Engine/Transmission (1998.5 - 2002) Funky fuel pressure observation

[KTobin]

I've recently installed a Fass fuel pump (pump only model w/o filter). After years of observing the stock Carter with 4600 pusher, I became accustom to an instantaious pressure spike to 17lbs upon start up and an instantainious pressure drop to 0lbs upon shut down. Fass and big fuel line are plummed to the stock fuel filter canister. However, when I installed the Fass last month, I noticed that it would build pressure slowly, to 17lbs over a period of 5-7 seconds and slowly bleed off to 0lbs over the course of say 25 seconds. Fine, I figured, it must be characteristic of the Fass pump, and acceped it. After a week, it went back to the old way of instantainious build up and bleed off. I thought that was odd but accepted it because the truck runs fine and the pressure is a steady 17lbs @ idle and 15lbs @WOT. This morning, I re-installed a 30 micron Stanadyne pre filter,post pump, that that was once on but hadn't got around to re-installing when I installed the Fass. Pressures are back to slow build up and slow bleed. It obviously has to do with openning up the lines. What would cause this? If the past is any indication, it will probably go back to instantainious within a week.



Kevin

 

[MeritBadge]

That would be air, air is compressible, liquid is not. A little air is commonly introduced during a filter change, and is usually worked out over a short period of time. I see the same thing on filter changes.



I would highly recommend you install a custom pickup tube for that FASS or a vulcan draw straw. For most cases its not a question of if you need it, but when.



Merit