Jump to content

FASS Installation on Monaco Dynasty

Recommended Posts

  • 6 months later...

OK.  I figured I would do a follow up from my FASS installation on our 2003 Dynasty with the Cummins ISL400.  I also feel like I probably need to eat a little crow as well but want to share my experience in hopes that others may have another option for the return line from the FASS.


For the past 3 years and 15k plus miles it has performed flawlessly with absolutely no issues or leaks.  When I installed my FASS there were several others doing this upgrade as well and most used a "T" fitting to tap into the fuel tank's vent port.  I believe one or two people opted to remove their fuel tanks and have another bung welded in but I really didn't want to go through that work so I followed suit and added the FASS return line to the vent port using a "T". 


I have had zero issues in over 15k miles, until our last trip that is.  Now that being said, there are a few variables from that last trip, but I still cannot make heads or tails as to why it started leaking now and hasn't leaked since.  First, I never allow our fuel level below half as that is my empty.  I always run on the top of the tank in the event we get rerouted or have other issues that prevent a fuel stop.  Also the road was absolutely garbage.  This is hands down the worst section of highway I have ever driven on and the last time we drove it was 8 years ago and it has not improved.  The section of Highway 89 between Flagstaff and Page absolutely SUCKS!  We were also just over 7k feet elevation going over a couple of passes so those are the variables that differed, below half tank, rough as hell road and around 7k feet in elevation.


The drive down on the same road was uneventful, no leakage.  However, on the return trip with just under half a tank (3/8 tank) is when we got some spillage out of the vent port.  We stopped to fill up just to the north side of Flagstaff at a truck stop on Highway 89.  During fueling I did my routine walk-around and noticed residue on the front of the Jeep.  I then proceeded forward and found residue around the side radiator of the coach, then around the drive axle.  As I moved forward there were drips hanging off some fasteners under the coach and upon feeling and smelling, it sure seemed to be diesel fuel.  I kept moving forward and noted it stopped at the vent hose from the fuel tank.


After filling we got back on the road and we stopped several times to check on things and never got another drop the last 600 miles to home.  


After a thorough cleaning of the underside, engine bay and radiator after removing the outer grille, I decided I would separate the FASS return from the vent/rollover valve.  However, I had to come up with another option as I really didn't want to remove the tank to do so.


I machined a flange style of mount for the 3/4" NPT bung in which the vent/rollover fitting could thread into and mount near the fuel sending unit.  That way the FASS return line could have the welded 3/4" NPT bung all to itself.


I grabbed some fasteners and round stock from my supply drawer and picked up a 3/4" NPT union from my local plumbing store.  Here I determined the circle pitch diameter in which to drill the mounting holes.image.thumb.jpeg.5eacf7c69f887f43b3ae81eeff6052f7.jpeg


I started by taking a few passes in the lathe around the circumference of the union to make sure it is round and squared off for welding.image.thumb.jpeg.8c05f0925a22e217058e36f09a97899b.jpeg


Next, I began machining the flange.image.thumb.jpeg.bdef2b1abb366a065a69ce5c6f72f51d.jpeg


Faced off, a skim cut around the perimeter and .875" hole drilled for the vent port.  It is now ready to part off.image.thumb.jpeg.7be640eec0f70fb58610e9e46670d6be.jpeg


Parted off and ready for welding.image.thumb.jpeg.8dd95159d668544844e529d8eb6d9abb.jpeg


After it was welded it was clamped in the mill vise to drill the bolt holes.  The pitch circle diameter and number of holes was plugged into the DRO.image.thumb.jpeg.bd9f3add7ed91d0bf93d177da4d38292.jpeg


With the holes drilled, I ran around the pattern again with an end mill to sink a .040" deep recess in each one to account for the head of the threaded inserts which will be used to secure the flange to the tank.image.thumb.jpeg.55eec96e57ea63004736d0e216588613.jpeg


Completed and at the ready for installation along with the fasteners and threaded inserts.image.thumb.jpeg.dec58da301af95fdeb6854092773d20f.jpeg



Coating a drill bit with grease catches probably 95% of the chips, but to make sure noting dropped into the tank I took one more precaution and created a catch for any chips that may get past the grease.  I used a piece of .062" TIG rod bent around a gear to create a hoop.  I then stretched a latex glove over it and secured with .020" aviation wire to ensure it didn't fall into the tank.  I then bent a few large passes on the end so the welding rod wouldn't fall into the tank and provide a handle giving me a means of guiding the glove where needed.image.thumb.jpeg.5f8e220d572e7d09f5018cdaca0d0e42.jpeg


With the chip catch inserted into the tank it was time to start drilling the five holes.image.thumb.jpeg.d617d7c1aa1f37c8caf16a08b27fba0a.jpeg


Holes successfully drilled in the tank and gasket at the ready.image.thumb.jpeg.2c8621fa8c9d7eea75c88ae72faedfda.jpeg


The grease easily caught the majority of the chips as the glove only had a few chips when it was removed.image.thumb.jpeg.a033cb9a01471a7e7d2b676b34b6547e.jpeg


The "T" that was used for the past 15k plus miles for the FASS return line and vent fitting.image.thumb.jpeg.07bdef46ead554a13b453ee246152e6a.jpeg


The vent/rollover fitting removed and cleaned for installation into the fabricated flange.image.thumb.jpeg.6f4557e5a1e146bea7f90e54cdeda3d7.jpeg


Thread sealant applied and screwed into the bung.image.thumb.jpeg.57bedcdbae1a868fb52c1fd6e1a53973.jpeg


Orientated so that the fasteners could easily be installed without interference.image.thumb.jpeg.7c3491c223407d8c7cca46439085a004.jpeg


With the threaded inserts installed in the tank, the flange and vent valve could be installed and the vent hose connected.image.thumb.jpeg.1a719bff0d6b066122b217abca88d252.jpeg


And the FASS return can now have its own dedicated port.image.thumb.jpeg.5238e9f78ce309b47ec91906aa167832.jpeg


Thanks for looking.  Hope this will help anyone contemplating how to plumb the FASS return line into the tank and not make the same mistake I did.  Although, I know there are several others using the same method of combining the vent and FASS return without any issues whatsoever.  



  • Thanks 1
Link to comment
Share on other sites

Posted (edited)

I'm just about to start this process... and at this point I'm planning to route the return through a tee with the vent like your original arrangement.

Maybe the low-fuel level in the tank allowed some part of the tank bottom to 'oil-can' in and out like a diaphragm, creating an unusually large volume of vent air blowing out and then sucking back in on that rough road?

I have a 3/4" tee for the vent (noticed that yours was originally 1/2" tee) to have a bigger cross-section through the fitting to try and keep the return fuel from getting caught up in the vent air and pushed out the vent hose. 

Also, I'm thinking about removing the rollover valve from the tank fitting. Run the vent hose vertical, up out of the tank fitting, and relocating the rollover valve to a bracket up high in the compartment.

What do you think about that plan? With a fitting like this:



Edited by wamcneil
Link to comment
Share on other sites

Posted (edited)

Walter, all I can say about my original plan is that it did not work.  I know several others that plumbed theirs the same exact way I did and they have not reported any issues.  Mine had issues and did not work as originally planned even though a couple of guys mentioned to me that it would leak and I did not believe them or thought mine would be different.  It leaked fuel out and created on hell of a mess that required several hours of cleaning.  


Ultimately, it's your rig and you can use whatever method you wish and hope it will work as I did, did but in the end it didn't work, and I wish I would have spent the time to add the second bung several years ago and it would have saved me a LOT of cleaning and stress.



Edited by zmotorsports
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...