Erratic fuel pressure?

[CoastyAV8R]

I know I'm beating a dead horse here, but here goes. Last month I installed a Westach fuel pressure gauge using the fitting at the bottom of the fuel filter housing using about 2. 5 feet of fuel hose to isolate the sending unit. At first everything was rosy, good steady pressure around 14 psi at idle and didn't drop below 7 psi WOT. In the last few days the readings have gotten erratic and I'm not sure if my lift pump is on it's way out or if my fuel pressure gauge set up may have a bad sending unit. By erratic I mean 10 psi then go down to about 3 psi while driving and about 1 psi WOT. I shut the truck off and restart and it goes up to about 13 psi then drops down to 10 psi at idle and then downhill from there. Has anyone experienced anything like this? Not sure if it's the lift pump or sending unit. What do ya think?



2001 QC ETH 4x4 25,000 miles

 

[LSMITH]

My guess would be that it is the lift pump from the history of them. You could get a new sender and put it in. You could also take your sender out that you have now and hook a regulated air supply to it to verify it's accuracy. Hope this helps.

 

[RJOL]

It may be the lift pump, but I'd check out the gauge installation first. Check all the wiring, especially the ground from the sender and the gauge. Also check the connections on the back of the gauge. The wires can slip off the pins very easily, or just have a bad contact. If that's okay, call Westach and see if they have any ideas, or see if they will send out a new sender/gauge. If that is not the problem, you may need to take it to the dealer and have them check the system out. If it is you lift pump, don't delay in having it repaired. The VP44 doesn't like a poor fuel supply...

 

[15w40]

It has been my experience that the lift pump gives a warning of erratic pressure before it fails. It will sometimes run with normal pressure for miles and then drop to low pressure. This is in, stop and go, city traffic and occasionally on the highway when climbing a hill. I have been watching my original lift pump pressure, in my '98, for about 50K miles before I decided to change it to the latest revision. The old pump never got to zero PSI and it may have gone another 50K miles but I did not want to get stranded at some inconvenient place. After changing the pump the pressure varies with the loud pedal but is much less erratic. Other posts have also said that the lift pump can cause the flakey pressure readings, but a bad sender can also be the problem. Maybe connecting a mechanical gauge will tell you which is causing the problem.

 

[tgbol]

CoastyAV8R

Did you change fuel stations? I found that when I bought fuel in a different station (same brand) that my lift pump would act like yours. Had a full tank and 7# I added a pint of Marvel Mystery Oil to the tank and cycled the fuel pump 4 times. My fuel pressure came right back to 14#. The station that I had bought fuel at was not a truck stop and I think the fuel was a little sour. I have found that every time the fuel pressure starts to waiver a little MMO seems jto take care of the problem. I use a pint every other tank now. Just turned 30,000 miles. I have come to the conclusion that a lot of the lift pump failures are caused by a lack of lubericty in the diesel fuel.

 

[CoastyAV8R]

Thanks for the replies. In the last 2 days the pressure has been great. 14 psi at idle, no lower than 10psi cruising down the road. Friday was a perfect example. Leaving work, 14 psi at idle 10-12 psi driving. Then all of a sudden I look up and it's at 5 psi at a light and went down to 1 psi driving. Even when it's in this condition, the psi I see is directly controlled by how much throttle I'm giving it, higher at idle and it drops when I give accelerate. About 2 miles later it 's back up to 14 psi at idle and 10psi driving. Damndest thing I have ever seen. PSI has been great on a drive to work Friday night and on 70 mile drive today. I'm trying to figure out in what manner the pump dies when it does die. Is it a gradual decline in PSI or is what Im seeing and lift pump on it way to the dumpster? I have checked the wiring for a good ground and the wiring on the gauge itself with no problems found. I guess I'll just keep an eye on it. Hard to fix it when it ain't broke... yet.



Pat

 

[Bigsaint]

Pat, I'll hazard a guess that it is the pump dying. I didn't have a FP gauge when the Carter was still in the truck, but did with the first Mallory pump. The Mallory was doing exactly what your pump is doing now, at times great psi's and at other times large fluctuations in the psi's. I thought it was the sender on the SPA, no way it could be the Mallory, I thought... Well, I left the truck at idle one rainy day for a few minutes only to return to find a -11 psi reading on the SPA gauge. I decided to shut her down and then bumped the starter to listen for the pump, all I heard was dead silence. At least you are aware of the problem, get yourself prepared to replace that pump IMHO. This is why I decided to add a mechanical pre-filter gauge, takes the guess work out of erratic pressure. Plus, it was a good excuse just to add another gauge .



Scott W.