Engine/Transmission (1998.5 - 2002) fuel pressure boucing around (more LP questions...)

[Ralph Heron]

Just finished the install of the FP, Boost (piers adapter) and Pyro (before the turbo) on my "stock" 1998. 5 24V (only engine mod is the AFE). Overall install went pretty smooth (took the better part of the day) but with the help of the post, things came together pretty well...



All guages Autometer Ultralites (yeah, they dont match at night but look great during the day... ). Triple A-Pilar mount (including tweeter). Actually did some surgery on the stock cover and then glued (epoxy) the triple guage cover onto the stock cover, with a little bit of work with the dremel and sandpaper, the install looks perfect (no screws, using the stock clip brackets to hold the thing in place).



Boost reporting 18 psi when under "enthusiastic" acceleration, EGTs around 700 max (pre-turbo, unloaded, driving with a heavy foot), Fuel pressure (post filter) at 13 psi.



Mounted a 90 degree needle valve on top of the filter (formost test port, post filter) and ran a 12" length of rubber fuel line to the sender (mounted on a simple home brew bracket). Opened the needle valve 1 turn.



On startup, FP is a solid 13 psi, then with a minute of idling, pressure drops to 7 psi (and then does some jumping, back and forth to 13psi... whenever I tap the throttle, FP jumps again to 13 psi (solid as long as I avoid idle). In taking a short drive, under normal pedal, FP is a solid 13psi. After taking a short drive then returning to idle, pressure always drops to a solid 7 psi.



Now the question that I've got: Is this the lift pump (replaced about 5k miles ago) going south?? Assuming that this is the case, what is the fuel pressure draw from the VP44 (just guessing that this may be my 7 psi).



Just for my curiosity, I put a voltmeter across the LP supply pins (unconected) and read 8. 9V. Is this expected ?? Note this was in unloaded conditions (ignition on but engine off, power plug removed from the LP)



Thx in advance,



Ralph-

 

[RustyJC]

Just for my curiosity, I put a voltmeter across the LP supply pins (unconected) and read 8. 9V. Is this expected ?? Note this was in unloaded conditions (ignition on but engine off, power plug removed from the LP)

Ralph,



The LP is controlled by the ECM. Just "bump" the starter (DON'T start the engine) and watch what the lift pump supply voltage reads for about the first 25 seconds after the "bump".



Rusty

 

[Ralph Heron]

Fuel pressure (LP supply)

Hi,



What you describe is exactly what I did...



In sequence, unplugged pigtail to the LP, connected these to a voltmeter which was visible from the interior. The turned the ignition to the place just before the started engages and read the voltmeter (read 9. 82 V). I was expecting a 12V pulse of 1-2 sec but see a DC 9. 82V...



Rgds,

Ralph-

 

[Ralph Heron]

correction...

as the original post stated, the DC voltage on the pigtail (taken across both pins) was 8. 9V (not 9. 82V)...



Rgds,

Ralph-

 

[Warpig]

FP gauge

Is your fp gauge transducer electric or mechanical? If it is electric then I would guess that pulses from vp44 are causing erratic readings.

 

[Ralph Heron]

fuel sender

The sender is electric... I was anticipating that the needle might work as a good "snubber" to dampen these out (havent really had enough time to check this out at different openings of the needle value).



When the guage bounces, it goes between the 7psi point directly to the 13V point (and back again). This, in combination with the facts:



1) at warm idle, the pressure reads 7 psi

2) when driving, the pressure reads 13 psi



has gotten me thinking...



The odd thing is that during after startup, 13 psi is measured and sometime later the guage settles to 7 psi. Beyond the guage bounce, I am worried about the low LP readings at idle.



I thing I was thinking about was hooking up a mechanical guage just to check things out (during warm idle conditions).



Any other ideas very welcomed !!

 

[R-N-R]

I cracked my needle valve barely open to stop gauge needle flutter. I do not think that it will prevent wide pressure swings.

 

[seagrave2001]

Just an idea but i know when i first installed my FASS II i got a unit with a strong spring in it and i was getting 10psi at idle on a cold start and whenever the grid heaters kicked in it would drop 1-2psi due to the voltage draw... I just upgraded the spring and now i'm gettign 17psi at idle & when the heaters kick in it still drops 1psi and when i have it pedal to the medal it goes no lower then 15. 5psi... Maybe theres a bad connection somewhere or it could be the large draw..... just a thought... the best way to check would be with a mech gauge...

 

[LarryM]

RHeron, That is exactly what my truck started doing today, EXACTLY! My original lift pump failed and this new one has only 5,000 miles on it. When idling, I'm getting 7 PSI, but when going down the road it's at 13. Basically, anything over 1,000 RPM will bring the pressure up. I'm checking the pressure with a mechanical DiPricol gauge post filter, and a Snapon mechanical gauge pre filter. Now, here's the hitch--- my old lift pump failed because of bad motor bushings that allowed the rotor to cock in the housing and bind. With the way this one is acting, with the engine idling and pressure at 7 PSI, if I tap the side of the pump motor with a wrench, the pressure jumps back to 13 PSI immediately then after about 5 or 10 seconds goes back to 7. I'm almost sure that the bushings are shot in this one as well. I read several posts about Carter having bushing problems with these pumps, and came out with a modification to correct it. I don't know what the numbers are for the modified pump, or how to tell it from the old ones. I have to replace this pump, so I hope someone can enlighten me on the new pump. Anyway, good luck with yours, I'll let you know how I make out with mine.



Larry

 

[seagrave2001]

Get a after market pump such as a FASS II that comes with a 4yr warrenty and gives you constant reliable service...

 

[Ralph Heron]

Hi Larry,



Thanks for chiming in... good to see that my install and guages/sender are not the issue. I've got another LP at home in a box... maybe I'll just throw it in for kicks and see what happens (dont know if it'll happen this week as Unlce Sam and our local tax collector (property tax) needs to be shown some love... )



If you get any info on the LP serial numbers with the "affected" bushings, please send some on.



Thx again,

Ralph-

 

[Ralph Heron]

is 7PSI post filter harmful to the VP44 at idle ??

Just trying to figure out the level of urgency in running 7psi into the VP44 during idle here (given that the pump doesnt get any worse). Once I get off idle, the post filter readings are a solid 13psi. As mentioned above, got another LP in a box but no time right now to get down and dirty (Actually the change is simple once things are done right... )



On the first LP change that I did, I came to find that the studs on the top of the installed LP had no loctite but whoever did the install(factory or service) had decided to loctite (RED) the nuts on... I had a hell of a time pulling the pump as the studs backed out, top of the LP came off and the spilled fuel and parts everywhere (anybody thats done this with the fuel filter in place knows that theres not a lot of visibility down there and not much room to swing a wrench (this in combination with the studs backing out, made the change a pain in the ***)... )



On installing the new LP, I found that the studs ALSO had no loctite so I pulled em and replaced them with loctite RED on the threads... this time it should be much easier...



Any thoughts on the 7PSI at idle would be appreciated...



Ralph-

 

[LarryM]

Hey Ralph, Here is some info on the lift pump that may be relevant. This lift pump has two modes of operation---1. It will run at 100% duty cycle when the engine is running, with a minimum of 10 PSI at idle. 2. It will run at 25% duty cycle when the engine is cranking with a minimum pressure of 5-7 PSI. This is all controlled by the ECM which sends full battery voltage to the pump for the 100% duty cycle and a lower voltage for the 25% duty cycle. My shop manual doesn't mention what that lower voltage is, so your reading of 8. 9 volts may be correct. Just for thought, the ECM uses input from various sensors to know when the engine is running or cranking, so it could be that there is a problem with the ECM or one or more sensors that it gets info from that could cause the pump to go to the 25% cucle. In my case, with the engine running and the reading low I can mechanically agitate the pump and it goes immediately to 13 PSI, so I'm sure it's the bushings again. I can't find anything that says that the ECM will throw a code if the outut of the ECM to the lift pump is low or improper. Anyway, for what it's worth... .



Larry

 

[LarryM]

Well, here's the answer, at least from my side. I replaced the pump with one from the local freightliner outfit--$157. 00. The parts man said that he didn't know of any newer/modified pumps so I looked at several and picked the one that had a "AA" stamped after the number. My old one had only one "A". This pump puts out 14 PSI at idle and 10. 5 to 11 wide open and 20 inches of boost. The problem is gone, at least for now. All I did was change the pump. This is the third pump in 76K miles, the last one only went 5K before it started going south. I tore the old one down, and it had bad bushings just like the other failed pump. The motor armature was rubbing and slowing it down. I don't know why the pressure comes back up at over idle rpm, maybe it helps to relieve the pressure on the pump and the armature realigns. Strange. . anyway, I hope this one lasts and the info is of some help to you Ralph. Let us know how you make out with yours...



Larry

 

[JohnMcIntyre]

FASS II info

In my experience, the analog readout pressure gauges are not so sensitive to actual pressure changes and keep a steady/average readout - most of the time!

The digital gauges however are MUCH more responsive to pressure changes typically and will jump around trying to give you an accurate, upto the second readout.

Most digital gauges have a paramemter setting that can be tweaked by the user which alters the sampling time, IOW, it'll take a value reading at pre-determined intervals and produce an "averaged" readout value.

It may be worthwhile experimenting with the range of parameter values available to see if you can get your gauge to be less jumpy?

Here's the FASS II details mentioned earlier in this thread. Hope this helps!

 

[Ralph Heron]

Closing... not the guage, just the F-ing lift pump again!

Okay this is number 4... original got 30k miles and then took out the VP44, next one did pretty good and took me another 70k (replaced just to be safe before guages were installed). Third one only lasted 5k miles which prompted this thread ( as all know, guages are money well spent!).



Installed my backup and all is fine... at startup or idle, post filter see 13psi, WOT around 10. 5 psi... Good to close this chapter (for now!)



Does anybody have a line on reasonably priced LPs ?? (need to get a backup)



Ralph-