try and fold it in on itself and pull it out the end of the filler neck.  
  There is a dimple in the side of the steel filler neck just below the mounting plate that works well for locating where to install the hose barb.  I used the largest drill bit I had then carefully reamed the hole by hand to get the hole just the right size to thread the 1/2" brass hose barb into the hole.  You only want to go 2 turns or so into the filler neck so that the fitting does not interfere with the fuel filler nozzle when fueling.  JB Weld the fitting into place.  

Installing the new Ford roll-over valves


The front valve is really easy to do.  Stick your head up under the bed in front of the driver's side rear tire and look at the top of the tank.  You will see where the fuel lines go into the tank and rearward of that is the grossly inadequate roll over valve/vent with a small rubber tube that is clipped to the frame rail.  You should be able just to reach in there and wiggle the valve out of the rubber grommet, and unclip the rubber hose from the frame rail.  The rear vent will require dropping the tank to get at it.  I would recommend removing the tank completely, install the new vent tube onto the tank, then re-install the tank, rather than try to struggle with the tank hanging there.  To do the rear tank, remove the spare tire, remove 1/2 of the skid/mounting plate, place a jack under the exposed half of the tank, then remove the other plate.  It helps to have an assistant, but it's do-able by yourself with the proper equipment. 


  For the front tank,  attach one end of the 5/8" fuel line to the new roll-over valve with a hose clamp.  Replace the grommet with the new one, if you so desire. Reach up on top of the tank and insert the barb into the tank with the fuel line perpendicular to the frame rail.  It's helpful to add some lubricant to the grommet and the valve to help the valve slip into position. Re-install the filler neck onto the truck,  install the three 7 mm screws before the hose clamps, but after the steel tube is fitted inside the rubber hose.  
Front tank vent mounted.      
  Then, run the 5/8" fuel line up to the barb on the filler neck, cut to length, and install with a hose clamp.  I used a zip tie to hold the new hose in place using the hole in the bed support, making sure that the hose slopes FROM the filler neck TO the tank. If you do not do this, the fuel line will create a trap and fuel will sit in the trap and the mod will be worse than when you started. You're done with the front.  
Front filler neck attached.      
  Install the remainder of the 5/8" fuel line to the other new roll-over valve with a hose clamp, and insert it into the grommet in the fuel tank.  Again, some lubrication would help. Then, re-install the tank, routing the 5/8" fuel line perpendicular to the frame rail so it comes out by the filler neck.  
Rear tank assembly.      
  Re-install the filler neck onto the truck, again start with the three 7mm screws after the steel is fitted into the rubber hose.  Cut any excess off of the 5/8" fuel line, and install onto barb with  a hose clamp. Make sure the fuel line runs FROM the filler neck TO the tank, zip tie as needed, and you're done.   
Rear filler neck attached.      



I was a little peeved after completing the project because there was a leak around the sending unit on the tank, and when I filled it up, I had diesel fuel pouring out the top of the tank.  So I needed to pull the tank and re-seal the gasket on the sending unit. 

Anyway, I went to the 'Big Truck' pump to fill up and was totally NOT impressed.  It actually seemed like it took longer to fill up as the nozzle kept clicking off.  However, I still have to use the 'Little Truck' pump for the real test since the fuel coming out of the 'Big Truck' pump just gushes out.  All in all, I'd do this mod again as it's not really that difficult, and didn't cost that much.

I found out, through other posts online that you should not use the 90 degree street elbow on the filler neck, only the hose barb. What was happening was the fuel line would dip, creating a trap and collecting fuel and no air at all was getting into the tank. After eliminating the 90 degree street elbows the system works great! I am pleased.

Happy wrenching.

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