I am reading 10 psi idle and 5 wot. I know it's low. Stock lift pump. Is it possible that I have too much air space in the coolant line between the isolator and the gauge? Or is it wise to assume that my lift pump is weak. Replaced a couple years ago with OEM pump. Never had a gauge in the truck before so I have no baseline to compare to.

 

Those readings sound pretty typical for a stock lift pump. If it has been a while since your last fuel filter change, you might want to change it. A restricted filter will cause low pressure readings under throttle. Also do yourself a favor and only use a good filter. I prefer Fleetguard or MOPAR.

 

Had to tie into the fuel system for the gauge, so I replaced the fuel filter at the same time. Napa filter though.

 

I would say the fuel pump is tired out and need replacement. Also I would consider on a upgrade at this point. I know the book value is 10-20 PSI normal span but I typically suggest 14-20 PSI for longevity. Remember Bosch states that 70% of fuel should be returned to the take from proper cooling and lubing of the VP44. But if the overflow valve is closed at <14 PSI then how can you have return flow... Hence my suggestion of 14-20 PSI.

 

I don't see anyone providing specs like that to help in making a decision on pump purchase. 95 gph Fass?

 

I would say any pump that can produce 90-100 GPH at minimum would be the best bet. You would also need to be able to keep 14 PSI or better to keep a happy VP44.

 

Thanks Michael! Good to see you here. I've had trouble getting logged into your site, so haven't spent much time there. But there is great info there.

Most guys are going with big HP upgrades. Me, I'm going mostly stock with upgrades that compliment the stock configuration. So it's hard to get a handle on how big to go with the lift pump. I'd like to get to the point where my pressure doesn't drop below 14, but don't want to spend anymore than necessary to get there.

I'm thinking that my lift pump is not as bad as my gauge reads. Seems that there is a lot of air in the pressure line downstream from the isolator. I'm going to figure out a way to fill the line without having to change syringes and get it flushed out. Seems that air bubbles would reduce pressure reading. Instructions said to leave an inch of air to allow for expansion, but when the air is segmented over the length of the tube it's hard to judge.

Also, when you turn off the truck, how long should the gauge hold a pressure reading? Seems like if there were no breaches the pressure would hold for a long time...even overnight.

 

I typically skip the whole idea of isolators and direct plumb and never worry again. I use air brake line and its typically rated for 600-800 PSI. Now nylon tube is a different story it get brittle and that where the old wives tale of fuel in the cab comes from.

 

How do you bleed off air from the line going to the gauge?

 

I normally don't worry about normally. Because after a few run cycles it will bleed out on its own. Then if you happen to get air in the system it will push air up in the line as well. So I normal don't panic about the air in the line. But what I do panic about is setting the needle valve properly so the gauge lags a full 1-2 seconds below making full pressure.

I had a nifty PDF article about water hammer and how it will replace a air space over time with a liquid be it fuel or water. But I'm sorry I lost it...