Engine/Transmission (1998.5 - 2002) Ever seen lift pumps do this?

[c-hawk]

On my third lift pump in about 4 months. The last two have died the same way...



Fuel pressure is good for a time. Then one day while going down the road the fuel pressure will drop from about 13psi at cruise to about 5psi. After a random amount of time it will jump back up to 13 where it will stay for a random amount of time. This will repeat, but it happens more often and stays at 5psi longer. (the wierd thing to me is ANY time it's at 5psi i can make it jump back UP instantly, by stepping on the go pedal) Eventually at cruise it will spend almost all it's time at 5psi and then start falling farther. 4psi, 3psi, 2psi... the end.



Anyone ever seen pumps go out this way? It seems wierd to me that at cruise it won't maintain pressure but by leaning on it the pressure comes right up. Also, two pumps in a few months this way is maybe more then a coincidence???



Thanks,

Chris

 

[johncameron]

My last 2 died the same way. One word, FASS. Problem solved.



John

 

[c-hawk]

John,



My other truck has a FASS... my experience with it (actually 2 complete pumps and one new motor) has been that it is unreliable. If i spend any money, ever from here out on an upgraded fuel system it will be mechanical, either a mechanical lift pump running off one of Don M's cams or possibly a rasp.



Chris

 

[Mad Bomber]

On my third lift pump in about 4 months. The last two have died the same way...



Fuel pressure is good for a time. Then one day while going down the road the fuel pressure will drop from about 13psi at cruise to about 5psi. After a random amount of time it will jump back up to 13 where it will stay for a random amount of time. This will repeat, but it happens more often and stays at 5psi longer. (the wierd thing to me is ANY time it's at 5psi i can make it jump back UP instantly, by stepping on the go pedal) Eventually at cruise it will spend almost all it's time at 5psi and then start falling farther. 4psi, 3psi, 2psi... the end.



Anyone ever seen pumps go out this way? It seems wierd to me that at cruise it won't maintain pressure but by leaning on it the pressure comes right up. Also, two pumps in a few months this way is maybe more then a coincidence???



Thanks,

Chris

Happens alot that way this is how you can tell they are going T/U

 

[dieselshu]

Check the electrical connection, spray it with water displacing compound and remove any salt or corrosion on the pins. It might be just that simple. I switched to Scotty Smart flow Fuel system, tried and proven for many years now. My pump is running strong and has great pressure 14-15idle wot 11psi.



Goodluck

Dave

 

[randrade]

I installed the Vulcan Big line kit and have had good luck. Pressure is 14 at idle/cruise and 11 at wot with comp on 5x5 couldn't be happier Oo. Oo.

 

[Shovelhead]

My last two were replaced after experiencing those exact same symptoms.

Pump #1 went 48,000 miles.

Pump #2 went 46,000 miles.

Pump #3 went 12,000 miles.

Now on pump #4 and my LAST Carter-style pump.

I'll be changing to something else (other that the HIGH $$ FASS) in the spring.

 

[SnoKing]

So have you guys pulled the fuel pump relay and put dielectric gease on the tabs and reinserted it. I did that when my truck was new in June or July of 2001, my fuel pressure has always been the same. 14. 5 at idle, 10-12 cruising and around 8 at WOT. Someone posted to do this way back then. SNOKING

 

[rubberneck]

The fact that the pressure comes up when you stab it makes me think electrical also. That is strange. I would look around a bit for a cause before condeming another pump. 3 pumps in that short of time sounds funny. Oh yeah, and im with you on the replacement options. I cant see spending that much coin on a replacement electrical option when for a few hundred more you can have a cam and 12v mechanical lift pump. I know that route isn't for everybody, but it will be for me.

 

[johncameron]

The fact that the pressure comes up when you stab it makes me think electrical also. That is strange.

My thoughts were if the pump is dying, it will pump better without any restriction. At no or little throttle the LP has to push fuel at 14PSI through the bypass valve. When you stab it th pump uses the fuel and it is easier for the LP to prime and push fuel with less restriction. As soon as you let off the flow reduces, pressure rises and the LP cavitates or looses prime. That would explain the rise in pressure under throttle. My last 2 stock LP did the exact thing and after installing my FASS pump WITHOUT touching the guage setup I have not had fluctuation problems.



John

 

[Texas Diesel]

So have you guys pulledone the fuel pump relay and put dielectric gease on the tabs and reinserted it. I did that when my truck was new in June or July of 2001, my fuel pressure has always been the same. 14. 5 at idle, 10-12 cruising and around 8 at WOT. Someone posted to do this way back then. SNOKING

I am having the same problem and will try this ASAP. Good suggestion!

 

[c-hawk]

Check the electrical connection, spray it with water displacing compound and remove any salt or corrosion on the pins. It might be just that simple. Goodluck

Dave

I'll double check this. Thanks for the suggestion. I'm assuming you mean the connection where the wiring harness plugs into the bottom of the lift pump?



Chris

 

[c-hawk]

So have you guys pulled the fuel pump relay and put dielectric gease on the tabs and reinserted it. I did that when my truck was new in June or July of 2001, my fuel pressure has always been the same. 14. 5 at idle, 10-12 cruising and around 8 at WOT. Someone posted to do this way back then. SNOKING

Thanks for that idea. I never thought to try that. Now i've just got to locate the fuel pump relay and i'll give that a try.



Chris

 

[c-hawk]

The fact that the pressure comes up when you stab it makes me think electrical also. That is strange. I would look around a bit for a cause before condeming another pump. 3 pumps in that short of time sounds funny.

That's what i'm wondering. Maybe i just got some really bad pumps, but it's starting to seem like something else is causing them to live a short life. I'll try the suggestions above and hopefully get a few more miles out of this latest one.



Chris

 

[c-hawk]

My thoughts were if the pump is dying, it will pump better without any restriction. At no or little throttle the LP has to push fuel at 14PSI through the bypass valve. When you stab it th pump uses the fuel and it is easier for the LP to prime and push fuel with less restriction. As soon as you let off the flow reduces, pressure rises and the LP cavitates or looses prime. That would explain the rise in pressure under throttle. My last 2 stock LP did the exact thing and after installing my FASS pump WITHOUT touching the guage setup I have not had fluctuation problems.



John

That could be too. Heck, i don't know. Just before i replaced this last one, i bumped the starter to run the pump. It built 15psi and held if for about 5seconds, then it dropped to 5-6psi and the sound of the pump changed. It did sound like it was cavitating.



Maybe there pick-up tube in the tank is restricted? I wish i understood this stuff better so i could do some logical troubleshooting.



chris

 

[blacksheepdiesel]

My Airdog has 100,000 mi with ZERO issues!



PM me for a price on a new one.



Trent

 

[Jeff Knight]

Hey Chris,



I'm with you on this. Understanding whats going on is a big plus. I just had the in tank pump installed under warranty. At this point its DC's problem. With 22000 miles left on my warranty i'll let them pay. After that i'll take steps to help my VP44 survive.



Jeff

 

[Gary - K7GLD]

Personally, I suspect you are seeing a fuel cavitation issue.



At about 16 PSI, the stock LP is at the ragged edge of cavitating and beginning to severely foam the fuel - which then causes loss of PSI/flow until system PSI is reduced. The clue here is, that as soon as you hit the accelerator, engine demand allows the passage of fuel thru the LP, with a corresponding drop in internal PSI *at the LP!*.



Of course, in this scenario, the cavitation and loss of LP PSI simply returns once low engine fuel demand again causes fuel PSI to rise to about 15 PSI - and the whole scenario is again repeated.



SO, why does it take a while before a new pump starts showing the symptoms? Maybe because a NEW pump has yet to loosen up, pump vanes to seat, and brushes to fully seat and wear in - in other words, the pump new out of the box isn't yet operating at maximum power and flow, so is more tolerant of operating pressures - it isn't yet as close to the ragged edge of cavitation.



How to correct or avoid the problem? Maybe install a pressure regulator at the LP to slightly reduce top PSI it sees down to around 12 PSI or so. OR, do as I and others have done, and install a pusher pump down near the fuel tank that adds input PSI and fuel flow to the stock LP, helping avoid cavitation.



I've had a Carter 4600 7-9 PSI pusher pump installed to assist my stock LP since my truck ('02) was new, and in over 4 years and 40,000 miles, had NO pump failures of any kind, and an idle PSI a bit above 20 PSI, full load PSI around 15 PSI.



#ad




BUT, I have ALSO duplicated the tendency of the stock LP to cavitate while doing some bench test research:



#ad




As fuel PSI exceeded a bit over 16 PSI with the LP under test, SERIOUS cavitation resulted, vast fuel foaming in the pump as visible inside the clear tubing used in my test - and normal flow could NOT be restored until the LP was shut down, foam subsided, and clear solid fuel again at the LP.



Naturally, the acceleration of the engine creating fuel flow thru the LP, OR the added fuel supply of an added pusher pump WILL restore fuel flow without need of a total system shutdown in normal vehicle operation.



At least, that's what my own testing has shown, and my personal solution of the added pusher pump at the cost of under $100 and a mornings work has so far avoided any related fuel or pump problems.



YMMV

 

[R-N-R]

Is there a chance that when your pressure drops (due to a failing LP) and you increase throttle, the VP is sucking fuel thus effectively bringing the LP temporarily back to life?



Replacing the stock LP is a very temporary solution, replacing the fittings and/or lines with free flowing style will increase the life, moving it from the rail to the frame will increase lifespan as well. Adding a 2nd pump to the frame is also a way to help increase longevity. Doing all of that, lines/fittings/2nd pump will come close to if not match the cost of a FASS type unit. If your going to spend that kind of $$ on a fuel pump, you might want to consider stretching a bit more and check out a RASP (mechanical) pump. The best part about upgrading the carter setup can be done in $$ increments, but you have to be aware that eventually the carter pumps will fail, it is just a matter if when. FASS seem to have a good track record, if nothing else at least you can replace brushes much cheaper than replacing a Carter. The RASP is still fairly new to our market, but logically the mechanical pump should be more reliable than any electric model, however you will have a belt that needs to be replaced at some point in time. The Rasp also retains the stock lift pump so you most likely will not be stuck on the side of the road or injure the VP when the primary LP fails. btw, I have a duel Carter setup and no $$ interests (gain) in any of the above.

 

[rweis]

Originally Posted by johncameron

My thoughts were if the pump is dying, it will pump better without any restriction. At no or little throttle the LP has to push fuel at 14PSI through the bypass valve.



The lp (whatever you have) only supplies fuel to the VP vane pump. The overflow valve is AFTER the vane pump over in the feed of the high pressure section and works somewhere above the vane pump operating range of around 116psi - 362psi. I do not know exactely what the psi is, but it is WAY above the 14 psi of the lp system. Source is p 214 - 215 Bosch "Diesel-Engine Management" 4th edition. Good book BTW describes exactely how the Bosch pumps work and includes the P7100 and VP44 series.



I have a RASP that is designed with a bypass return to the tank to keep the fuel moving unlike the Carter OEM.



Gary has done the experimentation.



Bob Weis