Erratic fuel pressure

[d90boy]

While checking my fuel pressure on my new truck a month ago the fuel pressure read a steady 16 psi at idle. Now the needle jumps up and down around 14 and 15psi. Is this a sign of a lift pump going south? I'm using an OTC fuel pressure gauge and all the air has been bled out of the line from the schrader valve to the gauge.

 

[Ramboy]

I wouldn't worry too much about it. I think you would have to see some readings closer to zero for it to be a pump problem.

 

[TowPro]

My last one would have 12-14 at startup, then drop to 0-2, then up to 12 again several times, then drop to 0.

 

[Bigsaint]

I agree with the above replys. Unless you are seeing huge swings in FP under throttle, or a dramatic loss of at idle psi's, I wouldn't be too concerned right now. The Mallory pumps I ran in the past, along with the pump I'm running now, all seem to show a +/- of 1 psi at idle, day to day.



Keep an eye on it, if the pump is dying the psi's at idle will continue to sprial downward and you eventually start to see the huge fluctuations that TowPro report, especially under throttle. When the pump starts showing the real erratic psi's, you are on the short list for a new pump, in my limited experience.



Scott W.

 

[d90boy]

Thanks all, I'll keep an eye on it.

 

[andrewkoperdak]

d90,



My lift pump would make good pressure until: 1) my fuel level was at or less than 1/2 tank 2) the engine had been run about 20 minutes or more. Just before it failed completely I could only get 5 psi at idle after 20 minutes of driving. Testing it cold in my driveway I could always get 15 psi but once the lift pump got warm from running or engine heat transfer it would bite the big one.

Andy

 

[Steve Campbell]

AKoperdak, I think you have....

figured out why most dealerships can not find the bad lift pump.



Example, owner brings truck in at 9:00 to have problem diagnosed. Tech starts it up at 1:00 to pull it in shop and connects fuel pressure guage. All readings are normal. "Can not duplicate symptoms" is the official prognosis. If he had hooked the guage up when the truck first arrived... . bingo!!!



Rereading all of the responses makes an even stronger case (in my mind) for the installation of a fuel pressure guage.