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Help with FASS PUMP - no pressure

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I need help figuring out the next step. 

I installed my FASS pump about 2 weeks ago (along with 2 fuel pressure gauges -1 manual at the last filter & one electronic), everything has been working perfectly until now. I have driven the rig about 18 hours since the install. It worked great today, we parked in a campsite (slightly nose down), and a bit later I decided to completely air down. (We are only planning on staying here tonight). The last time I recall seeing the pressure gauge was about 15 min before parking- it was fine then 18 psi at a startup. 

When I turned the key on to air down - I noticed the fuel pressure gauge was saying only 2 or 3 psi - instead of the 18 like it normally does! Is it time to panic yet?

The FASS pump is vibrating like it’s running  but the vibrations are weaker than I remember when I installed it.

Things I have done so far…

Verified I have 12v going to FASS pump when the key is on.

Both pressure gauges say the same - an electronic (at dash) and mechanical at last fuel filter  

My tank return goes back into the tank - right in front of the diesel fill. There is no fuel stream dropping into the tank- maybe a drop or two. 

Because the pressure was fine just before the problem-  I don’t believe I have water in the fuel (I filled up 40 gallons ago - fuel gauge is just above half).

My guess is the pump died.

I have not tried to start the rig.

I do have an extra set of FASS fuel filters  - should I replace them? 

Help! We are near Missoula Mt and supposed to be in Banff on Monday.


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Some how lost it's prime??? Try going thru the first startup procedure I think I remember something like leave the second filter loose?? It's been a while since I did it...

So try loosening the second filter and run the pump... it should start leaking out that filter and the pump should get a 'normal' smooth sound...

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if we ever meet - I owe you a drink!

That fixed it. It would have gone perfectly- except when the pump primed I started loosening the filter instead of tightening it. (My brain got it backwards…). So I covered my hands and arms in diesel. Oh well… at least I am not stuck!

Thanks to you and everyone here!

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Installing the FASS filter is a smart thing to do.   You are polishing your fuel before you ever burn it.    Make sure that you always have spare spin on filters for your FASS.    Also….   You can stop changing the filters on the Coach as frequently.    My FASS setup is 15 micron primary, 2 micron secondary—- then it sends the fuel to the original filters which are I think 10 micron and 2 micron.   In theory….the 10 micron filter should never stop up.  And the second 2 micron filter ought to be able to stay a long time too

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  • 2 weeks later...

Hello everyone!

I am still having an occasional issue with the FASS pump losing prime on startup. 

It has happened four (or so) times since I installed the pump. it has happened on a 1/3 tank and full tank. I have completely redone the FASS startup procedures (including the WD40 spay into the pump). That’s how I got it restarted w/ Cubflyers advise (above).

Last time, Full tank, we were at a campground for 5 days, startup twice no issues (air-up, then move off wheel risers), drove 6 min to dump station, at dump station for 20-ish min. When going to leave the issue happens. 
when it turn the key to ignition on, my fuel pressure gauge starts up, and then over the next few seconds I can watch the pressure drop to zero. What I have been doing is hitting the starter as soon as I see the pressure start to drop. The engine starts up and then a few seconds later I can see the pressure rise to the normal 18-ish psi. Once running- never an issue.

I have checked for fuel leaks and I don’t see any.

I also have a mechanical gauge in the engine compartment and it always matches the electronic one in the dash.  

Any ideas…

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Might be time to call FASS and see what they recommend.

Not sure on the internal construction of the pump I would assume that there might be some sort of check valve to prevent fuel from draining back to tank.  Maybe there is some trash in it that might dislodge and move resulting the random problem. 

You might tire removing the hose/fitting and see if you see anything? 

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The symptoms you have sounds like the pump is losing prime somewhere between the pump and the fuel tank.  All it would take is a pinhole in the supply line, bad seal at the first filter, or a loose fitting on the supply line.  It would not leak fuel externally, because that line is not under pressure, actually either neutral or vacuum.  If you can disconnect the hose at the fuel tank and put pressure on it, fuel would leak out,? Or maybe just put some pressure on the fuel tank and go hunting for fuel leaks?

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There's a consensus of losing prime . . . .

I "seem to recall" a check valve at the FASS pump outlet that's part of the fitting (maybe it's a regulating valve).  In my case there was a temptation to replace it with a 90 elbow.  Glad I didn't.  Anyway, if there's a leaking check valve in the pump that would allow fuel back into the tank.  A leaking suction line between FASS pump and tank will do the same, and do it without showing any fuel leaks. 

You should probably call FASS and get their input also.

- bob

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My understanding is the check valve in the return line fitting is what creates the up to 19 psi of back pressure when the pump is running. As long as the filters allow enough flow, the engine pulls 1 to maybe 35 gallons/hour from the pump flow, the rest of the diesel returning to the tank and the check valve keeps the pressure up. as the filters clog limiting flow, the engine pulls from the diesel flow and the pressure goes down. the 2-3 psi is typical of air in the pump and it will not build enough pressure to push the air thru the return check valve/pressure regulator. Loosening the 3002 filter lets the air and as you found diesel "leave the system" allowing diesel to get to the pump gears. Once the air is gone, the pump builds enough pressure to open the check valve and diesel/separated air returns to the tank. If the check valve had an issue with not closing, not sure you would get the 18 psi. the other thing is that if the check valve is not closed, the 2 psi might push the air past and eventually fill the line with diesel. you probably have a pretty good size air leak if in 20 minutes you lost prime.  The 3001 pre-filter being loose or a bad seal would let air in very quickly as well as others have discussed about fuel line pin hole leaks.  

Do your fuel lines go thru the old CAPPS pump? They can develop a pretty good leak and would be in vacuum letting air in. The fact that pressure seems to be normal (18 psi) when you turn on the key and goes down may mean that the pump has fuel and as it flows to the engine and return to the tank, it sucks air from someplace in the hose a ways away from the pump.

Just a few thoughts on solving your issue.

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