Engine/Transmission (1998.5 - 2002) Weird LP/FP problem

[tmacc]

I'm getting some weird FP readings. When I start the truck in the morning, I get 14-15 psi. After about 20-30 min. it may drop down to 3-6 psi at idle or steady cruise, but if I accelerate it will go up to 13-14 psi. The Cummins pump is only two months old. I don't have access to a mech. gauge. Any idea what kind of problem I have here?

 

[Gary - K7GLD]

For starters, try unplugging the power connector to the LP and replugging it - maybe a couple of times to see if you have a bad or deteriorating connection at that point...



DO get back to us to let us know if it works or not - trying to play out a theory of mine on bad LP connectors...

 

[Tom L]

tmacc/Gary,

My 02 is doing the same thing, just started it. For the last 5000 miles the new lp performed flawless, then yesterday on comes the warning light as to low fuel pressure, I changed the fuel filter, unplugged the electric connection at the lp to clean contacts, took it to Dodge, no computer readouts, installed a fuel pressure guage and it idles at 13 lbs for a few minutes, then drops to 8, then back up to 12. 5, then back to 4. 5 and stays there til you rev up the motor. All the time at idle you can hear the motor itself changing rpm, this is I beleive, when the alternator kicks in and out. When this happens the pressure readings change, until it rests at 4. 5. Truck ran fine even though this was going on. I will road test it again tomorrow ???? Tom

 

[tmacc]

Tom,

That's about how many miles my lp has on it. This afternoon, I could only get about 7 psi if I "bumped" the starter to get the lp to run. I will check connections tomorrow. I wonder if it's junk in the tank blocking the pick-up? I'm afraid to check the fuel filter till I have a spare pump in hand.

 

[fmj]

I'm having the same problem, but have 70K+ on LP. I thought it was either a lift pump or sending unit on gage. It only does this once in a while, which is frustrating as I can't put a mechanical gage on it to find out if its a pump or sending unit. I now have in hand both new sending unit and LP. The guys at Harbor Diesel seemed to think its the LP . The pressure typically reads 14 lbs at idle and towing a long hill will drop to 12. Then all of a sudden it goes to 9 lbs, and then right back up. Under load it has never dropped, only when I get to downhill or where its getting very little pedal . I will probably change LP this weekend, as I have to tow trailer next week and don't want to chance it. If it still does this after changing LP, then I will change sending unit. One way or another I'll have a spare something. Will let you know results over next week or so.

 

[Gary - K7GLD]

As far as PSI when "bumping" the starter - I assume you mean not actually starting the engine - mine with the added pusher usually only goes to about 6-7 PSI - rarely up to around 10 for that brief instant before it shuts back down - but once the engine is started, 23 PSI at idle, 20 at cruise, and about 16 fully loaded up a steep grade... Sure wish we could get a voltmeter down on those LP contacts to see what the voltage is doing during those PSI swings you guys are seeing...

 

[tmacc]

OK gang, I was just about to give Gary the nod for suggesting elec. connections, but it was the pump. 6K I picked up a new pump at Cummins ($125 BTW) and a couple of filters just for grins and commence to trouble shooting. I have my LP back at the tank BTW. Even at 3 PSI I could feel the pump running. First I wiggled the connections and that seemed to get the gauge up to 14 psi. I checked the connections at the relay and that seemed to narrow it down some, so I change the relay. (I keep a spare in the glove box) The gauge held a steady 14 PSI, so I took it for a test ride. Less than mile down the road, it fell back to 3 PSI. So..... I un-plugged the LP and the gauge went to ZERO! Plug it back in and it goes to 3 psi. With the system I have I had the new pump in in about 15 min. and the gauge read 15+ PSI. I took the truck for about a 3 mile test ride and it stayed up in the 14-16 PSI range. 11. 5 PSI when the comp is on 5/5. ... ... At a 130K this thing is starting to become a money pit! At least the PSD waited till it was paid for before becoming one. Tomorrow's 130 mile comute will tell the tale.

FWIW, the gauge always reads 15 PSI when I "bump" the starter and will hold it for about 20-30 seconds till the pump shuts down, unless of course the LP is shot.

 

[mrander]

tmacc, what cummins dealer did you get your LP from?

Last one I bought from cummins was 168, and that was from the one near BWI.



Hope mine lasts longer then 6k.

 

[JPLB22]

fmj



I had the same sort of problem you describe with my original LP ( I changed out the original at 30,200 ). On mine I would be driving down the freeway unloaded, at 12 psi, then all of a sudden with no accelerator pedal change at all, FP would drop to 9 psi. Then a minute or so later it would be back up to 12 psi. This is all on flat freeway with no change in engine load.



At that time with the original LP my idle psi was 14 and WOT was 6 psi ( no fueling BOMBs whatsoever ).



My hunch was that the check valve spring in the old LP had gotten weak. So at times the check valve would stay open allowing pressure to be lost. In the next couple weeks I'm going to open up the old LP and mod the check valve to stay closed all the time. Then I'll put the old LP back on the truck and see if that was the problem.



Jeff

 

[Gary - K7GLD]

Permanently closing off the check valve MIGHT create problems due to working the pump constantly up against maximum losd - as well as internal LP cooling issues, since fuel flow thru the internals of the drive motor would be lost...

 

[tmacc]

Well, today was only 120 miles, but the FP gauge stayed steady all day. 14. 5 PSI at cruising speeds.

mrander, are you a member of Mason-Dixon TDR? Glen Burnie (MD) Power Systems is where I got my LP also. We have a club discount there. (We can thank HAMMER for working his tail off for that. ) Worth every penny of the ten dollar annual dues.

Well off to replace the brake pads. Every 15 k like clock work.

 

[JPLB22]

Thanks for the heads-up Gary. I wonder if I can find a suitable replacement spring ?? But as I said in my previous post, I haven't opened up the old LP yet. So I don't even know if the check valve spring was the problem. Sure would be nice to just replace a spring to save a $135 pump ( from Freightliner ).



Jeff

 

[mrander]

Thanks for the info, tmacc.



No, not a member. I didn't know they existed.

I found them on the TDR Chapters page.



I think I will spend 10. 00.

 

[Tom L]

Hey all,

With my mechanical guarge to back up my electic light sys, I watched again today! Same thing as last night, so I went ahead and removed the new pump with 5ooo miles of use and re-installed the old with 39+ thousand miles, guess what, the fuel pressure stays steady at 11 lbs running 62 mph and 7 psi at mash the peddle to the mat! Imagine that, 5000 miles and the pump takes a s***! I am looking into engineering a new pump system by passing the lp and going straight to the filter. Tom

 

[fmj]

Just an update. Changed out transfer pump today. Idle is at 14, same as before, and lowest I can get unloaded up a hill is 13, did go to 12 before. I won't know if this fixed my problem until I get to tow and outside temp is 90+, as thats where the problem surfaced before. I'm towing car hauler this next week, so I'll keep my eye on gage. Spare sending unit still in box waiting, as if I get repeat of problem above, I'll change it out. Only real difference I did notice is that it returns to 14 lbs immediately now, where as before it too a few seconds.

 

[EricBu12]

I have had no problems since I did my pusher pump with a regulator and a Racor 30 micron filter before it.



Heres the diagram



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As you can see, I have the Racor filter just off the outlets of the tank, then goes to a 7# pusher with a bypass incase the pusher fails... Then to the stock pump, Now off of the stock pump, I go to a "T"... one leg goes to the filter, one leg goes to the mechanical regulator (Below)



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the through leg of the regulator goes to the FP gauge sender. the return leg goes back to the Racor filter inlet so the pusher has a small head pressure as well. The regulator needs Vaccume to steady the fluctuations



Now, when I am driving, I adjusted the pressure to 13 psi. (it is able to go over 20... . )



I have 13 idle,

12 cruise,

and no less than 10 or 11 WOT.



I have a edge EZ and a 4 wire bluechip for my mods. 331 hp at the wheels

 

[Tom L]

Hey All, I took my old(new) lift pump to Cummins for a pressure check. They hooked it to fuel supply, put a test press gauge in and connected the battery. Guess what, it pumped and showed the correct psi/gpm per spec! Now what? Well they said re-install the tested pump and drive the truck back to us and we'll test it on the truck. If it shows a problem it will be warrenteed. I am disgusted, if you have read my earlier posts pertaining to my lp and its eratic behavior. I've tried all the ideas as suggested. I also would like to know how one gets to the ball and spring bypass check valve system, my pumps seem to be factory sealed except where the cover is attached with the 3 stud screws that let you access the screen and pump vanes/rotor. Now according to the service manual there is a seperate bypass system but it is no way connected to the lp, unless you call fuel lines a connection that is. Is this the check sys that is being referred to? And why would it act up with one lp pump and not another? Any ideas guys? Tom