Lift pump = negative pressure at bleeder when running??

greiswig · **Posted:** Fri Nov 23, 2007 3:18 pm

Hi,

For those of you who have installed lift pumps, I have a question for you: I installed a Carter P74029 pump from NAPA with a 120 mesh strainer in front of it. All seems well when the engine isn't turning: the pump clicks, and if I open the bleeder, fuel starts coming out in short order. The pump is rated at 72 gph and 4-7 PSI.

If I run the engine for a bit, then open the bleeder, it sucks in instead. I even opened the fuel cap to see if that made a difference, but it didn't. So if you've installed a lift pump of any kind, I'd be really interested in whether you see similar behavior or whether you see what I would have expected...positive pressure at the bleeder even after running the engine. The whole self-bleeding circuit seems dependent on that latter fact. I could have a defective pump or strainer, I suppose. But I'm wondering even which side of the filter the bleeder is on...if you have a filter that is even partly clogged, will you end up with decreased flow, and hence vacuum at the bleeder? I've only got about 15k on the current filter, and the fuel I run through it is filtered well before it even goes into the tank, and has biocide in it. But with positive pressure on the other side of the filter element, I would expect pressure at the bleeder no matter what.

Puzzling...Thanks for any input.

BlackLibertyCRD · **Posted:** Fri Nov 23, 2007 4:13 pm

greiswig wrote:

Hi,

For those of you who have installed lift pumps, I have a question for you: I installed a Carter P74029 pump from NAPA with a 120 mesh strainer in front of it. All seems well when the engine isn't turning: the pump clicks, and if I open the bleeder, fuel starts coming out in short order. The pump is rated at 72 gph and 4-7 PSI.

If I run the engine for a bit, then open the bleeder, it sucks in instead. I even opened the fuel cap to see if that made a difference, but it didn't. So if you've installed a lift pump of any kind, I'd be really interested in whether you see similar behavior or whether you see what I would have expected...positive pressure at the bleeder even after running the engine. The whole self-bleeding circuit seems dependent on that latter fact. I could have a defective pump or strainer, I suppose. But I'm wondering even which side of the filter the bleeder is on...if you have a filter that is even partly clogged, will you end up with decreased flow, and hence vacuum at the bleeder? I've only got about 15k on the current filter, and the fuel I run through it is filtered well before it even goes into the tank, and has biocide in it. But with positive pressure on the other side of the filter element, I would expect pressure at the bleeder no matter what.

Puzzling...Thanks for any input.

It sounds like the cp3 pump suction is stronger than the output of the Carter P74029 or there is a restriction. Also there is no guarantee that you will get the air out without the bypass installed in the filter head. I run both the lift pump (not Carter) and bypass to be sure there is no problem.

greiswig · **Posted:** Fri Nov 23, 2007 5:24 pm

BlackLibertyCRD wrote:

It sounds like the cp3 pump suction is stronger than the output of the Carter P74029 or there is a restriction. Also there is no guarantee that you will get the air out without the bypass installed in the filter head. I run both the lift pump (not Carter) and bypass to be sure there is no problem.

I agree that it sounds like a restriction, but if the bleeder acts the same on other people's rigs with lift pumps, maybe it isn't restricted. I'm really interested in whether those with lift pumps can see pressure at the bleeder after running at load. One thing of note is that your Facet pump specs out as flowing less than this Carter unit does... 34 gph versus 72 gph. So I'd be REALLY interested in hearing whether you always have positive pressure at your bleeder.

BlackLibertyCRD · **Posted:** Fri Nov 23, 2007 6:11 pm

The automatic bleeder works while the engine is running with air going through the return. My CRD has to have positive pressure to do that.

truckbouy2 · **Posted:** Fri Nov 23, 2007 7:31 pm

The cp3 is designed to pull fuel from the tank. Your added lift pump has to be a FLOW-THROUGH design pump, so the cp3 is free to pull as much fuel as required. If it is not a flow-through the cp3 could be starving for fuel. Which won't take long to kill it. The 3rd generation Dodge Cummins with the cp3 operate as our Libbies. The in-tank lift pumps are a flow-through design so the cp3 can pull as much fuel as needed without relying on the lift pump. Plus with a flow-through design pump, if the lift pump craps you get to drive home. Without a flow-through you ride the flatbed home....

greiswig · **Posted:** Fri Nov 23, 2007 8:28 pm

truckbouy2 wrote:

The cp3 is designed to pull fuel from the tank. Your added lift pump has to be a FLOW-THROUGH design pump, so the cp3 is free to pull as much fuel as required. If it is not a flow-through the cp3 could be starving for fuel. Which won't take long to kill it. The 3rd generation Dodge Cummins with the cp3 operate as our Libbies. The in-tank lift pumps are a flow-through design so the cp3 can pull as much fuel as needed without relying on the lift pump. Plus with a flow-through design pump, if the lift pump craps you get to drive home. Without a flow-through you ride the flatbed home....

This Carter claims to be a flow-through pump, too.

From the Federal-Mogul web site:

"Carter Solenoid pumps allow fuel to flow through the pump even when the pump is not energized. The Solenoid pump uses a piston actuated by an electromagnetic coil, rather than a motor, to generate fuel pressure and flow."

Serendipity · **Joined:** Mon Nov 20, 2006 10:51 pm **Posts:** 45

I'm going to quote myself. It'll save some typing and boost my ego at the same time. :wink:

Serendipity wrote:

Haven't posted in a while, so I thought I'd offer an update. I am still very happy with my lift pump. I revamped the wiring to take advantage of the factory relay, and I added a pressure gauge by drilling and tapping the banjo bolt in the outlet port of the factory filter head. The gauge is on a short length of hose. With the gauge held down by a wiper arm, I went for a drive. With the pump on and engine off, I have 7.5 PSI. Started and idling, I have 5 PSI. Cruising I have 3 or 4 PSI. WOT, I have 0 PSI. The CRD really moves some fuel at WOT. A compound gauge which would show vacuum also would be slick, but I'd rather put that money toward a Kennedy pump if it will maintain positive pressure at WOT. My pump is mounted in the engine compartment and is pushing through a WIX primary filter and the OEM as a secondary. The filters are fairly fresh, and while it is not impossible, I don't suspect that they are restricted. I'd be curious to get some comparative pressure numbers from others running lift pumps. By the way, I found out the hard way that you have to either leave an air pocket or put some kind of flow restriction ahead of the gauge or the needle really vibrates when the engine is running.

greiswig · **Posted:** Fri Nov 23, 2007 9:37 pm

Serendipity wrote:

I'm going to quote myself. It'll save some typing and boost my ego at the same time. :wink:

Serendipity wrote:

Haven't posted in a while, so I thought I'd offer an update. I am still very happy with my lift pump. I revamped the wiring to take advantage of the factory relay, and I added a pressure gauge by drilling and tapping the banjo bolt in the outlet port of the factory filter head. The gauge is on a short length of hose. With the gauge held down by a wiper arm, I went for a drive. With the pump on and engine off, I have 7.5 PSI. Started and idling, I have 5 PSI. Cruising I have 3 or 4 PSI. WOT, I have 0 PSI. The CRD really moves some fuel at WOT. A compound gauge which would show vacuum also would be slick, but I'd rather put that money toward a Kennedy pump if it will maintain positive pressure at WOT. My pump is mounted in the engine compartment and is pushing through a WIX primary filter and the OEM as a secondary. The filters are fairly fresh, and while it is not impossible, I don't suspect that they are restricted. I'd be curious to get some comparative pressure numbers from others running lift pumps. By the way, I found out the hard way that you have to either leave an air pocket or put some kind of flow restriction ahead of the gauge or the needle really vibrates when the engine is running.

Good info. What pump were you using when you typed this?

Serendipity · **Joined:** Mon Nov 20, 2006 10:51 pm **Posts:** 45

It is one that I have had for quite a while, and I can't find a reference for the model number. Near as I can tell, it is the smaller of the two solenoid type pumps used with the GM 6.5 diesel.

Lift pump = negative pressure at bleeder when running??

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