Engine/Transmission (1998.5 - 2002) Fuel pressure gauge near 0 at very low fuel levels...

[hammersley]

OK - since fuel prices have been going through the roof I've been running on the 2nd half much more than the 1st half..... I have an electronic FP gauge that is hooked up to my FASS. The last two weeks I've noticed that if I get my tank low 0 light on - I'll get an aratic FP measure - off the 15# max that my guage shows - flutters at 14+ to 15 and then all of a sudden twice I've seen under 3 and than at 1. 5. RIP (Source Automotive) put me on to the fact that I may have some stuff in my tank that only shows up when fuel is very low. He suggested that I shut it off and see if the guage goes back up to normal... ... his thoughts were that maybe the fuel pickup has something on it and when you stop sucking - it'll drop off. That seems to be the case.



Now - what can be done - there is no low point drain on the tank - so has anybody retrofitted one? How do you get all the CRAP out of the bottom? Also is there anything like a cloth screen that may be loose?????

 

[DavidC]

I have not had mine apart but somewhere here there is a post where someone came up with a sock like screen on the pickup tube. Be pretty hard to put a drain valve on the plastic tank. Most that have to drop the tank run till almost empty but I imagine if you removed the fill tube you could fit a hose down there and siphon or pump into 5 gallon jugs. As for cleaning the tank there is also a post lingering around here on that subject. Your best bet may be to remove the pickup and check the sock or look for a crack in the pickup tube at the low level, I understand it is all plastic.



Dave

 

[hammersley]

Plastic Tank - I would have thought there would have

been a very convenient super solvent typ glue that would allow a drain to be installed... ... .

 

[steved]

OK, this is what I was told about the FASS:



I have an LED low pressure light in the cab... and a guage out by the CP3... the light will sometimes flicker when I hit a bump, which could be the same symptom you are experiencing with the guage... I obviously can't see my guage...



I was told that the way the FASS is built, you can have air build up in the particulate filter that will "shake" loose when you hit a bump... this pocket of air will then hit the return checkball, and since air has different physical characteristics than fuel, it will cause the pressure to suddenly drop (air doesn't compress like a fluid) which in turn causes the light to flicker...



I buy this since I ruled out an electric problem and the fact that different fuels cause it to act up more than others... you might be getting to the point of being so low on fuel that you are starting to suck air... or possibly have a pickup issue within the tank...



Just my thoughts...



steved

 

[nps]

... and since air has different physical characteristics than fuel, it will cause the pressure to suddenly drop (air doesn't compress like a fluid) which in turn causes the light to flicker...

Air compresses far easier than a fluid - that's why your brake system is fluid-filled, and air brakes require compressed air to operate.



Hard to believe an air bubble could cause a measurable drop in PSI.

 

[steved]

Air compresses far easier than a fluid - that's why your brake system is fluid-filled, and air brakes require compressed air to operate.



Hard to believe an air bubble could cause a measurable drop in PSI.

I think we are talking more than a small bubble... I got the impression that the outer portion of the filter would get partially filled with air (doing what it is supposed to do) and then that trapped air would escape through the media into the return line catastrophically... there really isn't anything to "force" the air through the filter media since the whole system is pressurized at that point... liquid tension on the media surface would be enough to prevent the air from passing through the media.



And there again, I don't think it is the fact the air compresses differently (the air is already under pressure in the FASS system), but that it passes the check valve differently than fuel does... sorta like having air in the waterlines of you house... the air will escape far quicker out of the same size orifice than the water... creating a drop in pressure and a "spike" when the air is exhausted and the water hits the restriction...



In a mechanical guage you might not see it "look" as bad as it comes across an electrical guage or light that can react faster... might only be a "vibration" in a mechanical guage... I mean again, it only causes my light to flicker once in a while...





steved

 

[hammersley]

Mine is an electrical

guage running right off the FASS... ... . Full tank no problem - less than 1/4 fluctuation starts.....