Weird fuel pressure problem

[JJMiller]

I posted a while back about a fuel pressure problem I was having. My guage would all of a sudden start bouncing down to 0, then back up to 10, then play around in the 3-7 range, then BELOW 0 then up to normal again... .



I took another trip today, this time I paid more attention to what was happening:



First 60 miles--normal, ~10psi



60 to 100 miles--Normal at slow speeds (less than 55mph), bouncing all around and even into suction at highway speeds (55+). Even when dropping below 0, it would bounce instantly back to 10psi when I let my foot off the pedal.



Return trip after truck sat off for several hours:



Same thing--first 60 miles or so were normal, then it started acting up again. --But then I tried something when it started to dip and dive again. I pulled to the shoulder, popped the gas cap (i know ... it's not a gas cap. ) put it back on, and started driving again. The pressures continued to drop erratically, but would not fall as low.



I have double-checked all electrical connections, and they are rock-solid. I'm going to take it to the dealer when I get back in town after a week, but in the mean time, does anyone have any suggestions on what else to look for besides a dying lift pump? Is it possible that a suction is being created somewhere which is why it takes 60 miles to have an effect?



Thanks,



---J

 

[Mill Rat]

Just a thought

John, is it possible that either the sender or the gauge have an internal electrical fault? Could the lift pump be losing it's power feed momentarily? I'm no expert, but, being this erratic I'm still somewhat suspicious of an electrical fault. Good Luck.

 

[johnrv4]

I would be suspicious of the fuel pressure sending unit also. I've seen quite a few of the electrical pressure senders go bad after a short time. I would try a mechanical gauge just to confirm it.

 

[OILBRNR]

Below zero? How is a Westach unit showing that? If it is, you have a wiring problem, which I'd suspect anyway from your description. Where did you ground the sending unit?



I've had mine on for over a month now, which is not a real shakedown, but no problems what so ever. Other than showing me that my fuel filter needed to be changed about a week ago. Went from 10 psi at 65 to 3-4 psi the next day after a fillup. New filter, and I'm idling at 15 with 13 at 65. 10 under WOT.

 

[BIG BOB]

My truck is BOMBed, I was having the same problem with my lift pump. I installed a Mallory 140 and a Racor 645R30 fuel filter back by the fuel tank, removed the stock lift pump and filter. I now is solid as a rock. Here is my post;



https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=20450

 

[Bigsaint]

I would hazard a somewhat educated guess that your lift pump is on the way out. I experienced the same type of all over the page readings for about one week before my first Mallory started pushing up daiseys. When you see negative psi's, you are seeing the VP44 sucking in fuel, rather than it being pushed forward by the lift pump. When mine died, the truck was idling at -11 psi's, according to the SPA gauge. My first thought was the same as other's here, bad sender. I was wrong. I don't know whether your FP gauge uses an elctrical sender or is mechanical, if it's electrical, sure, it could be the sender. My guess is on the pump, though.



Scott W.