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FASS Pressure Question


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I installed a FASS system (see my sig) on the weekend as per instructions except that I installed the supply to the stock filter housing (with gutted filter) to maintain my heater & pressure sensor. The other supply end connected to the stock sending unit and power directly to the battery. It did quiet down the engine a bit, less rattle.I have a small marshal 15psi mech. pressure gauge attached to the stock filter housing with a snubber. Before the install I saw 7psi @ idle with a slight drop @ wot, after the FASS install, I'm seeing only 10.8 or 11psi. I expected the advertised 16psi or at least 14. Has anyone else seen numbers this low after install? I haven't contacted FASS yet until I investigated a little further. Any comment or thoughts of where to look would be appreciated.

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id try checking FP with a 2nd gauge to verify the first one...if pressure is that low you'll prob have to modify the spring behind the return fitting on the LP or request a diff one from fassmake sure your return line from LP to your tank doesnt have a kink or tight bend in it....i have heard that will affect the psi as well.A restriction in the fuel module(in the tank) or fuel line will also affect the PSI.

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Thanks Cody.

This diesel thing is new to me, but I'm learning. I've checked the lines .. no kinks. I'm going to do the bucket test on the weekend to rule out the sending unit supply and I'm going to get a permanent in cab gauge to replace the temp one I have under the hood. If all clear, I'm going to start shimming the return spring.

Thanks again.

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Another possible option is that you are sucking air on the suction side of the pump somewhere at a fitting or cracked line. That air in the pump would greatly reduce its head.

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  • 2 weeks later...

i had the same problem contact fass and ask for the longer spring. i don't know why they just don't install it instead of the short one. they sent one out and i had to take the pump back out just to change springs. kinda sucks if you have to pay a shop cause i bet they don't reimburse you for paying twice

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Just installed an AD2-165 and had the opposite issue. Standard was 25 psi and thats too high for me. Contacted Pure Flow and they said to clip one coil off of the spring and get the pressure down around 18. Did so and its now 18-19 psi and WOT of 17 psi, never lower. You want your pressure above 14 at all times to keep the fuel bypassing the VP-44, so either get a new spring or put a couple of washers under the existing spring to get the pressure up. 10 is far too low...

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Did you big line the section between the stock filter can and the VP44?

Not yet ... I'm going to order that kit from FASS and get a proper pressure gauge before I do anything further. I haven't had the chance to investigate more .. life's getting in the way.
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