Engine/Transmission (1998.5 - 2002) Riddle me this...3 LPs doing weird things

[Sticks]

LP on the truck was doing fine, 15psi idle, 13psi cruising at 70, 11. 5 psi WOT on an issopro mechanical FP gauge post filter, and the LP mounted on the frame rail by the tank... upside down.



One day I am on the highway and I noticed it dropped to 7psi. I get off at the next exit, and it is idling at 9. OK I rev the motor up to 2k and it drops back to 7 then it jumps to 14, back to 7, I let off the throttle and it can't make up it's mind to be at 9 or 14psi at idle. I get out, disconnect the LP pigtail and reconnect it, back to 14 and it stays. This continues for a few more days until yesterday I had time to swap out with a couple of used spares that I had at home.



First one. Hooked up and I bump the starter. Starts out fine then gets louder and it pegs my 30 psi FP gauge. WTF?!? I shut it off, disconnect the plug, put it back in and try again. Same thing. OK, we will try the next one.



I get it on and it only reads 9 psi. That wont work either so I put # 2 back on and it jumps to 30 again. I whack it a few times with the 22mm wrench I had and it drops to 12 psi and holds it. I start the truck and it pegs the gauge again. I whack it again and it drops to 12. I rev the motor up to 2k and it drops to 8. Well that is not going to be acceptable.



I take all three LP in the garage and pull the top covers off. All three are very clean, no debris. All three spin about the same, and none of the vanes on the impeller are sticking. I wipe them all off (the impellers and vanes now that my greasy hands have handled them) and re assemble all three. Next I pull the Fuel Filter to check it. It has been in for 4 tanks of fuel and it is unusually clean. Still mostly white confused: isn't the cetane supposed to turn them black? I add staydyne (sp) every fuel up) and no debris in the bottom of the canister.



I reinstall # 2 and it starts at 12 psi and stays there, but still drops to 8 at 2k RPM and no load... Too weak and I don't trust it to jump to 30 at an inconvenient time. #3 gets installed, and now reads 10 psi, still too weak. I put #1 back in and get 15psi. I rev the motor up and it drops to 14. Good enough to get me out to Cummins to buy a new one for $171 mad: what happened to $150 that I have been reading about???)



The whole 20 mile trip out and back my FP stayed where it is supposed to. The new LP is in the tool box waiting to be justified.



Question #1 - What would cause an LP to jump around between 2 different pressures? Was something causing one of the vanes on the impeller to stick, or is this an electrical issue (I was not able to get a volt meter hooked up while this was happening)?



Question #2 - What would cause an LP to peg a 30 psi gauge and go back to reasonable after whacking it with a wrench?



Question #3 - Where is the relay and fuses for the LP? Not in the PDC, I looked and the fuel system relay that is in there is for the VP44 - and does not kill the LP when removed, nor do any of the other "might be related" relays.



NOTE - Changing a LP is only about a 3 minute job, and the only fuel I lose is what's in the pump. Valves are nice

 

[Tcolesanti]

I had my second lift pump do the same thing. It would idle at 15psi, WOT 10. 5psi and cruise at 11psi. Then one day, it jumped down to 9psi at idle, 6psi at cruise and 3psi at WOT. The lift pump was only three months and 10,000 miles old. It would jump between said pressures on a regular basis for about a week until I just replaced it with a FASS setup. I chalked it up to be the lift pump going out, but, a stock lift pump producing 30psi sounds strange since they are factory regulated at 15 or 16psi. That pump I would say has a regulator problem(i. e. sticking). Hope this helps, I would just wait and change the lift pump when the old one dies.

 

[Gary - K7GLD]

Sounds like the internal (infernal?) bypass valves are wandering in spring tension or physical contact - the one up at 30 PSI musta had the bypass ball stuck from one thing or another - did you try actually pushing in on the ball bypass when you had the pump apart?



Don't overlook the possibility of arcing and deterioration of power contacts at both the pump and plug - those can cause erratic voltage and pump operation too, even on a new pump, where only ONE set of contacts is renewed...

 

[Straight6Jeff]

Th other thing to consider is that your guage is going south, or, if it is electrical guage, the sending unit. I am on my 4th isspro guage, and it bounces all over the place. I hve a new Hewitt guage w/isolator coming from MassDiesel at the end of the week.

 

[Sticks]

Originally posted by Gary - KJ6Q

Sounds like the internal (infernal?) bypass valves are wandering in spring tension or physical contact - the one up at 30 PSI musta had the bypass ball stuck from one thing or another - did you try actually pushing in on the ball bypass when you had the pump apart?

Where is this bypass valve located? I see no valve looking in either the input or the output ports, and inside is just the screen, and the impeller hub with 4 vanes.

Originally posted by Straight6Jeff

Th other thing to consider is that your guage is going south, or, if it is electrical guage, the sending unit. I am on my 4th isspro guage, and it bounces all over the place. I hve a new Hewitt guage w/isolator coming from MassDiesel at the end of the week.

Well, since the gauge gave different yet the same readings for all three pumps, I can rule that one out.



I am iffy about the connections, and Friday, I cleaned both ends of the now 7 foot pigtail, and the pump, plus a dab of dielectric grease.



The 30psi one is a big question in my mind. Should not have happened. Is there a valve in the VP that could be sticking open, thus the low pressures?

 

[lmills]

check the electrical connections at the lift pump first

 

[djacy99]

Take the cover off and look inside the area where the banjo bolts thread. There is a channel from the in to the out . It has a ball and seat with a spring behind it. It could have dirt in there and that would cause the 30 psi to 15.

 

[Gary - K7GLD]

As to the ball valve, it's tucked up inside as mentioned above on the output side of the pump, and you can use a suitable allen wrench or similar to reach in and try to depress the ball you will barely see inside - it should be seated, with no spring visible, and should give decent resistance to movement - but not sticky - and return smoothly to the seat when force is removed...

 

[AKAMAC]

"LP mounted on the frame rail by the tank... "



"upside down"



I think that the pump needs to be mounted with motor down. My new pump is stamped "This side down" on base of motor. The devil is in the details. Mike

 

[AKAMAC]

Sticks, did you ever figure it out? I am curious if these pumps can be mounted wrong side up. Thanks Mike

 

[Sticks]

Right now I still have the one I started this whole mess out with... The one that could not decide if it wanted to run at 14 or 7 psi. The new one is sitting under the seat with the tools needed to do the replacement.



I don't know if it was the repeated plugging and unplugging of the pigtail, or popping the cover off and cleaning the innards that did it, but it has been running normal. 14. 5 at idle, 13 highway, 11 WOT.



As for the other two pumps, they are still sitting in the catch pan. I am going to look for this valve you guys were telling me about this weekend on the high pressure pump, and do a comparison of the impeller assembly to see if there is unusual wear differences.



If it wern't for the fact I already have too many projects on my plate to get me through the summer, fall and winter, I would request that anyone with a failed LP send it out to me to let me take a look at them. I am beginning to think that half of them are sill good, they just need serviced (READ - take the cover off and clean the impeller, screen, and rinse out the housing). Think about it. 1 tiny piece of debris (sand, metal shaving, dead bug, rust) gets between the vane and the impeller hub, thus causing to to get "wedged" in the closed position and can no longer move fuel. 1/4 of the pumps ability has been compromised, and the pump runs erratic. 2 vanes get stuck, and now it has lost half of it's ability. 3 vanes and it is no longer able to move fuel.



One of my questions never got answered. Where is the relay and or fuses for the LP circuit?

 

[lmills]

I think they are in the Power Distrbution Box

 

[KTobin]

Originally posted by djacy99

Take the cover off and look inside the area where the banjo bolts thread. There is a channel from the in to the out . It has a ball and seat with a spring behind it. It could have dirt in there and that would cause the 30 psi to 15.

I just dismantled my LP that was replaced this week and there is without a doubt, NO ball and spring in it. A small screen mesh on the in side and a direct line from the vanes to the out side. What's up with that?



Kevin

 

[Sticks]

AH HA! See, i'ts not just me!

 

[Sticks]

Originally posted by sticks

I am beginning to think that half of them are sill good, they just need serviced (READ - take the cover off and clean the impeller, screen, and rinse out the housing). Think about it. 1 tiny piece of debris (sand, metal shaving, dead bug, rust) gets between the vane and the impeller hub, thus causing to to get "wedged" in the closed position and can no longer move fuel. 1/4 of the pumps ability has been compromised, and the pump runs erratic. 2 vanes get stuck, and now it has lost half of it's ability. 3 vanes and it is no longer able to move fuel.

Well, it's been a week. Nobody want's to comment on this?

 

[Gary - K7GLD]

The ball valve is tucked well up into the output side of the pump channel - it IS hard to see with the pump in one piece. but it IS there, guaranteed!



It is somewhat recessed into the motor side of the housing, and you would probably have to "feel" you way into the area where it is located with am "L" shaped device like the allen wrench I mentioned further back in this thread - once located, try to depress the ball - it should have good resistance against the spring behind it - but move freely...



You guys aren't really gonna make me take ANOTHER one apart to show ya, are ya!

 

[Gary - K7GLD]

HOLD THE PHONE!



I am WRONG!



The pressure relief valve is on the INPUT side - and the ball is NOT accessable from the pump side - at least not easily - old age and Alzheimers strikes again...



I am posting a new thread on the construction of the LP - and will come post a pointer here when I get it done... Sorry for the misinformation...

 

[Gary - K7GLD]

Here's the pointer to "anatomy of a lift pump"...



https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=108275