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hawkeye213

Stupid fuel pressure gauge question

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Finally got my FP gauge installed correctly. Had been using the isolator but I could never get it to work right or for very long. Now I am direct to the gauge. 18.5 @ idle 16 @ wot. Stupid questions. If I start to notice a drop in pressure do you just pull over and shut it down? I think I read it takes 14 to open the overflow on the IP to cool the IP. So less than 14 you pull over and shut it down ? I know that sounds dumb but now that I have the FP just want to know what it is trying to tell me to do. If your LP went out completely the reading would be 0 correct ? You shouldn't be getting a reading from the IP sucking the fuel correct ?

Thanks Rob

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I would be concerned, but I would not immediately worry unless it went below 10 PSI, but I would swap the filters out the first chance I got, and take if from there. If the pressure remained low, then you have other problems to deal with.

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Ok... The factory minimum is 10 PSI...

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But now keep this in your head about the 14 PSI.

The VP44 has 1 fuel inlet for the lift pump. But has 2 outlets.

1. Overflow valve that opens at 14 PSI.

2. Injectors when there firing.

So if your driving along at 12 PSI its fine because there is still 2-3 GPH flowing past the electronics keeping them cool and through the VP44 pump moving parts keeping them lubed up.

But now lets change that up a bit. (Common occurance for me.) I'm out firewood hauling climb to a elevation of 7,500 ft and 35 miles of single lane dirt road. Now i'm loaded up for for the sake of the post I assume my pressure is 12 PSI (really not). Now I'm going to be jaking the whole time running down hill I will take me over 2 hour to get to the bottom of the traveling about 15-20 MPH and not using the throttle much at all. So now the overflow is CLOSED (less than 14 PSI) and the injectors are NOT firing because I'm coasting. So that mean the injection pump is not being lubed nor is it being cooled. There is little to ZERO flow. There is a tiny bleed hole in the overflow valve but its not going to help... Now for the thmubnails I used x8 magnifing glass to show how really small that hole is... :stuned:

So with my idle pressure at 17.5 PSI and WOT at 15 PSI I know the overflow valve is ALWAYS open and flowing cooled fuel through the pump. :thumbup2:

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You might see it be low when it is extremely cold and then goes up as the fuel gets warmed up from the returned fuel. You shouldn't see more than a 2-3 psi drop at WOT. I just got done pulling a fertilizer bin (10,000# empty and 18,000# full) over a bunch of hills and my pressure dropped only 1 psi. Sure is nice to have a proper fuel system (pressure gauge and after market lift pump) on these trucks! Just monitor the gauge and if you see a pressure reading that is abnormal, investigate it in a timely fashion. Our trucks eat up VP's with poor pressure.

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Well I been getting 13 psi WOT with the aidog 100 is that to low ,because the guy from pureflow told me that I could buy a spring and change it and I would pick up 1 to 2 psi so if thats to low maybe I could order that spring.

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As for the AirDog series pumps I've noticed the ball and spring have some issues. Like I found that over time of the fuel pressure pushing against the ball that the ball is literally pushed into the middle of the coil. About once a year I'll lay in the shop pull the fitting out and find the ball literally jamed into the coil of the spring. Once I pull the ball out and re-assemble the fitting and hoses the pump is back to 17-18 PSI like it was new but by the end of the year it will be dipping below 15 PSI. Solution... Make a collar for the top of the spring so the ball can't wedge it way down through the coils of the spring over time.

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Maybe this is a ignorant question, but wouldn't a VP44 temp. gauge be useful to monitor its health. If it isn't lubing, it would get hot.I know, I know, like we need one more gauge in these trucks. They are already looking like Peterbuilts with 17 gauges!

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Solution... Make a collar for the top of the spring so the ball can't wedge it way down through the coils of the spring over time.

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Well I got to find a solution to it... It's getting rather annoying... Like as I was traveling in the heat of the day the fuel would get warm and the pressure would fall way off but as the fuel cooled in the evening pressure is back up. But if I played with it bumping the starter I would see a nice high pressure and change of tone and pressure would fall... :mad:So I know its this darn check ball and spring giving me hell and the pressure would drop as low as 12 PSI climing a 7% grade... Not like the Mopar1973Man to drive with lower fuel pressure... :nono: :duh:So of course if I figure out something I'll post it... :smart:

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With the lift pump giving 12 psi there wouldn't be fuel flow through the pump for cooling but wouldn't there be fuel in the pump for lubrication?

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I'm running 2 cycle oil so I was too worried about the lubre part but knowing the overflow valve starts closing below 14 PSI and closed completely by 10 PSI I was getting a bit worried about temp. Now There was several grade where I was seeing 1100-1200*F pyrometer temps, Coolant up to 205*F, etc... So I know it was taking its far share of temp from the engine.I'm going to look into it today when I get a second...

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