Engine/Transmission (1998.5 - 2002) New Problem with L/P, filter and VP44

[Gary - K7GLD]

"Glad you can take a joke Gary!"



Wadda ya mean "take" a joke - some of the guys here insist I *am* a joke...

 

[Ncostello]

Big White Beast... you are totally on the right track except that if the lift pump is higher than the tank you can never have any kind of positive pressure at the lift pump inlet no matter where the fuel line exits the tank.



Not trying to change anyones mind on this but... . picture a full fuel tank and imagine a lift pump that is exactly 5 feet above the top of the tank. The total lift the pump would be required to overcome is exactly 5 feet. Lets say the lift pump pressure would show 12psi. It does not matter if the fuel line comes out the bottom of the tank or the top... and it dont matter if it comes out the top, goes down 3' and then back up... it still has 5 feet to overcome because the distance between the top of the liquid level in the tank and the inlet of the pump IS the distance to measure.



Now picture the tank has gone down to half full... . which means the liquid level in the tank is now 6" lower in the tank. The pump now has to overcome 5'-6" of lift. Now the pressure may show 11psi. Which explains why you might see a lower psi on the pressure gauge because some of the work the pump does is now required to lift fuel further rather than push it out the outlet. But the pump is doing the same amount of "net" work as it always does. So when you fill up, the pump only has to lift fuel 5' again... thus more of its work shows up as output pressure than suction side lift energy and the pressure goes back up to 12psi. This is why you see the 1psi difference Big White Beast. But it really does not matter how full or empty this tank gets... as long as the transfer pump inlet is above the top of the tank the inlet will never ever see any positive pressure regardless of where the line comes out of the tank.



The only way to make the lift pump get benefits of real positive pressure is to move it so that the inlet is lower than the liquid level in the tank. If its always getting positive pressure at all tank levels, it has to be lower than the bottom of the tank.



But for all practical purposes, its hard to get a pump that low without having to worry about it getting knocked off by a rock or dead possum in the road. I think Big Saint did it with a PE4100 however. So I'll do the best I can possibly do by mounting the OEM pump right along the frame where it sometimes will have just a hint of positive pressure and others just a hint of suction.



All in the name of making life easier on my little ole Carter lift pump and keepin it going. I think it will work... honest .



Sorry for ramblin on but theres nothing on TV tonight.

 

[Schmidt]

Guys

I am new to this ctd thing and to the TDR. I have been spending a little time getting educated on it through the forums. One thought that I have on this lift pump problem is that electric fuel pumps are made to push fuel and mechanical pumps are made to pull fuel. This is one reason why most all fuel injected vehicles have the electric fuel pumps in the tank. would it help eliminate these problems with the lift pumps if a guy would mount it on the frame as close to the tank as you can get it?????

Love this website and my CTD!!!!

Also, I drag race, a Mopar of course, and the first thing you do when you put an electric fuel pump on is put it within 12-18 inches of the fuel cell... .

 

[Big_Bad_Dually]

hole with water example

Okay, your digging a footing for a building right? But you forgot to call 1-800-digsafe. You hit the nieghbors water line and fill the hole beond full. Of course this is a massive footing for this building and it is about a ten foot deep hole.

Now that the water line is shut off you have to pump the water out of the hole. So you go to your buddy's and cobble up a pump set up. The whole thing works until the water level gets around five feet below grade. Then its like "Man I could take a leak faster".

That is because this weak little pump you and your buddy rigged up that used to be for the water bed cant "pull" enough because it is having to lift the water inthe hose five feet instead of right at grade, where with water level started.

Or because you have got this massive 8. 5 hp honda three inch pump sucking water through a garden hose that wont provide enough volume to the pump which is causing cavitation. (This is why when the l/p is providing 0psi @ idle (aka dead) you can get that little burp or loss of power when running hard above 2grand)



So to get rid of all these problems we need to start with pump close to the liquid sorce (length wise) and below most of the source (below the fuel level or level w/bottom). Pretty much like setting the pump in the water on the bottom of the hole instead of lowering hose to the bottom. Because we all know that pump (if big enough) would rather push the water out of the hole (because liquid can't be compressed) than suck the water out of the which allowes easier cavitation, loss of prime, and eventually death of pump.



Hopes this helps some people understand why our lift pumps A. go B. need to be there to deliver fuel to the VP44 clear up in the front and C. why they should be in the back below the fuel level to PUSH fuel forward.



BRIAN



cav·i·ta·tion The sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces

When a liquid begins to boil under low pressure



suc·tion A force that causes a fluid or solid to be drawn into an interior space or to adhere to a surface because of the difference between the external and internal pressures.

 

[Big White Beast]

Neil.....

I figured out where I confused myself. I ran my up and over theory through my depleted brain several times and figured where I erred.



You are correct in that with the pump higher than both sources the draw is the same, albeit different levels in the tank. My mind wouldn't let go of having to draw over the top of the tank. I kept seeing it having to draw higher than itself when in fact the top of the tank is still lower than the pump.



With my pusher mounted on the frame I suppose it would benefit from a bottom tap somewhat. The LP only sees the benefit of what the pusher throws it.



Thanks for making me think guys, only way to learn anything.



Garrett