Sporatic Fuel Pressure on SPA Guage: Lift Pump going bad??

[RobG]

While coming back from LA yesterday, with my camper loaded and towing a car on my flatbed trailer, I noticed something unusual...



On my SPA fuel pressure guage, which usually shows 10-12 psi at cruise, and 6-8 while accelerating, it started bouncing around all over the place... positive pressure, negative pressure, fault (FL), etc. But ONLY when I was on the throttle using 1/2 or more throttle. If I let off or just set the cruise, it would, after a minute or so, settle down and show it's usual 10-12 psi. ONLY under a heavy throttle condition does it freak out.



That being the case, I don't think it's the SPA sender. Instead, I have to wonder if it could be indiciative of an impending lift pump failure? The truck still accelerated okay, though I suppose it did feel a little weak (hard to say, really). Fuel mileage was lower this trip than in the past, in the high 10's to low 11's (shoulda been 12+).



Fuel system is stock (no custom banjo bolts or fuel lines). The rest of my mods are in my sig.



Whaddaya think?



Rob

 

[chuck3]

Is this the first tow with the gauge installed?

 

[jnewkirk]

Rob,



I had the same problem due to improper grounding. Make sure the ground wire to your gauge is attached at the same location as the ground to the sending unit. All grounds aren't equal with electric gauges. Good luck.



John

 

[Bigsaint]

Rob, if you have had the SPA installed for awhile and are just now seeing the fluctuations in FP, my take is that the pump is dying.



I had the same readings on my SPA with the first Mallory pump, I foolishly ignored them, thinking that it was a sender problem, it wasn't and I found out the hard way one day. I had parked the truck, left it running for a short trip into a local business and came back out to see -11 psi's on the SPA. I still thought it was the sender, so I decided to turn the truck off and bump the starter, to listen for the wine of the Mallory pump. Could'a heard a mouse fart at 1000 yds, dead silence after I bumped the starter, the Mallory was pushing up daiseys.



That's one of the reasons I added a second, mechanical FP gauge to my truck. It just takes one the concerns out of the loop.



When the second Mallory started to die, my mechanical gauge confirmed the SPA erratic readings. Started with a noticable drop in psi's at idle, varied between a drop of 1-2 psi's. I bumped up the deadhead on the Mallory, all was fine for a few days, then I lost more psi's. Several days later I started to see the huge fluctuations again, especially under throttle. This time I learned from my past experience and replaced the Mallory before I became stranded.



If I'm good at nothing else, I shine in the area of killing lift pumps .



Scott W.

 

[barryg41]

I had fluctuating problems with my boost sensor and transmission temp sender. Same problems as you did RobG.



Dielectric grease fixed my problems. I am not saying that you lift pump is not going south but it is worth the time to see if the connection is not the problem.



Several TDR members have had problems with the connections not holding up with time. The grease has helped.



Bigsaint..... "Could'a heard a mouse fart at 1000 yds"... LOL



I hope the connections are the problem not the lift pump.

 

[dresslered]

Fuel system is stock (no custom banjo bolts or fuel lines).

You need to give us some more information. How and where is the fuel sender mounted. If it is mounted on the test port of the injection pump, I'll bet "The Saints" next pay check that it is the sender.



Second bet (BigSaints next pay check) is the grounding as jnewkirk suggests. An electronic fuel pressure sender should have its own "isolated" ground coming off of the pressure sender rather than rely on the grounding through the threads. Being a SPA, I don't have a lot of confidence.

 

[Bigsaint]

LOL@ D. Dressler, betting my paychecks and such .



If Rob had the sender mounted on the inlet to the VP44, my guess is that he would have seen erratic psi's since day one. I've had my SPA for about one year now, approx. 4 months mounted prefilter,direct to a pressure port w/o a snubber, remaining time mounted post filter, again, directly to a pressure port at the outlet to the FF. I did have to change the display refresh rate to keep the numbers stable, but that was all I did at the time of the install post filter. I can't see how Rob would have the sender mounted at the inlet to the VP44 and see stable readings, unless he retarted the display rate way back.



I guess it could be traced down to the wiring inside the 90 degree elbow, which attaches to the sender. If you have every taken one of these apart, you will see that the contact area on the wires is very small. If you re-read Rob's post, he states that it only does it under heavy throttle, which still leads me to believe it's the pump. The only thing I didn't see with mine, that Rob is, is the FL (fault) on the SPA. Mine simply transitioned from positive to negative psi's as best I could tell, then again, I wasn't in the truck when it lost positive pressure.



I'll up the pot and bet 3 of D. Dresslers paychecks on the pump getting ready for a long dirt nap.



Scott W.

 

[BigDaddy]

Originally posted by RobG

it started bouncing around all over the place... positive pressure, negative pressure, fault (FL), etc. Rob

I'll bet both Bigsaint's and dresslered's next pay check that it is the plug connection Erratic readings or "fl" (fault) is normally associated with an open circuit. It could be signal, power or ground.



Do the dielectric grease as suggested by barryg41 and check the connection plug at the transducer for security.



If the problem/indication still exists get a mechanical gauge and double check your pressures.



If it's the lift pump PM dresslered on where to send his check. This should more than be enough to buy a new lift pump and a spare :-laf

 

[MCummings]

See about Getting Briar Hoopers FP Gauge.



I'll say it's the Fuel pump planting daisy seeds. It's gonna be pushin' 'em up soon.



My Lift Pump was doing the exact same thing,, "Normal" Then ,, ALL OVER THE PLACE.



Here's a little test you can do if you really trust your FP Gauge.



Drain some fuel out of your fuel filter, then Bump your starter and watch how fast your Lift Pump builds pressure,, You should build ATLEAST 15PSI ASAP. My Lift Pump would Peg my 0-15 gauge when bumping the starter for prime,, ASAP. Right before it's death, it took a little while,, maybe 2-3 seconds,, you could watch it slowly climb from 10 pounds to 14. 5 pounds.



I'd say replace it,, or atleast buy a spare for when it does go ALL the way out.



Merrick Cummings Jr