Duramax Diesels Forum Truck of the Week
  #1  
Old 07-05-2018, 10:28 PM
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Default Fass HD Runs but low pressure

About 3 months ago the my fuel pressure gauge that usually reads 10/9 psi jumped up to 21 and then started doing weird stuff bouncing all over the place. I chocked it up as a bad sending unit (Autometer) the other day I changed both the filter and water separator, along with the sending unit and no change. Gauge reads max 6psi and then after driving it goes to 0 if I get on the throttle.
I added a mechanical gauge today and when my digital reads 6 they both read 6 but when the digital reads 0 the mechanical reads 2/3 roughly. Both are in the FASS filter replacement bowl.

Its a pretty old unit SN 27305, should I buy a new pump or just a motor or a whole new unit?
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  #2  
Old 07-06-2018, 12:24 AM
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Sounds like a good time as any to see how FASS stands behind their product. I know they have a warranty card that says it must be turned in for warranty in the future, but you never know until you try.
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  #3  
Old 07-06-2018, 02:28 PM
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Talked to tech support they think its the return spring, advised I pull the fitting and stretch the spring out 1/4" and see If that helps any.
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  #4  
Old 07-16-2018, 10:26 AM
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Got a new spring and ball from FASS, and the new spring is 1/4" shorter than the spring I stretched, and it made the fuel pressure worse. Now it never shows positive pressure, on the stretched spring it showed 5-7 at times and then would go to 0. Should I try and rebuild the pump or just buy a new setup?
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  #5  
Old 08-05-2018, 09:06 PM
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Well its not the Pump, we replaced the pump motor today and same thing no pressure
We went on to build a new feel line to make sure the old line wasnt sucking air. Still no pressure. Wondering if there is some issue in the oem filter housing? My hunch now after we took one of the return springs in the FASS system and i stretched it out pretty far and it made 5psi for a bit then went to 0. Truck wont make any fuel pressure with just the pump running an the engine off. To make sure we put a physical gauge on and the same results.

Is it possible the factory filter head is just returning the fuel? Pressure is being measured from the FASS filter delete bowl things.
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  #6  
Old 08-06-2018, 05:10 AM
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Bypass the filter head. If you have a Fass you don’t really need it.
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  #7  
Old 08-07-2018, 04:26 AM
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Trash it and go with Kennedy!
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  #8  
Old 08-07-2018, 10:02 AM
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I may be way off but check the voltage to the pump.
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  #9  
Old 08-07-2018, 12:41 PM
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I recently worked on a customers truck which had a poor GROUND for the pump. Power was good but the ground was bad and was causing issues.
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Old 08-07-2018, 08:47 PM
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Pump runs great, power wise its 13.3 at idle
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  #11  
Old 08-07-2018, 09:03 PM
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Is that to the pump? Because I think the posts are referring to the "from the pump" wiring. If you have voltage om the negative wire then you have a bad ground connection
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  #12  
Old 08-08-2018, 01:59 AM
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Is it reading no pressure with the truck running?
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Old 08-08-2018, 09:42 AM
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Quote:
Originally Posted by 2004LB7 View Post
Is that to the pump? Because I think the posts are referring to the "from the pump" wiring. If you have voltage om the negative wire then you have a bad ground connection
^ this
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  #14  
Old 08-19-2018, 09:23 PM
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Pump is running great, after replacing the filter head with a new one and running pressure sensor off the bleed screw we got the same thing no pressure unless I pumped the priming plunger. Now today we just so happened to crack the fuel tank fill cap and noticed a seriously high amount of fuel being returned to the tank. Is there something we are missing?
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  #15  
Old 08-20-2018, 04:11 AM
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Collapsed fuel line...
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