A buddy of mine has 99' 3500 5 speed cummins that is completely stock (as far as we know). We can't get it to hold fuel pressure. just put a new airtex lift pump on it and has around 18psi at idle, when you rev it up in neutraul it drops below 6 psi. Put in new fuel filter, blew thru lines, took the plate out of bottom of sending unit thats in tank thinking it might be holding fuel from flowing thru. Don't have actual FP gauge in truck, just have warning light that comes on when driving it, which comes on in about every gear except when you really ease into it. Any ideas what might be restricting this pump and the pressure.
Note: Already exchanged two new airtex lift pumps thinking that was the problem.

 

I would put a manual gauge in to see how much fuel psi is, it just might be the sensor it self do that then see what happens

 

its easy to put a cheep mechanical on it just to test it out

 

Yeah we have a gauge that screws into filter housing, thats where we got our readings. We do need to get one in cab so we can see what its doin when he's driving. But i'm pretty sure warning light is working cause it does pull down just sitting in neutraul when you rev it up.

 

A few ideas come to mind here if you have already changed the fuel filter. First off is pulling the tank and checking to see if the pickup tube is cracked or the screen on it plugged up, and second is, what are the specs on this Lift Pump you installed. Are you sure it has a high enough flow rate and is rated for Diesel Fuel.

 

Just attache this gauge to a piece of AN4 stainless hose and run it up through the top of the hood so it sits down by the windshield wiper. It makes a nice test guage that you can see while your drivin and you don't have to alter anything to do it.

 

Yeah this pump is definitely rated to handle this stock truck, I ran it on an 02' I had with superchip in and it held up pretty good. We dropped tank and everything is fine. When we first put lift pump in it is fine right at first but after just a little bit of driving it's like it gets wore out immediately and can't keep up, we've been through 4 lift pumps, (thanks to o'reilly's for exchanging them for free.) Is it possible It may not be gettin the right power it needs to the lift pump? Guess we need to check with volt meter.

 

Might check the line coming from the LP to the bottom of fuel filter, could be kinked some since the airtech hooks up different than the stock LP. Just a thought.

Did the LP that was on the truck when he first bought it go out and did it run fine until the new airtech was put on?

Call if you need help.

 

You can also test the pump by just running a hot wire to the battery and driving around for a while to see if it keeps pumpin. That gives a dependable 12V no matter what. If this works then you know it's the trigger wire.

 

We tested the power coming to the lift pump and had 11-12 volts. We think we may have fixed the problem. We put in new overflow return valve on side of the injecor pump and so far so good. We think maybe it was stickin open and causing the lift pump to have to work so hard. His light is staying off and fuel pressure is good. Thanks for the input guys.