Archived slowly decreasing lift pump pressure on trip out west, need ideas.

[dillrepair]

2002 6 speed manual 2500 cummins with fass ddrp mounted on frame rail, edge jwa on conservative mode for towing, lift pump wired direct to battery with relay, otherwise stock except for bigger air filter and aftermarket exhaust.

pulling 36' rv trailer to my travel job in nevada... before trip replaced fuel filter, oil, diff fluids, transmission fluids... no issues noted. had fuel pressures consistently at 16 at idle before and during the first part of the trip.... began to notice slowly decreasing lift pump pressures down from normal ~13 while towing at 55mph and ~45% load.... began to slowly creep down so that it could not maintain greater than 8-9psi towing at those speeds. appeared to be progressively getting lower over time no egt or other issues.

replaced fuel filter first.... no change. replaced fuel strainer before fass... no change. installed spare stock carter lift pump (brand new)... now even lower lift pump pressures (cannot stay above 7 psi under any normal load or accelleration without the trailer hooked up). had a dead pedal condition once after getting off at rest stop 400 miles ago, but had been running cruise control for a long time and apps immediately reset itself upon shutdown/restart.

no drivability issues at all. no change in power, just a slowly decreasing fuel pressure... now stopped in cheyenne for fear of getting into remote area and possibly killing an injection pump. i'm at a loss at this point any ideas? i have to be at my job in reno in 5 days and have at least 2 solid days of driving to do yet so any help or advice is much appreciated.

edit: forgot to mention i checked voltage at the lift pump as well and my alternator.

-thx, pat.

 

[GAmes]

Could your pressure gauge be failing?

 

[dillrepair]

Could your pressure gauge be failing?

It's possible of course. I had a mechanical gauge as well but it has long since failed. The Guage I'm getting these readings from is part of the edge kit. This mechanic says the apps is bad and there's a dead spot in it. And wants to replace it but will also throw a mechanical gauge on it quickly to check my concerns. It's hard for me to reconcile what the fellow on blue chip diesel writes about dead pedal and the vp44. I'd certainly rather have it be the apps bad but I also don't want to die on a mountain pass when my fuel pressure drops to zip.

 

[dillrepair]

Do electric fuel pressure gauges die a slow death like that?

 

[GAmes]

I only suggested the gauge because you haven't had any power or drivability issues. I know next to nothing about computer controlled engines.

 

[GAmes]

My electric fuel pressure gauge is an Isspro EV2. I have it connected via a toggle switch to another sender for oil pressure. Recently, when I had oil pressure selected (because my factory gauge kept dropping to zero) I found that the pressure readings got higher as time passed. I'm blaming it on a poor connection at the toggle switch because I rarely select oil pressure and I have no other idea. So as for "slow death". I wouldn't be surprised.

 

[petersonj]

no drivability issues at all. no change in power, just a slowly decreasing fuel pressure...

You didn't mention the place of origin of your trip. If your trip began at a low elevation and if the suction side of your fuel system has a low level restriction, the dropping fuel gauge pressure may be a result of a slow climb in altitude, as your are now at about 6,000 feet in elevation in Cheyenne, Wyoming.

Even though you have a frame mounted pump, it is atmospheric pressure that pushes the fuel into the pump. At 6,000 feet there is significantly less air pressure to do that, so a small restriction in the suction part of the circuit may go unnoticed at a lower elevation. For an example in the hydraulic field, many hydraulic pump manufacturers require their pumps to have larger suction line diameters at higher elevations (above 5,000 feet) for the same reason mentioned above.

This may not be your problem at all, but if it is, you can expect the symptoms to get worse when you leave Cheyenne and if you continue west on I-80 going over a 9,000 ft pass and then staying at 7,000 to 8,000 feet for a good part of Wyoming.

If this is the case, I would not be too concerned, as the fuel pressure should stabilize and then slowly rise as you drop in elevation. Sometimes gauges can be your enemy.

- John

 

[dillrepair]

guy at the shop says he hooked a mech gauge up to it and fuel pressure is fine... in the mean time i have a new overpriced apps which seems to help thus far... while he was working i went and got a mechanical pressure gauge of my own to put in the old broken one's place so we will see shortly whats up. i guess my fass ddrp probably isn't bad so i will put that back on as well.

 

[dillrepair]

john - thanks for the reply... i am coming from wisconsin at a bout ~700 feet elevation. i was worried about the scenario you suggest as well, hoping it wasn't a restriction on the pickup. hopefully the hydraulic gauge i'm about to install will verify things one way or the other. i had the alarm on my edge set to go off at 8 psi so when it kept going off more and more frequently i became more and more paranoid... anyway i'll report back in a second once i get my own first hand pressure data.

 

[dillrepair]

yeah he flat out lied about the pressure... new mech gauge reads just like the old one that broke.. shows about a half a psi lower than the electronic gauge that came with the edge module. so in case anyone's passing thru cheyenne id say stay away from "Mr Diesel' . he did the APPS like he said, i pulled the cover and checked that, but why lie about checking the hydraulic pressure like that? i'd rather have honesty: "its still low, i don't know why" . anyway got a nice new fluid filled industrial gauge for only $25 from "industrial distributors" downtown... those guys are worth a visit if you need something to get a job done.

 

[petersonj]

Hopefully the rest of your trip will be uneventful. Please keep us posted as to how the truck performs and the final outcome of the low fuel pressure issue should you figure out what is going on with your fuel system.

- John

 

[dillrepair]

thanks j. yeah i think i'm going to get a couple banjo fittings tomorrow and some fuel line and run both pumps in series for the rest of the haul to reno to keep my pressures up. i don't think it would have been very good to be towing this 36' bumper pull camper in the wind as its been today anyway... its gusting to 45-50mph.

 

[dillrepair]

running pumps in series with no issues, idle pressure at 25-27 and running under heavy load uphill haven't seen it drop below 17. maybe it was as john said.. just a combination of altitude and throttle position sensor and a weaker than needed lift pump for the conditions. she does seem to be running stronger now with more reserve pressure and the new apps. i'm leaving my edge on drive mode because it does so much better uphill ... i watch the EGTs really carefully though and let off if i get close to 1000... seeing ~60-70% load at times uphills with egt topping off around 1000. now all i have to do is get some weather conducive to escaping this state. sorry wyoming residents... not a big fan of your state, esp in winter.

 

[petersonj]

Thanks for the feedback - sounds like you should be able to complete your journey without any more fueling problems.

Running your pumps in series is probably benefiting you because even though you have doubled the pressure the flow has remained the same, but flow volumetric efficiency has improved because each pump is operating in a more efficient range. Example: "Under heavy load uphill haven't seen it drop below 17" would mean (if both pumps are equal) that each pump would be operating a 8 1/2 psi.

Good luck in your travels,

- John