Engine/Transmission (1998.5 - 2002) oddly erratic fuel pressure

[BK]

I have a 99 2nd Gen (from new), with my 3rd LP replaced about 2 years ago. Way back when, when replacing the 1st, I used the Geno's relocation bracket and put the new one I sourced from Cummins back by the tank. At the time I worked for a company and we had an account with Cummins, and I was able to get 3 pumps super discounted on the campaign they had way back when for an industrial application.

I presently have an ISSO FP gauge.

Just a month ago, on occasion the FP gauge starting showing erratic reduction in pressure at idle, say when sitting at a red light, but was perfectly fine when cruising on the highway or around town.
The FP gauge's needle would jump down and up and down, making it's way down to 5psi erratically.
Take off, slow and easy, or hard, makes no difference and it would nicely work it's way back up to 14-15psi fairly linearly, and stay there till the next red light.
The tank is filled with diesel (so thinking it's not a pickup issue??).
The truck runs just fine, nothing strange. Idles well, and nothing sounds strange when the fuel pressure changes.

Finally happened when I was pulling into my driveway, and left it running, ran out checked the sender, gave it a wack, ran back into the truck and the pressure jumped up to 15psi from 5psi.
So thinking GREAT!
Did it another time.. ok great.
It's the original snubber hose from 20 years ago. Outer jacket was all cracked up, so really needed to go.
So figured, new sender and snubber hose.
Just finished installing both, fixed the leaks in the new plumbing after a few test drives, all leaks gone, and sitting in the driveway, it started doing it again.
MF! Mood took a hit.

In parallel of the Geno's ordered, I ordered a mechanical gauge and hose, so I could co-witness the pressure (if it happened again). So installed the mechanical gauge in a port in the tee I installed with the new hose, and damn, the pressure is doing funny things again at idle-both gauges agree. The ISSO and it's sender were telling the truth.
The hose had to go anyway, but the ISSO sender was only a couple of years old and 95bux, oh well.

The issue is intermittent when it started, seems more persistent now maybe.??
When I was test driving, and it wasn't doing it's thing, with the new sender, it was at 14-15PSI at a stop and at WOT go down to 11-12. LP seemed strong. Than WACK, thought I was done for the day, and it started it's dance down to 5PSI when just sitting there in the driveway at idle.

So when I installed the last LP few years back, I bought a stock replacement FASS and associated parts from Genos, so plan installing it tomorrow (Sunday).

Any thoughts???
If it's the LP , I could see it being erratic at idle and at cruise. Not this strangeness of seemingly fine at cruise, but comes and goes with the erratic behavior at idle down to 5psi.
Can't handle the head pressure at idle? Cruise reduces head pressure so the failing pump is better?
(Fuel filter was just changed in the fall)
I'm hoping for a good reason to sleep well tonight before doing the new LP in the cold tomorrow.


TIA,
Bob

 

[Ozymandias]

I think there is something else wrong with your fuel system if you are changing lift pumps like socks.

But I've no clue what it could be... Sorry

 

[petersonj]

At the time I worked for a company and we had an account with Cummins, and I was able to get 3 pumps super discounted on the campaign they had way back when for an industrial application.

You do not mention the brand or model of these lift pumps. Some electric lift pumps return unused fuel internally - over a relief valve and back to the suction side - all done inside the pump. Other electric lift pumps pass unused fuel over a relief valve and back to the fuel tank with a dedicated external line.

If it's the LP , I could see it being erratic at idle and at cruise. Not this strangeness of seemingly fine at cruise, but comes and goes with the erratic behavior at idle down to 5psi.

The electric lift pump will show its highest pressure at engine idle because the VP44's fixed displacement internal vane pump is turning its slowest rpm at engine idle. At engine idle, fuel returning to the tank from the VP44 is approximately 18 gph - much less than the lift pump rated output. Consequently, most of the fuel from the lift pump is being returned to the suction side of the lift pump or to the fuel tank via a relief valve depending on pump model.

Although I have not personally experienced this, some 24-valve truck owners have had problems with the relief valve inside the lift pump, and chattering at engine idle is one of the symptoms.

- John

 

[BK]

22 years (will be 23ys Spring '22) and 3 LP (well 4 counting the OG under warranty), isn't out of the ordinary for the 2ndGen and that crappy OE LP.
Dodge obsoleted it with all the failures, and for the 2nd Gens with external pumps, Dodge offered an in the tank solution after "obsoleting" the Cummins external pump.
Cummins own design notes , stated even with that (Lift) Pump, was to be gravity feed. It was not to be used as a "Lift Pump".
Never mind 8ft away from the tank on the side of an engine. Bad design. The relocation kit gave it some additional life. I got 2-3 years out of the ones on the engine (2nd one to die on the side of the engine, first was under warranty), and about 12-13years on the frame. Now 3 years later, might be bad again.
This lift pump that I'm thinking is presently bad sat in its box in my basement for ~15 years before going onto the truck ~ 3ys ago. Lasts of the campaign pumps.

You do not mention the brand or model of these lift pumps. Some electric lift pumps return unused fuel internally - over a relief valve and back to the suction side - all done inside the pump. Other electric lift pumps pass unused fuel over a relief valve and back to the fuel tank with a dedicated external line.

The LPs were the same exact ones as the OE from Dodge that came from the factory...just from Cummins directly, not Dodge.
Same PNs. Just 1/3 the price via the replacement campaign I was able to get them through as a registered service shop. We worked on critical power applications, data center UPSes, station battery, switchgear batteries, and generators.
These last two lived on the frame by the tank, not on the engine as OE design.

What I have to go on next is the FASS replacement. FASS_RP_DRP
https://www.genosgarage.com/product/fass-replacement-stand-alone-fuel-pump-rp-drp/fass-fuel-system

Although I have not personally experienced this, some 24-valve truck owners have had problems with the relief valve inside the lift pump, and chattering at engine idle is one of the symptoms.

So "chattering" as in chattering of the pressure, like what I am seeing?

tnx
-Bob

 

[BoeingDiesel]

A few years ago DavidC gave me a diagram for rewiring the lift pump that would, in effect, bypass the load on the ECM. The symptoms you are describing were very similar to what I was experiencing on my '99 and this did solve my problems. I'll look through my files to see if I still have the schematic and post it for you.

 

[Bromiley99]

I recently had a similar a very similar problem with my 99, took me a while to figure out but end of the day my tank breather was plugged. Simple to check just drive around with your fuel cap off an watch your fuel pressure gauge.

 

[petersonj]

@BoeingDiesel and @Bromiley99 offer good suggestions, especially relaying the lift pump circuit. The rewiring of the lift pump using a relay is crucial as it removes a considerable electrical load from the ECM. Even if this is not your problem, I highly recommend that you perform this mod.

A bit of history on my truck - I purchased the truck in November of 2001. In 2005 at 87,000 miles, the VP44 was replaced under warranty (timing piston failure). The original engine mounted lift pump was still operating fine. However, Dodge's fix at the time was to install an in-tank lift pump which was also covered under warranty and installed at the same time as the VP44 replacement. A fuel pump relay was part of this conversion.

Fast forward a few years and miles. In 2016 at 251,000 miles on the odometer, I removed the in-tank lift pump, which was still working fine, and installed a used FASS DRP-02 on the frame near the fuel tank. I did this conversion just because I wanted an easy roadside repair should I have a lift pump failure. I currently have 360,000 miles on the odometer and the used FASS DRP-02 is still working fine.

I think the FASS lift pump from Geno's that you referred to would be a good pick for a frame mounted pump. I also recommend an in-line suction strainer just before the lift pump if you don't have one already. I use a Napa 3270 140 micron strainer.

And yes, the "chattering" could be a ball and spring rapidly opening and closing in the relief valve which could cause erratic pressure.

Let us know what fixes your problem.

- John

 

[BK]

Thanks all, great input.
The breather isn't one I heard about, tnx Bromiley99.

BoeingDiesel, would appreciate the schemo. I probably have everything I need to wire it up. I've had my own ideas of wiring it up, and always like to see what has been done.

petersonj , I bought a few from Geno's when I bought the FASS, and some other plumbing parts.

Just out to pick up some different thread sealer, after reading some last night, seems others have the same leaking issues I just had with the same permatex sealer I used and had me chasing leaks with most of yesterday. Replacement was quick chasing leaks was not. So redo all of that once I get the pump plumbed in.

 

[BK]

So all done, mystery solved,,,, T'was the LP under the frame with a rattly relief valve.
Idle about 17-18psi.
WOT hit 12psi, general acceleration, 14-15psi.

Diesely mess, but done in just over an hour.
Keep the old relocation bracket that was just for the OE pump. Used that to support the online filter.
Put the new bracket about 6" behind the rear of the Tcase skid plate.

I put a valve inline between the fuel tank output and the input of the strainer filter for future inliine filter changes.

I read somewhere, and can't find it again, about the inline strainer filter before the FASS pump, and that inline strainer filter should be changed once a year (which I change the main fuel filter once a year anyway, so one more filter to do at that time.)
Swear I read it on Geno's site, including the fact their "latest" relocation kit can be used with the FASS pump, just gotta flip the bracket around.

All the plumbing for the FP sender was leaking still.
(it was dry from sitting over night, but after the LP test drive, all wet again)
After the LP success, took all that FP sender plumbing apart.
I had used Permatex 80632, and it does not call out for a setup time.
Permatex states 80632 is good for air, hydraulics, oils, and diesel fuel systems. I'd only use it for air, maybe after a 24hr setup time, maybe.

The Permatex on the outside was hard, but inside the threads it was as thin as water and very wet. I threw out the fittings I had more of, cleaned up the onesies well.
Put it all together with the Blue Monster 76009 from the HD run this morning. And now it's all dry on one take.

There was a Permatex from HD I used when I first did the FP gauge install ~20ys ago, it was blue like this Blue Monster, and it worked first time with no setup. That permatex crap made me feel like I couldn't turn a wrench, lost 2.5hrs to that crap.

Like Anti-seize, the Blue Monster gets all over, but it's very tough to clean off anything. Brake cleaner was having a hard time getting it off my nitrile gloves. Tools clean up fine with brake cleaner and some scrubbing.


BoeingDiesel , if you can find the schemo would appreciate it. Interested if the relay was driven off the normal pump source, of if picked a "run" 12v source.

Again, thanks all for the replies and input.
Always learn something on TDR.
I'll give up my subscription once I finally sell the old oil burner red sled.

 

[BK]

So just a follow up, the FASS replacement has been doing great.

I finally made a harness and sourced the LP mating connectors.
Fire up the relay off of feed that normally would run the OG LP.
And fused line off of the battery and out of the my relay, to the FASS.
And I fused the + line from the ECM.
This is the drop in harness before I realized, need to add just one more thing...

I've had the 99 since new, and the one thing that bothered me was bleeding the fuel lines after changing the fuel filter.
I used to call out my wife "Mary" to bump the starter, so I could crack the banjo bolt.
And 15sec didn't always seem it was long enough to fiddle blindly with getting the wrench on the banjo bolt, and open , and closed.

So WIAI (While I;m At It) in my harness, wired in an extra socket across my FASS LP relay.

So I could install a "Mary" switch.. flip the switch, get the FASS LP going, and bleed.
That worked great.
(no picture)

But after some thinking, I didn't like just using a manual switch that requires to be shut off.
Not too safe of ECO friendly.
I carry the "Mary" switch in the truck incase something goes wrong with the output from the ECM, or my relay.
I can jump it, and keep going.


So created a "Mary" box (I), momentary switch fires timer and 15amp rated relay built in.
The momentary switch, when pressed starts a 25 second timer, with a 15amp built in relay, jumps out the relay in the harness, and shuts off after 25seconds.
If the momentary switch is pressed during that 25 second period, it stops the count down, opens the relay, and stops the pump.
That worked great.

But I like data and WIAI, Rev3 and "Mary" box II.
Measures Pump running current and applied voltage to the pump , well at least at that point in the harness, relative to the negative of the battery terminal.

Meters are "under glass", as well as status lights. Needed switches are mounted to the "glass".
The large middle switch starts or stops the timer.
Smaller switch to the right Zero's the Amp meter, it uses a hall effect device to measure the current in the + lead.
So now I get to see the health of the pump and it's source of power per say.

Should have done this years ago, at least in part. The meters now a days are soo inexpensive and work well.

 

[Gary - K7GLD]

"BK" posted earlier:

What I have to go on next is the FASS replacement. FASS_RP_DRP
https://www.genosgarage.com/product/fass-replacement-stand-alone-fuel-pump-rp-drp/fass-fuel-system

IT should be noted by prospective buyers, that this pump is NOT recommended for use with Biofuels, or those with added vegetable oils. Here in my area 20% diesel biofuel is available - and I use it regularly, since biofuels provide diesel fuel lubricity that is lost in later reduced sulpher diesel fuels.

 

[petersonj]

@Gary - K7GLD , that information is interesting regarding no biofuels for their lift pump. I did not know this. In April, 2016, I installed a used FASS DRP and modified it to be a frame mount. That was 7 years and 110,000 miles ago. I live in Baker, Oregon, so I have been using the state mandated 5% biofuel for years. I also have filled the tank with B20 many times. So far, no lift pump issues.

- John

 

[BK]

@Gary - K7GLD
I had read that in the instructions.
We have just regular old Diesel fuel here. So didn't give a 2nd thought.

 

[petersonj]

I just edited my previous post. I said, "So far, lift pump issues". I meant to say, "So far, no lift pump issues".

- John

 

[BK]

So while we are in the subject, what other choices are there for a replacement pump that will deal with the BioFuel spiked diesel ?

@petersonj , that's great to hear... wonder what the limiting factor in the design is that would make FASS make the statement?

 

[petersonj]

... wonder what the limiting factor in the design is that would make FASS make the statement?

It's hard to say. Remember, my FASS DRP is used. I don't know when the pump was new. Maybe FASS began seeing some pump failures over the years related to bio-diesel. This could have resulted in writing in the disclaimer for warranty. Just a thought.

I actually bought a second used FASS DRP a long time ago because I thought the first one would have given up by now.

- John

 

[BK]

It's hard to say. Remember, my FASS DRP is used. I don't know when the pump was new. Maybe FASS began seeing some pump failures over the years related to bio-diesel. This could have resulted in writing in the disclaimer for warranty. Just a thought.

I actually bought a second used FASS DRP a long time ago because I thought the first one would have given up by now.

- John

tnx, me too, was thinking of buying a 2nd one to keep on the shelf just in case.